THE EXPANSION OF TAXONOMY WITH SPECIAL REFERENCE TO SPERMATOPHYTA

342

THE EXPANSION OF TAXONOMY WITH SPECIAL REFERENCE T O SPERMATOPHYTA

BY W. B. TURRILL (Royal Botanic Gardens, Kew)

(Reckwed I October 1937)

CONTENTS

I. 11.

111. IV. V.

VI. VII.

VIII. IX. X.

XI. XII.

XIII. XIV.

Introduction . . . . . The logic of classification . . . Alpha taxonomy . . . . . Cytologyand taxonomy . . .

Ecologyand taxonomy . . . . Palaeontology and taxonomy . . . . Phylogenetic classification . . . Biometrics and taxonomy . . . Physiology and taxonomy . . . Development towards an omega taxonomy Summary . . . . . . Acknowledgement . . . . References . . . . . .

Geneticsand taxonomy . . .

PAGE . . . . 342

. 345

. . . . . 350 . 355

. . .

. . * . 346

. . .

. . * . 358

. . . . 362

. . . . 365

. . . . 366

. . . . 367

. . . * 369 . 370 * 371 . . . . 371

. . .

. . .

I. INTRODUCTION

TAXONOMY, from the derivation of the word, is “ the law of order” (Turrill, 1 9 3 6 ~ ) . In biology, its subject-matter is usually restricted to the classification of animals and plants, mainly or entirely according to their morphological (including, especially for some groups, anatomical) characters, into groups known as families, genera, species, etc. Such classification involves, of course, much more than the mere arranging of specimens in pigeon-holes. The characters on which any classification is based have to be studied as intensively and extensively as available material allows; the correlations of such characters have to be determined, though they are unfortunately not often expressed in statistical terms ; comparisons have to be made over a wide range with similar organisms (i.e. in taxonomic phraseology, “the affinities have to be determined”); precise descriptions of the groups have to be prepared on a comparative scheme indicating the relevant distinguishing characters ; drawings, especially of dissections, have to be prepared ; nomenclaturally valid names have to be found or made. These and other methods of orthodox or “alpha” (Turrill, 1935) taxonomy have resulted in the accumulation of a vast literature, of floras, manuals, monographs, revisions, and papers scattered in a large number of periodicals. Even with the help of such general guides as the extremely valuable

The expansion of taxonomy 343 Index Kewensis, Index Londinensis, Die Pflanzenfamilien, and the Genera Plantarum, and within a limited group such as the Spermatophyta, it requires years of experience to become even relatively familiar with the published results of plant taxonomy. One may, however, say with confidence that the results achieved can, considering the difficulties, be regarded by taxonomists with a considerable degree of satisfaction. While the interpretation to be placed on many facts may remain a matter of dispute, the facts themselves have been discovered and made available on a large scale and in an increasingly precise form. The amount of agreement is really much greater than the amount of disagreement, even as between two such contrasted systems as that of Bentham and Hooker and that of Engler and Prantl.

It is not possible to give any accurate figures for the number of taxonomic groups of organisms now accepted, especially at or below what is usually recognized as a mean “species level”. Perhaps it would be a fair basis for discussion to say that about a million species of plants and animals have been classified, described, and named by taxonomists. For the seed-bearing plants alone the figure may be very roughly estimated at 250,000. The number of new specific names published in recent supplements of the Index Kewensis, and for the Spermatophyta only, averages 35,000 for every five years. These names, however, include new combinations and nomina nuda. My colleague, Miss M. L. Green, who is in charge of the compilation of the Index, tells me that about 2000 new (or supposed new) species of seed-bearing plants (excluding fossils) are named and described per year, These figures are sufficient to indicate the practical need of classification, if only for the purpose of reference. The widely accepted taxonomic species is here taken as a convenient unit of classification, but every species, it must be remembered, is represented by a large, often an enormous, indefinite, and for all practical purposes an infinite number of individuals. T o find with ease any one of a number of things some arrangement is essential. This applies to plants in a garden, to specimens in a herbarium, and to written or printed data. To enable reference to be made for purposes of correlation, attesting, or even common understanding, classification is necessary, and, usually, is simplified by such devices as names or other symbols. One wonders if the cytologist, ecologist, or geneticist sufficiently realizes his debt to the pioneer work of the taxonomist. Taxonomy, as it mainly is at present, has developed without their aid, but it is difficult to imagine that they could have advanced far had not the main “lines of latitude and longitude” been laid down over the existing flora and fauna by the taxonomist. An important function of taxonomy is, then, the frankly utilitarian one of providing means of identification by placing organisms into diagnosed groups and thus enabling biologists to tell one another what organisms they are studying, of enabling them to procure the material they want, and so to state the results of their researches that they can be attested and utilized for further research. It may well be emphasized here that another practical result of classification is to indicate gaps, whether in material collections or in knowledge. It may be suggested that more attention should be paid, in the future, to this aspect. Very frequently taxonomists (in common with other biologists) refrain from a public acknowledgement of ignorance. To set out, concisely, what we

344 W. B. TURRILL want to know but have not at the present means of knowing, not only clarifies thought and practice but may lead to valuable research. Similarly, “ negative ” results should also be published. It may be just as important to know that two species will not cross, or will not grow near each other, or will not fit rigidly into a simple classification, as to know that they will.

To the primary utilitarian aim of taxonomy various authors have added others, and the result has sometimes been confusion rather than clarification. Thus it has not infrequently been urged that the ultimate aim of taxonomy should be the production of a phylogenetic system. This raises questions of fundamental importance, and these are discussed at some length below. Again, it has been suggested that classification should be based on some even more idealistic or abstract concept than phylogeny (see examples given by Bather, 1927).

Taxonomy is one of the oldest branches of biology. Very early in his history man must have commenced to recognize different kinds of animals and plants, to group them, and to give names to the groups. Much of the grouping was no doubt on the basis of the real or supposed relationship of the organisms to man-food- supplying, protection, harmful, medicinal, sacred, etc. Later, obvious differences of habit-as trees, shrubs, herbs-were used as diagnostic of groups. Still later, less obvious characters, internal and external, were described, and utilized for making categories which gradually became expressed in terms of species, genera, families, etc. The development of physiblogy in the narrower sense has had, on the plant side at least, very little influence on taxonomy. Increasing refinement in morphological research and, with the advent and wider use of the compound microscope, a widening of morphology to include anatomical details have, of course, modified descriptive terminology and increased the number of characters which are studied and used. They have not, however, profoundly altered either taxonomic theory or practice. During the last three decades, several branches of biology, and especially cytology, ecology, and genetics, have shown phenomenal development. Perhaps their growth is largely due to their being biological, and not merely botanical or zoological, and also to the fact that they cannot be clearly classified as morphology or physiology, but are intimate compounds of both. Their development has certainly brought them h t o closer and closer contact with taxonomy. Sometimes, indeed, there has been a rather violent impact. Further, many taxonomists have along their own lines been extending their field, not only by the exploration of floras and faunas more and more remote both in time and space from those previously studied, but also by biogeographical investigations which have led them to consider many matters new to them. There are many signs in biological literature published recently (that is, within the last ten years) that taxonomy is “at the cross-mads”. None, whose opinion on this matter is worth considering, will deny that a very great deal of most useful research has still to be done along the traditional taxonomic lines. Nevertheless, there is an increasing desire amongst taxonomists to consider their problems from wider view-boints, to investigate the possibilities of closer co-operation with their cytological, ecological, and genetical colleagues, and to acknowledge that some revision or expansion,

T h expansion of taxonomy 345 perhaps of a drastic nature, of their aims and methods may be desirable (cf Stojanoff, 1936).

What follows is an attempt to describe, rather more in detail than has so far been done, what changes are taking place in taxonomic outlook, and also to formulate what may be the lines of future development.

11. THE LOGIC OF CLASSIFICATION

In a recent contribution Gilmour (1937) has drawn attention to the need for a reconsideration of the purpose and general principles underlying the process of classification. He points out that in its simplest terms, classification consists in grouping individual objects (and, ultimately, sense data, expressed as qualities and relations) into classes, so that all the individuals in one class have certain attributes in common ”. Again, he writes : fundamentally, the purpose of biological classification is the acquisition of ordered knowledge regarding living things, and logically any grouping of plants and animals should be considered a taxonomic process.” This is certainly true, and a very large number of classifications of animals and plants are constantly being made for special purposes. It may be suggested that while it would increase precision to have agreement as to whether the name Taxonomy be kept for a special type of classification or used in a broader sense, it matters little which is done so long as a clear and workable definition be given and maintained. Of greater importance is the able analysis given by Gilmour of the meanings which have been attached to the phrases artificial classification ” and “natural Classification”. He notes that “it is usually stated in logic that a system of classification is the more natural the more propositions there are that can be made regarding its constituent classes.. . .Thus a natural classification is one founded on attributes which have a number of other attributes correlated with them, while in an artificial classification such correlation is reduced to a minimum.” The difference between a natural ” and an artificial ” classification is thus one of degree. All classifications are man-made abstractions and, in this sense, can be said to be artificial ”. Since, in biology, classifications are based on the attributes of natural objects-animals and plants-they must in some degree be also natural ”. Actually the words “artificial” and “natural” have been used with so many connotations that it might be best to replace them in taxonomy.

There is no doubt that discussions on the basis of classification soon touch some of the deeper issues of philosophy. Not only do epistemological theories come under consideration, but fundamental beliefs of the nature of existence must be accepted or rejected-at least in any given practice. Any one of a dozen oft-repeated sayings of biologists can, on analysis, be shown to involve the acceptance of some broad philosophical dogma, which if expressed in general terms and its logical consequences set out plainly would rather shock the biologist. Fortunately for his peace of mind, the biologist rarely worries about either logic or philosophy. It may, indeed, be argued that the biologist has a scientific right to use any philosophy he likes, as he likes, temporarily or permanently, so long as it gives him a “method

346 W. B. TURRILL of dealing with (his) available data (enabling him) best to correlate and to increase our knowledge”. The logician and philosopher would say that there can be no permanent advance in knowledge unless the research is founded on a logically and philosophically correct basis. This is no doubt true in the long run, and analyses like those of Gilmour must prove of great value in clarifying biological and especially taxonomic concepts. On the other hand, logic and philosophy are not themselves absolute ; they are continually being modified and the biologist can point to many imperfections in any given system of philosophy. If this statement meets the eye of any logician or philosopher perhaps he will regard it as an appeal for help. A number of mathematicians have turned their attention to biological problems and are building up the important “ science ” of biometrics. We biologists would welcome a full logic of biology, or a I ‘ biologic ”, or at least such help from trained logicians and philosophers as would give us new tools to replace such as we have worn out, or to use along with those still serviceable. An inspector-general of biological methodology is badly needed. Until he arrives we have to do the best we can with rather an undue amount of dissatisfaction and superficial quarrelling amongst ourselves.

111. ALPHA TAXONOMY

There is no need here to review in detail the great contribution taxonomists have made to biological knowledge. Not only have they provided classifications and more or less ready means of taxonomic identification of plants and animals, they have also, in the process of classification, provided an immense mass of facts of the most diverse kinds. That many of these facts are “locked away” in essentially taxonomic literature is not, in the main, the fault of the taxonomist. It may well be urged that the time is ripe for biologists in general to familiarize themselves more with this literature and to abstract and reclassify from new standpoints many of these facts. It is true that up to the present what is usually regarded as orthodox taxonomy has been based on structure, external and internal, macroscopic and microscopic. One may say that there are only two subsciences of biology: morphology and physiology, the study of form and the study of function. All other divisions are further limitations or, more often, ill-definable mixtures or compounds of morphology and physiology. I ‘ Traditional ”, I ‘ orthodox ”, or .IL alpha ” taxonomy is based on morphology, but since even museum or herbarium specimens, within broader limits than are often realized, represent the structure of living organisms, the taxonomist is frequently confronted with problems which have a physiological aspect. This is particularly true when the taxonomist ceases to be content with mere description, identification, and naming, or with “ practical ” classification, and commences to explore the broader problems of distribution and evolution or to speculate on origins and causes. Many of the younger taxonomists have had some training, even if only academic, in physiology sensu stricto, cytology, ecology, genetics, etc. This is certainly one reason for renewed interest being shown in taxonomy, but from new standpoints. Turrill (1935) has suggested that while accepting the older invaluable taxonomy, based on structure, and conveniently designated ‘‘ alpha”, it is

The expansion of taxonomy 347 possible to glimpse a far-distant taxonomy built up on as wide a basis of morphological and physiological facts as possible, and one in which ‘ I place is found for all observational and experimental data relating, even if indirectly, to the constitution, subdivision, origin and behaviour of species and other taxonomic groups ”. Ideals can, it may be said, never be completely realized. They have, however, the great value of acting as permanent stimulants, and if we have some, even vague, ideal of an “omega” taxonomy we may progress a little way down the Greek alphabet. Some of us please ourselves by thinking we are now groping in a “ beta ” taxonomy.

There is one very important matter which needs discussing before we proceed to enumerate some of the attempts being made to expand taxonomy. Briefly, it is whether an “omega” system of classification is or is not possible, and if it be possible would it be useful. As a general alternative which can be further analysed, should alpha taxonomy be entirely cleansed of everything except morphology, to this completely morphological classification the traditional categories (of families, genera, species, etc.) and nomenclature be limited, and subsidiary classifications be formed based on cytological, genetical, etc., attributes, and with their own categories and nomenclatures? It should be noted that, whatever the decision, the advisability of experimenting with subsidiary classifications is not in dispute. They may be valid logically and of very great use for special purposes and within definable limits, Since alpha taxonomy is essentially based on attributes derived from a study of structure, the following argument takes morphological classification as a standard of reference. With the necessary and uniform replacement of a few terms it should apply to any classification.

A classification will only fulfil its practical functions if, at any one level, class attributes contrast. Further, unless the same general type of attribute is used throughout the classification (i.e. at all levels) there is grave danger of fallacies arising in its use. There is, for example, a particular danger in phytogeographical studies if one uses a classification in which the units (e.g. species) are defined, even in part, by their geographical isolation. If the danger be known certain safeguards can be applied as has been suggested elsewhere (Turrill, 1925). Sprague (1925) had this danger in mind when he wrote ‘‘ No category of characters, whether morphological, anatomical, chemical, or physiological, should be neglected. Differences in geographical distribution, however, should not be treated as taxonomic characters, except in so far as they can be shown to be the result of special climatic or edaphic requirements-otherwise the resulting groups will be relatively unreliable so far as any phytogeographical deductions are concerned. Floristic geography is based on taxonomy, not vice versa.” In conversation, Dr Sprague has pointed out to me that his sentences can be paraphrased by saying that no characters of the plants themselves should be neglected in classification. The geographical distribution of a taxonomic unit is not necessarily, however, a character of its constituent members, as it may be due to causes purely external to the plants themselves, such as chaqges in the distribution of land and water. An exception is made where distribution can be correlated with such characters as favour survival under given climatic or edaphic conditions. An extension of the idea underlying Sprague’s remarks seems to me

348 W. B. TURRILL to need consideration. Surely if one includes as attributes in a classification biochemical characters, for example, deductions from such a classification regarding these characters will be unreliable to the extent to which they were used in the original classification. Thus, to say that alkaloids are common in the Solanaceae gives a correlation between a biochemical character and certain morphological characters, but if one deliberately. places in the family certain plants because they contain alkaloids, while if they did not one would place them in another family, then such a statement is either not a new deduction or is unreliable to the extent to which the character of containing alkaloids was used. Somewhat similar fallacies abound in phylogenetic speculation in plant (and probably animal) groups where palaeontological evidence is absent or meagre. A taxonomic group is said to be primitive because its members show primitive characters, while the characters are said to be primitive because they occur in a primitive group. All phylogenetic schemes are not, of course, based on such “attempts to prove two propositions reciprocally from one another” (Mill, 1919, p. 538), but it is surprising how often a careful analysis reveals such arguing in a circle ”.

One could, then, reach the conclusion that a classification should be based on one kind of attribute only, if it is to be validly used for drawing deductions regarding correlations with attributes of another kind. In our standard example, a morphological classification of plants or animals should not involve chemical, physiological, geographical, etc., characters at all. The matter, however, is not quite so simple as it may appear from the standpoint of strict logic.

morphology”, and what excluded? Morphology (plant or animal) nowadays includes a good deal more than the mere “ study of shape ”. Anatomy is certainly morphology by dissection often with the aid of a compound microscope ; much, but not all, cytology is morphology with a twelfth-inch oil-immersion lens ; ontogeny. embryology, palaeontology, etc., are very largely morphology ; genetics is in theory physiological, but in practice largely uses morphological characters, and so on. Even within morphology in a very strict sense the taxonomist is forced to consider matters which involve physiology. The degree of correlation of characters and the constancy or plasticity of characters are so closely related to internal and external factors that the taxonomist must, to a certain extent, consider the physiological working of these last. Morphological characters are generally the most visible signs of inward and physiological grace and, indeed, it may well be concluded that even the useful classification into morphological and physiological characters (attributes) is artificial, in the sense that it is purely a device of the biologist. Yet other difficulties appear in practice. In a classification based entirely on morphological characters only phenotypic characters could be used. Thus no greater or less value could be given to characters with a distinct genetic basis than to such as represent mere plasticity due to different environments acting on the same genotype. In an individual tree sun-leaves and shade-leaves would belong to different taxonomic units, as might ramets of a clone, if differently treated. Good examples of phenotypic variation with clones will be found in the reports on the Transplant Experiments of the British Ecological

Exactly what is to be included under

The expansion of taxonomy 349 Society at Potterne (Marsden-Jones & Turrill, 1930, 1933, 1935, 1937). The plasticity of one genotype of Plantago major is particularly striking in this connexion. A classification of phenotypes, irrespective of their genotypes, is often valuable and even essential, but as a classification for general purposes it may be misleading. This would be particularly true in biological applications to hodculture, agriculture, etc. Phenotypically, heterozygotes showing complete dominance are the same 3s

the corresponding dominant homozygotes when kept under uniform conditions. It may be said that only morphological characters can be used to distinguish the

higher taxonomic divisions (phyla, families, etc.). Certainly experimental breeding by hybridization appears at present out of the question because of sterility barriers. One must, however, note that many “characters” given high value in orthodox taxonomy (which is mainly morphological) are essentially either physiological or an indissoluble combination of morphology and physiology. Examples are wqrm- bloodedness, vivipary, lactation, the seed-habit, and angiospermy. One must also admit the possibility of improved serological and biochemical methods coming into general use. In addition, there is always a high degree of correlation between morphology and physiology, however one limits these terms.

Lastly, but very important, is the fact that there are many different morphological characters and that the taxonomist often chooses for use those which his experience leads him to accept as “ essential ” or likely to give the clearest classification or that most easy to use. Sometimes the taxonomist frankly acknowledges that he has chosen arbitrarily. The genera Celsia and Verbascum have recently been monographed by Murbeck (1925, 1933). Considering together the species placed in Murbeck’s works in the two genera one notes that the following (inter alia) character opposites could be used as primary taxonomic characters: 4 versus 5 stamens; flowers solitary versus flowers in cymose glomerules; an androecium of stamens all with not decurrent anthers versus an androecium with 2 or 3 stamens having not decurrent anthers and 2 stamens with decurrent anthers. The character of stamen number was chosen (by Linnaeus in his Genera Plantarum) to separate the genus Celsia (stamens 4) from Verbascum (stamens 5 ) . A choice of other differences would have given other classifications, yet in species of both Cehia and Verbascum (as accepted by Murbeck following Linnaeus and most intervening taxonomists) species occur with the other above-mentioned contrasting characters. Numeroils other comparable examples could be given-in Cyperaceae, in Liliaceae- Amaryllidaceae, etc.-amongst plants, and zoologists could probably give corresponding examples from animal taxonomy. This possibility of alternative morphological classifications is important and will be referred to again.

That taxonomists often choose (abstract) the characters they use as diagnostic for their groups and give them, more or less arbitrarily, higher or lower taxonomic values has one important practical bearing which is not always recognized. Often several, equally valid, classifications can be made of the same organisms, entirely on morphological attributes. In all probability one of these will be more in accord with cytological, genetical, etc., classifications than the others. Alpha taxonomy is conservative but it is not inflexible.

3 50 W. B. TURRILL As the result of a considerable and very recent correspondence on taxonomic

problems I deem it desirable to interpolate here a paragraph on the use of anatomy in plant classification. As' already stated, anatomy, as understood in modern botany, is essentially morphology with a compound microscope. Some plant anatomists (or histologists) seem, however, rather offended unless their important contributions to taxonomy are recognized separately from those of the morphologist who deals only or mainly with macroscopic features. The use of the compound microscope is obviously essential for the classification of many of the Thallophyta, whose morphology is anatomical (e.g. many Algae and Fungi). The taxonomist has frequently to use details of structure in classifying even vascular plants. In the identification of Pteridophyta, Coniferae, and many Angiospermae the compound microscope is frequently used. Further, the alpha taxonomist usually welcomes additional microscopic characters, and in some taxonomic works they are now given as part of the complete description of families, and even of genera and species. Modem histological methods, in section-cutting, staining, etc., have made considerable advances in knowledge possible, and the establishment of institutions and depart- ments largely devoted to studying the anatomy of economic products, such as timber, has resulted in the accumulation of much knowledge which must finally be incorporated in taxonomy. The alpha taxonomist has long recognized the value of anatomical characters and the omega taxonomist will still further use such data.

Since, then, we meet a practical difficulty in defining morphology, and since morphology and physiology are so intimately linked we can biologically justify a consideration of the opposite extreme to special (" multiple ") classifications. Is it possible and desirable to aim at a general classification in which all attributes shall be weighed in the balance and given a taxonomic value? The logical correctness of such an attempt may, to a certain extent, remain in doubt, but if a sound practical result can be obtained, logic must accept it.

Before attempting to see the dim outlines of a full general classification, a far distant, if not visionary and unreachable goal of the most optimistic taxonomist, it will be well to attend to the possible contributions which other branches of biology, and especially those of considerable recent development, may be expected to make, or even have already made, to taxonomy. It will be most convenient to use as headings the names given to more or less distinct divisions of biology, though it must be remembered that much of the most useful and instructive research has used combinations of methods or has been at the border-lines where the artificial divisions meet.

IV. CYTOLOGY AND TAXONOMY

A considerable amount of cytological research is essentially morphological. From one standpoint the intimate details of cell structure, and even such karyo- logical features as the number, sizes, and shapes of the chromosomes, might be considered taxonomic characters to be treated equally with other morphological characters. The early realization that the chromosomes carry the entire or at least main hereditary equipment of the organism gave cytology (more strictly karyology)

The expansion of taxonomy 35’ a unique position in biology. As Darlington (1937, p. 562) has said: “Cell genetics led us to investigate cell mechanics. Cell mechanics now compels us to infer the structures underlying it. In seeking the mechanism of heredity and variation we are thus discovering the molecular basis of growth and reproduction. The theory of the cell revealed the unity of living processes; the study of the cell is beginning to reveal their physical foundations.” With such an authoritative statement before us we are forced to acknowledge that cytology is much more than high-power morphology.

It must suffice here to indicate, with a few examples, the relationship of taxonomy to cytology, and the possible modifications which it may be necessary to make in the former as a result of the impact of the latter. In the early days of cytological research a laborious and often unsatisfactory technique prevented more than a few organisms being investigated cytologically by any one individual within a reasonable time. Nowadays one investigator can relatively quickly and precisely examine the chromosomes of many species of a taxonomic group, or of many individuals of a species. This statement involves an important postulate : that the cytologist starts by using the concepts of orthodox taxonomy. The Latin names and, one must assume, the taxonomic conceptions for which they are symbols, are taken over from ordinary systematic botany or zoology. It is difficult to imagine any other scheme that would work. The cytologist must define his material, as carefully as possible, for purposes of general reference and comparison, and he naturally does so in the most generally and easily used and understood terms available. The cytologist, whether he acknowledges it or not, is largely dependent on taxonomy at the very commencement of his studies. The taxonomy he then uses is conveniently referred to as alpha taxonomy- it gives him at least some idea of the grouping and relationships of the material with which he is working. When his piece of research is completed he may find that the alpha taxonomy has misled him, at least partially. He is then in a position to help the taxonomist to improve the classification of the organisms investigated- that is to advance towards an omega taxonomy. It is convenient here, as a result of the great recent development of cytology, to consider certain data under two headings : ( I ) cytology and the classification of taxonomic groups of higher grades than the species; ( 2 ) cytology and the problems of species. This is, of course, a purely artificial arrangement.

Cytology and the classification of higher taxonomic groups than species. Certain taxonomic groups show no or few readily detectable cytological differences between their members. Thus the Gymnospermae have, with few exceptions, 1 2 haploid chromosomes (Sax, 1933). The Pomoideae have also a uniform chromosome complement, except for polyploidy (Darlington, 1937, p. 7 9 ) . In many genera, too, there is general agreement between chromosome number and other cytological details and a classification based on gross morphology. Examples are: SiZm, Rhododendron, Prunus, Ribes, and Antirrhinum (Darlington, 1937, p. 79). Marsden- Jones & Turrill ( 1 9 3 7 ~ ) find that the taxonomy and interspecific genetics of British species of Centaurea agree well with the findings of cytological studies. At other times cytological investigations have shown the chromosomes to be so variable

24 B R XI11

3 52 W. B. TURRILL within a uniform taxonomic group tlwt it is difficult or impossible to correlate cytology and morphological classifications. The genus Carex (Heilborn, 1924) gives an example of this. Some cytological’research has resulted in valuable criticism and modification of the details of morphological taxonomy. This is well illustrated by the Hooker Lecture of W. Wright Smith (1933), in which the cytological results of Bruun (1932) on the genus Primulu are co-ordinated with taxonomy. Other work suggests the need for taxonomic reinvestigation, and, it might be added, for the extension of cytological research. Thus Frankel & Hair (1937) find that the classification of species into Hebe and Veronica, as at present largely adopted, is not in accord with their results. They say “although from the viewpoint of systematic convenience the New Zealand Veronicas belong to Veronica, according to the cytological evidence they must be grouped with Hebe. Here, then, we have probably the first example in which cytological findings are clearly contradictory to the systematic evidence.”

Cytology and the problems of species. The “species problem” is often stated as if it were a single problem-an answer to the question “what is a species? ” It is, however, rather a whole mass of problems involving consideration of identity, characters in common, distinguishing characters, choice and valuation of characters, correlation of characters, origins, barriers, distribution in time and space, fertility, habitats, etc. The taxonomist is probably right in stressing the fundamental importance of his grade “ species ”. Not only is this grade of great practical importance and convenience but the majority of well-known and generally accepted taxonomic species really correspond to natural groups, that is to populations of plants or animals which can be recognized as keeping distinct under natural conditions. Darlington (1932), writing as a cytologist, says that “species are important, to systematist and geneticist alike, in two respects : in their external relations and in their internal relations. In regard to external relations, it is clear that discontinuity between species is due to isolation being followed by independent variation. Isolation is doubtless often brought about by distribution, by change of habit, or by genetic sterility factors; it is also brought about by structural and numerical changes in the chromosomes, which at the same time determine genetic change.”

Cytology helps in the taxonomic study of species in many ways. Differences in chromosome complements may be treated as additional “ characters ”. Usually the taxonomist does without such characters, but they sometimes help, especially in determining interspecific hybrids or in estimating the value to be placed on certain identifications. Thus the record of a hybrid between Centattrea nemoralis Jord. and C. Scabiosa L. is doubted since the cytology of these plants has been studied. Again, cytology may justify not only the giving of specific rank to a morphological entity but even confirm speculations as to its origin. The well-known suggestion of Stapf (1927, and the earlier literature there quoted) that Spartina Townsendii was a species of hybrid origin, is most strongly supported, if not completely confirmed, by the researches of Huskins (1931). It is often said that species differ one from another “in size”. This is obvious if the word “size” be taken literally. Since species as objects of study are populations, whatever else they are

The expansion of taxonomy 353 or are not, everyone must admit that some contain more and some fewer individuals. Probably, however, by “ difference in size ” is meant that species differ one from another in the degree of distinction. Some have many common and many clear-cut differential morphological characters or/and are completely isolated entities. Others have fewer common and fewer clear-cut differential characters orland are less completely isolated entities. In other words, the species is not a uniform grade, at least as accepted by different taxonomists. This is certainly true, and it may be a future taxonomic task to redivide and rename our categories. Our knowledge is still too incomplete to do this at present more than very tentatively and partially. The cytologist can already suggest from his standpoint that “kinds ” or “ types ” of species can be recognized. Six “ kinds ” have thus been already suggested (Darling- ton, 1932, p. 481)~ and, to quote from the same work (pp. 482-3), “the description as species of all these different types has been found convenient by the systematist, and clearly systematic convenience is the sole criterion that can as yet be applied universally. Consistency, whether morphological, genetical or cytological, if possible in theory, would be impossible in practice. Rules can, however, be applied to the study of an individual species when its systematic character, method of reproduction and chromosome constitution are known.”

The determination of the degree of constancy of association (correlation) of characters is important in taxonomic studies. Such correlation as occurs is largely determined by barriers to crossing. The taxonomist has most often and primarily to study what he finds in nature and soon realizes that such barriers are, at the species level, of various kinds and degrees. A most important type of barrier, because it is often, in nature, complete, at least over wide areas and for long periods of t h e , is sterility due to differences between the chromosome content of gametes of different species. Such sterility means that the species populations must keep distinct from one another. Of course, this is not the only cause of sterility, of not breeding together, or of isolation. It is, however, a widespread and important one. A wide field experience shows that in the British Isles Centaurea Scabiosa L. and C. n@a L. (including the microspecies C. nemoralis Jord.) frequently grow together in quantity. No hybrids have, however, been seen or made (Marsden-Jones & Turrill, 1937) and cytological examination showed that the main species have numerically very different chromosome complements. In this connexion it may be again urged that both geneticists and field workers should publish their negative results more than they do, and, better still, should draw the attention of cytologists to them.

A great deal of evidence regarding the occurrence, and in plants the very wide occurrence, of polyploidy, its origin, and its diversity, has been accumulated in recent years by cytologists. Much of this is of fundamental taxonomic importance. Some results involving polyploidy have been correlated with distributional studies, as, for example, those of Manton (1934, 1937) on Biscutella. Polyploidy has usually some, and often a marked, effect on morphological and physiological characters. With the advent of allopolyploidy there is, possibly often, the production of what, at least under natural conditions, the taxonomist must regard as a new and distinct species, as in Spartina Townsendii. Muntzing (1936) has shown the evolutionary

24-2

354 W. B. TURRILL importance of autopolyploidy. The following extracts from his summary indicate someof his conclusions so far as they are relevant to the matter here under discussion. “ Intraspecific chromosome races are characterized by quantitative morphological differences and positive correlation between chromosome number and gigas characters.. . .The chromosome races are generally ecologically different.. . . Intra- specific chromosome races are generally separated from each other by barriers of incompatibility and sterility.. . . The formation of autopolyploid chromosome races is an important factor in the evolution of new species.. . . Autopolyploidy may have secondary effects, of which aneuploidy and the formation of new basic numbers are the most important.. . .The similarities between allo- and autopolyploids are more important than the differences and are due to the fact that all allopolyploids are partially autopolyploid. Purely quantitative chromosome alterations, whether absolute or partial, have played a very important role in the evolution of the higher plants.”

Taxonomists have not yet considered, in such detail as to reach satisfactory conclusions, the reaction of such cytological discoveries as the widespread occurrence amongst Angiospermae of polyploidy and its implications and effects. The matter has, however, been broached (Hall, 1937a, b ; Blackburn, 1933). Thus Hall (1937a, b), referring to Tulipa, says: “ I t still remains a matter of dispute whether the polyploids should be assigned specific names of their own. For example, T. Clusiana is pentaploid, the corresponding tetraploid from the N.W. Frontier of India, though it has been named T . chitralensis, cannot be distinguished macro- scopically from the pentaploid : are the diploid and triploid forms to have specific names? The distinction between diploid and triploid is absolute, something different in kind from that between two closely related species both diploid.” The mere presence of extra chromosome sets may be considered in itself of minor classificatory importance, except for special purposes, but the results of polyploidy may be such that the taxonomist is forced to consider not only the effects but their cause. When polyploidy results in morphological differences (as it does, at least most often) and still more when it is the cause of sterility barriers, it becomes a matter of taxonomic concern, at least to those taxonomists who aim to perfect their system by a knowledge of underlying causes.

Gates (1936) includes cytological features in his genetical and cytological studies in Oenothera. Babcock and his co-workers (see Babcock & Cameron, 1934, hnd the earlier references given there) utilize chromosome characters in studying the relationships of species of Crepis. Chromosome numbers are to be given where known in the proposed New Students’ British Flora ” The periodic publication, especially by Tischler and Gaiser, of lists of chromosome numbers of plants, makes it relatively easy to trace the known cytological facts for any given species. A glance through any of the periodicals devoted to genetics and cytology (Journal of Genetics, Genetics, Genetica, Hereditas, etc.) shows how rapidly cytological data of a kind useful to the systematist is accumulating. It might be held that till cytologists have fully investigated many more genera it is undesirable to modify existing categories and nomenclature. The time, however, appears to be rapidly approaching when such

The expansion of taxonomy 355 an argument will be invalid within a wide range of genera. Already it is desirable that some tentative practical conclusions should be reached by joint discussions and co-operative research between taxonomists and cytologists.

V. GENETICS AND TAXONOMY

Genetics is now so intimately related to cytology (cytogenetics) that much research could equally well be classified under either heading. Genetics is essentially experimental. It follows that its taxonomic importance is greatest at and below the species level. Certain taxonomically wide crosses have, however, been made artificially or found in nature. Thus Fernald (1918) records a hybrid, found in the wild and completely sterile, between Cyperus dentatus and R(h)ynchospora capitellata. The genera of the two parents are placed by Pax (1887) in the two different sub- families of the Cyperaceae. In the Orchidaceae intergeneric hybrids are not infrequent, both in the wild and in cultivation. The hybrid genera Potinara and Burrageara are remarkable as having been built up artificially each from four different species belonging to four different genera (Orchid Hybrids : Sander’s List, 1931). These examples are, as far as the writer knows, extremes. In view of suggestions put forward by Lotsy and others it might be a very useful experiment for a large number of wide crosses, even such as on taxonomic or other grounds might seem absurd, to be attempted under diverse conditions. Negative, as well as any positive, results should be published. Darlington (1928) has shown that a certain inverse correlation is to be expected between the fertility of a diploid and the fertility of the tetraploid to which it may give rise. He adds “it is perhaps for this reason that most sexually reproduced polyploids are of the allopolyploid type ”. More recent research has tended to confirm the wide cytological truth underlying these statements. It follows that if Fl plants can be made by very wide crosses and from them (by somatic doubling or otherwise) allotetraploids be obtained, these would probably be fertile true-breeding plants and might be of strikingly new kinds, while some might even match or give a clue to the origin of existing genera and families. It remains true, however, that genetics is likely to be of most service to taxonomy at and below the species level.

It is worth reminding taxonomists that genetics involves the rearing and study of very large numbers of individuals of plants or animals. Much taxonomic work has to be done in herbaria and museums with dead material and inadequate samples (above all from the statistical standpoint) of species populations. Co-operation with geneticists would often enable the taxonomist to study, from his own standpoint, large numbers of living organisms and to describe some of the “characters” on a sounder statistical basis than is possible by the usual methods. Further, the taxonomist sometimes has to deal with organisms (especially plants) which he suspects to be, or is fairly certain are, hybrids. A preserved collection of hybrids made under full control and fully documented would be of extreme use even in routine taxonomy for comparison with material received for determination. Some collections of this nature are already preserved at Kew and more would be welcomed

356 W. B. TURRILL (Marsden-Jones et al. 1930). The taxonomist should be only too willing to give his help in return by checking taxonomic identifications and by giving references to publications.

From actual experimental research two intimately related kinds of results may be expected to have special taxonomic significance : the demonstration of sterility barriers or their absence and the determination of the degree of constancy of correlation of characters.

Sterility, in the broadest sense of the term, is of many degrees and, as based on causes, of different kinds (Crane & Lawrence, 1929). As a barrier between species it is often, but by no means always, associated with other barriers, such as differences in geographical distribution. When sterility is complete it obviously results directly in isolation between differential genes, and the characters for which they are responsible, carried by intersterile organisms or groups of organisms. Sterility is not, it must be again emphasized, the only cause of isolation, but it is a most important one, and without it the evolution of the vegetable kingdom at least must have been very different from what it is. If gametes carried in wind-borne pollen grains of grasses fertilized gynoecia of Quercus the acorns would scarcely grow into oaks! The distinctness of buttercup species in a meadow would not be maintained without sterility barriers and the occurrence of ‘ I hybrid swarms ” involving Centaurea nkra, C. jacea, and C. mora l i s is a consequence of their absence (Marsden-Jones & Turrill, 1931). Species in the genus Nemophila have been shown by Chittenden & Turrill(1926) to breed true to definite characters, to be intersterile, and to be morphologically distinct but closely related. Varieties within the species are fertile one with another. The recognition of barriers, their degree, and nature are tasks of the taxonomist in conjunction with his colleagues in other branches of biology.

Danser (1929) has proposed a taxonomic scheme, with the introduction of new categorical terms, based essentially on sterility-fertility criteria. Detailed criticism of this scheme is difficult because the author gives practically no actual examples of its working. The whole scheme appears very hypothetical, though from the nature of the criteria used it should be almost entirely experimental. According to Danser, a comparium is the sum total of individuals which can be combined directly or indirectly by crossing; a commiscuum is the sum total of individuals which can be combined directly or indirectly by mixture, i.e. by crossing which results in the production of fertile hybrids; a convivium is, within a commiscuum, a group of individuals which can be distinguished from other groups by more or less sharp characters and which is maintained in some degree of isolation by conditions other than cross sterility (ecological, phytogeographical). Two difficulties are likely to be encountered in putting this scheme into practice. Firstly, there is the difficulty, if not the impossibility, of proving complete sterility, i.e. of proving the distinctness of comparia and commiscua. Sterility, in the broad sense, that is actual failure to produce offspring, has a great variety of causes, some of which can be temporarily or artificially removed. It follows that while it is possible to prove that two organisms belong to the same comparium or commiscuum mere failure to “bastardize” or

The expansion of taxonomy 3 57 “mix” as a result of a limited number of experiments would not prove that they belonged to different comparia or commiscua. It might be said that the basis of classification is “bastardization” or “mixture” in nature, and that these can be shown without controlled breeding experiments. Introduction of the term

convivia ”, however, is a tacit acknowledgement of separation of groups in nature by other barriers than sterility and to prove sterility or fertility between them experiment is essential. Secondly, the difficulties involved in self-sterility, incompatibility, and apomixis are mainly or entirely ignored. On a logical basis there is much to support Danser’s main ideas. This is because the greater the abstraction the easier it is to make a classification which does not lead to logical fallacies when used for deduction. Some of Danser’s ideas will have to be included in a broader scheme leading to omega taxonomy. The study of barriers, their degree and nature, will be much more important in omega taxonomy than it is in alpha taxonomy, and sterility-fertility criteria are of very great importance in the recognition of more or less permanently separated and naturally occurring groups. Barriers of an ecological or distributional nature may, however, under natural conditions, be just as effective as cross-sterility in keeping groups distinct. It is with what actually occurs rather than with what can or might occur under artificial or hypothetical conditions that the taxonomist has to deal.

Sterility between different groups must (like other forms of isolation) tend to increase correlation of characters. Such correlation is complete when the characters are due genetically to a single gene, and is also more or less high when the responsible genes occur in the same chromosome (linkage), the actual value depending on the degree of occurrence of certain phenomena such as crossing-over. A great need of the plant taxonomist engaged in the study of species is for more information regarding the genetics of wild plants. This is particularly true with regard to common, polymorphic species of temperate floras. Their taxonomy, in broad outline, is often fairly well known, but the study of the species from the inside has only been attempted for a very small minority. There are excellent reasons, economic and technical, why so much detailed research has been done on cultivated plants- wheat, maize, Primula sinensis, fruit trees, etc.-and there is no doubt that many of the results obtained apply also to wild plants. There are, however, certain different problems involved. In the wild, selection is probably more erratic, competition is different and more complex, and isolation is different and more varied. All of these ultimately affect the occurrence and correlation of characters and the character composition of populations. Genetical work with a limited number of genera of the British flora has clearly indicated that species are much more polymorphic than is generally realized and that many differential characters “ break down ” sooner or later. It is very easy but of doubtful value for the herbarium botanist to make dogmatic assertions about correlation of characters without studying very large and fair samples of wild populations covering the range not only of one species but also of ‘ I allied ’’ species, and also examining the characters and their correlation experi- mentally. Such assertions should at most only be made tentatively with a clear indication of the limited data on which they are based.

I 1

358 W. B. TURRILL One other matter may be briefly referred to here. Taxonomists have to evaluate

the characters they use. Some characters are considered “generic ”, others “specific ”, and others “ varietal ”. On the whole genetical research has not so far given much help in providing a more objective basis for this evaluation of morphological characters. Marsden-Jones & Turrill (1928-37) in Silene find no essential difference in kind or in genetical behaviour between specific and intraspecific morphological characters. This, of course, is apart from their selection by different habitat conditions. Kristofferson (1926) writes : “ in Muleru, the crosses show a series from the most simple to the most complicated types of segregation, and the only difference between species and variety crosses is the complexity of the species segregations ”. The matter is, however, one that requires more joint research, in which both the taxonomic and the genetical aspects are fully investigated. The valuable work of Harland on Gossypium is no more than mentioned here because he published a summary recently in this periodical (Harland, 1936).

VI. ECOLOGY AND TAXONOMY

The ecologist, like the cytologist and geneticist, is dependent on the alpha taxonomist, and must, at least in the early stages of his special research, utilize the concepts and nomenclature provided for him. But he also can in return contribute much towards improving existing taxonomy and helping in its progress towards an omega perfection. I t is particularly from autecological studies that the taxonomists may expect help, the more so that these must often combine wide field studies with controlled experiment. There is an almost unlimited number of subjects in this line of research to which the taxonomist could direct the ecologist and much of first-class importance is awaiting attention in a flora (or fauna) taxonomically so well known as that of the British Isles, and with common species. One may express a wish that much of the energy now being spent on alpha taxonomy by those studying British plants might be directed to more intensive studies of the autecology, in the widest sense of the term, of the species within one genus. The work is fascinating and need not involve great expense.

The ecologist studies living plants, examining them in their natural habitats, in large numbers, at all seasons of the year, and at all stages of their life histories, He records many characters, especially of behaviour, ignored by, because unknown to, the alpha taxonomist. T o the taxonomist, with his method of concise description, the ecologist may often seem somewhat verbose. It might, indeed, be suggested that the ecologist, and especially the autecologist, should extract and publish in summary form such data as indicate ecological differentiation between species. Such characters include : habitat preferences, community occurrences, phenological data and general seasonal behaviour, reproductive phenomena (pollination, seed setting, seed germination), structure and behaviour of seedlings, plasticity under different environmental conditions, duration, and variation in habit.

Ecological barriers are sometimes very striking in their action. Thus, the writer was much impressed when studying the distribution of the genus Silene in the

The expansion of taxonomy 3 59 French Alps, by the sharply marked ecological ranges of Silene alpina and S. Cucu- balus (S. vulgaris). “ In the French Alps S. alpina occurs on screes. S. Cucubalus penetrates practically continuously from the lowlands as a woodland-edge plant and as a constituent of the hay meadows. One typical instance may be given of their distribution. Below the Meije, opposite La Grave, the valley of the Romanche has meadows with Silene Cucubalus as one of the dominant constituents.. . .The tall erect habit of S. Cucubalus enables it to compete successfully with other tall herbage. On an outcrop of schists the vegetation is very open, but the chief species is Silene alpina with low habit of growth, small leaves, and I-flowered inflorescences. The two distinct habitats are separated only by a foot-path but S. Cucubalus never invades the territory of S. alpina and the latter with its low habit of growth could not survive competition with the tall, close herbage of the meadows. A few hybrids were found intermediate in habit characters, but only in intermediate habitats. Experiments have shown that the two species can cross reciprocally, and they do cross naturally, as is shown by the occurrence of hybrid plants in the wild and by analysis of plants raised in considerable numbers at Kew from wild seed. Seeds collected 50 to IOO m. above the meadows yielded mainly pure S. alpina, but 12 % of hybrids. Yet the two natural populations of adult plants are specifically quite pure and the two species are certainly not, under the existing conditions, amal- gamating to one more polymorphic species. The physiographic, edaphic, and biotic conditions of the two habitats act as a sieve” (Turrill, 1936). The detailed yet extensive research on Silene maritima and S. Cucubalus (S. vulgaris) which is being carried out by Marsden-Jones & Turrill (1928-37) prove that comparable (though in detail different) ecological barriers alone preserve these as distinct taxonomic units in north-west Europe. When ecological (or phytogeographical) barriers between species break down, or as often happens are broken down by human agency, and the species are interfertile, hybrid swarms and finally even the amal- gamation of two or more species to one more polymorphic one may occur (Turrill, 1929, p. 476). Yet the taxonomist has to acknowledge that, in nature, ecological barriers can be as effective as genetical and cytological ones. In the course of evolution such long-continued isolation is likely to lead to the development of correlated barriers (such as cross-sterility), just as other (for example, gross morphological) differences tend to accumulate under isolation. For precise taxonomic work it is at least highly desirable to know as much as possible about habitat distribution and ecological barriers and on these and kindred subjects co-operation between the ecologist and taxonomist is essential.

The work and schemes of Turesson have attracted a good deal of well-deserved attention. A useful summary of his earlier papers is provided by Barton-Wright (1932). As a result of very extensive experiments, mainly with plants of the North European flora, Turesson (1922) concluded that “ The mass of genetically distinct forms which make up the Linnean species do not distribute themselves indis- criminately over an area comprising different (i.e. ecologically different) types o f localities, but, on the contrary, are found in nature to be grouped into different types, each confined to a definite habitat. Further, these ‘ecotypes’ do not origin;itr

360 W. B. TURRILL through sporadic variation preserved by chance isolation ; they are, on the contrary, to be considered as products arising through the sorting and controlling effect of the habitat-factors upon the heterogeneous species-population.” Including “ecotypes” Turesson has proposed a graded series of terms, but his original definitions of these are none too clear or practical. They have been somewhat modified to advantage by Gregor et ul. (1936). There can be no doubt of the extreme importance to the taxonomist of the facts which are accumulating under the term genecology . It must also be allowed that experimental taxonomists can scientifically justify the making of new categories of classificatory units to meet their own needs. The final test must be the pragmatic one, of whether these categories really help to a clearer understanding of facts than could be obtained without them. Even if they do their use may be only tentative. Orthodox taxonomy is justifiably conservative in its terminology of categories and in nomenclature. Sometimes, however, rigid adherence to established rules may not be a scientific advantage, especially for research on lines very distinct from those of alpha taxonomy. It is evident that at present the genecologist feels a need to use names and terms with connotations different from those associated with the species, subspecies, varieties etc. of existing floras and monographs. At the same time he cannot, apparently, avoid using alpha taxonomy ! Turesson, for example, lists and describes his ecotypes under taxonomic (“Linnean”) species and in practice his scheme results mainly in a reclassification of intraspecific units, with, of course, the recording of extremely valuable and hitherto often unknown data. Probably a complete break away from alpha taxonomy would be unfortunate, since it would make correlation between the new and the old knowledge extremely difficult. Yet, by an intentional mixture of metaphors, one might suggest that in attempting to put new wine into partly old and partly new bottles the genecologist runs the risk of falling between two stools ! Further, it is evident that genecologists do not always understand the alpha taxonomy of the-groups with which they work. That taxonomic-ecological units, corresponding to Turesson’s ecotypes, exist appears to be certain. It is easy to select conspicuous examples in many angiospermous families and possibly in a majority of species. There is, however, a doubt if, without undue stretching of definitions, all individuals of species populations can be accommodated under clear-cut ecotypes. The writer and his colleague, Marsden- Jones, have so far found it impossible to apply the ecotype concept, using the published terminology and definitions of Turesson (and Gregor), to and within those species of Silene with which they have been working intensively and extensively for over 12 years. The planned experimental work has not yet been completed, and this statement is therefore made tentatively.

The above criticisms are intended only to indicate possible limitations of genecology from the taxonomic standpoint, or, more precisely, the limitations of the concepts, terms, and definitions so far proposed for the classification of gene- cological units. The actual experimental and observational research on which genecologists are engaged is frankly acknowledged to be of the utmost importance. Further criticisms have been made recently by Fiegri (1937).

The expansion of taxonomy 361 Many taxonomists are deeply interested in problems of distribution and the

origin of floras and faunas. Phytogeographical and zoogeographical studies necessitate a very wide investigation, far outside the cnnfines of pure taxonomy, if any real insight is to be obtained into the underlying causes of the composition of floras and faunas. The distribution of every species is in some respects different from that of every other, and tabulation of actual geographical distribution and, still more, of special (ecological, altitudinal, etc.) distributions is a dficult matter. What appear in theory to be ideal schemes break down in practice, and, as in other branches of biology, only concentrated effort by trial and error can lead to the production of fairly satisfactory methods for phytogeographical research (Turrill, 1929, p. 249). There is, however, no doubt of the value of ecological data in attempting to trace the history of faunas and floras and even in describing their present composition, especially when descriptions are made with the intention of comparing different faunas and floras one with another. Probably only botanists or zoologists working in large institutions devoted, mainly or entirely, to taxonomic studies can appreciate the real practical importance of a knowledge of distribution. Even in botanical identification, to know “where a plant comes from” may be an enormous saving of time. Much taxonomic work is geographically divided into regional floras, and summaries of the ranges of taxonomic units, in addition to descriptions of them, are included in the best monographic work.

Du Rietz (1930) published a valuable critical summary of various applications of fundamental units in taxonomy, with special reference to species and intraspecific groupings. In his definitions of form, variety, subspecies, and species, he lays very considerable stress on continuity or discontinuity in the distribution of biotypes. The biotype is “a population consisting of individuals with identical genotypical constitution”; a form occurs sporadically, a variety locally, and a subspecies regionally within a species range, while species are “ the smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotypes”. Du Rietz’s scheme is essentially an attempt to define the general practice of alpha taxonomists in more modern language, while retaining their usual terms of species, variety, etc. It is evident to anyone who has attempted to apply Du Rietz’s definitions on a fairly large scale to actual plant material that all intermediate grades between his “ forms ”, “ varieties ”, “ subspecies ”, and“ species ” occur. His emphasis on species as actual populations is valuable (though this has been pointed out by other writers, e.g. Turrill, rgq), as is also his discussion of some of the practical consequences of accepting this standpoint (p. 389).

Rensch (1929), mainly for zoology, uses geographical distribution, together with fertility and sterility, and “ inherited ” morphological characters for his conceptions of geographical race, “ Rassenkreis ”, and species. The emphasis here is also placed upon geographical distribution.

362 W. B. TURRILL

VII. PALAEONTOLOGY AND TAXONOMY

The great advance, in recent years, of knowledge of past faunas, vertebrate and invertebrate, has no exact parallel on the plant side where comparable research has yielded results of very unequal value in different groups. For the Pteridophyta and Gymnospermae we have considerable data, for the other groups of plants the known facts are, for the greater part, either very isolated or of such a nature as to give little help in taxonomic studies of existing plants. The reasons for this lack of balance in palaeontological knowledge of plants are several. The great bulk of sedimentary rocks was laid down under seas and the conditions of deposition were unfavourable for the preservation of plant remains. The best preserved fossil plants (excluding those of post-glacial, and therefore :elatively very recent, age) are in Palaeozoic strata and contain no undoubted angiosperms. The accepted geological divisions are diagnosed almost entirely by their animal fossils, and consequently geologists, apart from specialized palaeobotanists, pay little attention to plant remains. As a result the study of plant fossils has, with few exceptions, little practical importance, whether by “ practical ” is meant use in determining stratification or human economy. There are, of course, exceptions, above all for coal-bearing strata, whether of Carboniferous or other age. There can be no doubt that if our knowledge of plant fossils were very much more complete than it is it would be of very great importance in taxonomy, and before proceeding to discuss examples of what can even now be used it is advisable to consider briefly the taxonomic possibilities of a more complete palaeobotany.

Palaeontology, based on the geological sequence shown by the stratification of sedimentary rocks, gives a relative time-scale. Approximations to a scale in terms of solar years are probably becoming more and more accurate. We can, at least, accept the accuracy of the general working principles by which geologists “date” fossil-bearing rocks, and hence can “ date” other than the zone fossils they contain. Certain conclusions can be drawn from even relative dating and are of extreme importance in any classification based on phylogenetic considerations. A geologically earlier group may have given rise to a later group, a later group could not give rise to an earlier.

In using known palaeontological facts as the basis of a phylogenetic scheme many precautions must be taken. Scott (1924) says “the succession of species in a continuous series of beds, does not necessarily represent the course of their evolution. What we actually find may rather be the result of migration, and the origin of the new species may have taken place elsewhere.” The name of any other taxonomic group than species, can validly be used in the above quotation. There are also difficulties in determining the exact geological age of some deposits and the risk of what are in effect circular arguments must be guarded against. All this is, however, well known and leaves fundamentally untouched the importance of the relative time-scale provided by palaeontological studies.

For classifications based on phylogenetic considerations the discovery of “missing links”, i.e. of organisms (or groups) intermediate in morphology and

The expansion of taxonomy 363 (presumably in) physiology between organisms (or groups) already known, is of great importance. Even the continued non-discovery of such links may be of significance in attempting to trace the origin and determine the essential constitution of taxonomic groups. Palaeontology may also throw light on the nature and permanence of barriers between such groups.

A considerable amount of activity has recently been devoted on the botanical side to a study of the nature and evolution of organs (organogenesis or morpho- genesis). It is not possible yet to say how far this will react on taxonomy. Certain terms will have to be used more carefully, some of them will have to be redefined, and some new terms will have to be introduced. Descriptions will be fuller and, one hopes, more precise. Much of this research in organogenesis is not at present linked with palaeontology but in the Pteridophyta and Gymnospermae palaeobotanists have started, so to speak, at their own end and tend to work up to the structures found in existing plants.

One other general line of research of importance to the taxonomist, though of indirect use in the actual classification of existing plants, has been considerably developed by palaeontologists. Studies on the changes in and the evolution of floras have resulted in discoveries which prove that migration has been at least as important as in situ modification in producing past and existing phytogeographical differentiation.

The dominant plant group in most existing land floras is that of the Angio- spermae. It is, therefore, particularly unfortunate that we know relatively very little of their fossil history, and especially few of such palaeontological facts as might be of use in taxonomy. Certain generalizations are possible. The Angiospermae are younger (as a whole) than the Gymnospermae. They may, therefore, have been derived from the latter. They are also younger than all the major groups of Thallophyta and Pteridophyta and than some, at least, of the Bryophyta. They assumed dominance in the Cretaceous period and retained it through the Tertiary epoch. In the earliest strata in which they are known as fossils they are already highly differentiated both in gross morphology and in anatomy, into the major taxonomic groups of Mono- cotyledones and Dicotyledones, and into a considerable number of families (on the basis of accepted classifications of existing plants). Beyond these statements the only generalizations we can make are negative ones. We have no certain evidence as to the group or groups from which the angiosperms evolved. We do not know when or where they originated. We cannot say with certainty which are primitive families or (with a few possible exceptions) what are primitive characters. Even more important taxonomically, we do not know, from fossil evidence, the time-scale relationship of monocotyledons to dicotyledons (as groups) or of one family to another within either group. These rather depressing statements are not intended as disparagement ofthe thought-provoking hypothesis of Arber & Parkin (1907), or of the invaluable discoveries and discussions of Wieland (1906, I929), or of Hamshaw Thomas (1925, 1931). These and other still more recent attempts to fill the gap between the Angiospermae and their ancestors have certainly indicated the possibility of filling the gap by palaeontological discoveries, though they also show that we do

364 W. B. TURRILL not know where the gap lies, except vaguely that it is between some group or groups of angiosperms and some extinct or existing group or groups of pre-Cretaceous ancestry. Indeed we do not know whether the Angiospermae are rnonophyletic or polyphyletic, even if the former be the more probable, and, therefore, whether we should refer to one gap or to more than one.

Within the angiosperms themselves we have very little fossil evidence of lines of evolution. Evolutionary series, lineages, lines of divergence and convergence are almost unknown. Most often fossil angiosperms have been placed in families or genera based on the existing flora of the world, or in genera closely compared with existing genera. The incomplete nature or poor preservation of many angiospermous fossil remains is undoubtedly one reason why their study has so far not led to results comparable to those of animal palaeontology. Often, too, the taxonomist is suspicious of many of the identifications made on very imperfect material. There are, of course, some exceptions. Chandler (1923) has been able to trace an evolutionary series (a main line with a subsidiary branched line) in the seeds of Stratiotes from the Upper Eocene to the present time. That many broad-leaved woody plants represent geologically old types seems established from fossil evidence. Seward (1926) has described from western Greenland a cretaceous flora which includes plant remains placed in Palmae, Liliaceae (?), Fagaceae, Moraceae, Menispermaceae, Magnoliaceae ( ?), Lauraceae, Platanaceae, and Leguminosae. Berry (1923) has given an interesting summary of the evidence available for tracing the ancestry of various well-known genera of trees. Nevertheless the palaeobotanist is not able to indicate with certainty either what are the primitive families or what are the main lines of evolution within the Angiospermae. Nor has he, so far, discovered extinct synthetic types. Seward (1931) has said that the “oldest known samples of petrified [angiospermous] wood confirm the conclusion, based on the still older leaves of the Arctic plane trees, that the oldest dicotyledons which clearly reveal their kinship with flowering plants are old only in a geological sense and astonishingly modern in their anatomical features ”.

The difficulties of making a satisfactory classification of plants, and especially of Angiospermae, on a phylogenetic basis (a matter discussed later) have beer. a partial cause of botanists attempting to trace the origin and history of organs-the “ Merkmalsphylogenetik” of Zimmermann (1930, 1934) as distinct from Sippen- phylogenetik” or race phylogeny. This has led to useful discoveries of detailed structure and to interesting speculations which must be taken into account by taxonomists (Hamshaw Thomas, 1932, 1934). Arber (1937) has, however, very truly pointed out that the Merkmalsphylogenie” of Zimmermann “for angiosperms slips back constantly into Sippenphylogenie ”.

The great floristic changes which have occurred in the course of geological time are of taxonomic importance because of the interest shown in and use made of geographical distribution by taxonomists. Studies such as those of C. and E. M. Reid and E. M. Reid & Chandler (1926, 1933 and references in these works to earlier papers) have thrown much light upon the great changes which led to the gradual production of a temperate modern flora in north-west Europe and its

The expansion of taxonomy 365 modificqtion by the oncoming of the Ice Age with its glacial and interglacial periods (various authors, 1935). Similarly, the study of po:=:-glacial peat deposits, especially by means of their pollen content, has resulted in an enormous scattered literature and conclusions of floristic and phytogeographical importance (Godwin, 1934).

VIII. PHYLOGENETIC CLASSIFICATION “Phylogeny” is a word with an attractive sound. It is also true that if the

theory of evolution be accepted it seems, on first considerations, that it should be possible, with complete knowledge of ancestry, to produce a classification on an objective phylogenetic basis. A considerable number of authors have, indeed, made serious attempts, in different groups of animals and plants, to use phylogenetic facts or assumptions as the principles underlying their classifications. Where sufficient palaeontological evidence, giving a relative time-scale, is available, some justification may be found for considering such classifications as indicating probable phylogeny. There are, however, various difficulties which have been either ignored or inadequately considered even by some recent writers. Gilmour (1937) has concisely and clearly pointed out the contrasts between I ‘ artificial ”, I ‘ natural ”, and ” phylogenetic” classifications. He also considers that ‘ I the criterion of degree of relationship applicable to individuals cannot be applied in practice to groups” and that “similarity of attributes cannot be regarded as a certain indication of such relationship ”. As a corollary to his logical arguments it may be pointed out that evolution has, in various groups of plants and in some groups of animals, resulted in a criss-crossing of characters (attributes), probably from a variety of causes which include similarity in mutations, or rather in their results, the selective action of similar environmental factors, and hybridization. The reticulation of characters which is so evident in any published classification of angiosperms has long been recognized by systematists. Thus J. D. Hooker wrote to Arber: “ I hold to Robert Brown’s view of the orders being reticulately not lineally related” (Huxley, 1918, p. 22). What is true of orders is also true of other taxonomic units. The example of Verbuscum and Celsia, given above in another connexion, illustrates reticulation between species of two usually accepted genera. Hayata (1921, 1931) has given many examples of reticulation, especially as between families of angiosperms, and has outlined a I ‘ dynamic system ” on principles which are not easy to understand, partly, perhaps, because he was writing in, to him, foreign languages. The conclusion he reaches appears to be that any simply linear, branched or unbranched, system of classification cannot bear more than a superficial resemblance to the actual course of evolution and cannot, therefore, be phylogenetic. He proposes a system in which “ affinities ” are shown by the grouping of families around a framework (for convenience Engler’s system is used for this). The immediately relevant point is his clear demonstration that among the families of angiosperms repetition of characters in different combinations is of frequent occurrence.

The whole question of “ parallelism ”-a very ambiguous term-awaits thorough investigation. The many and varied examples of what have been or may

366 W. B. TURRILL be considered parallelism need extraction from taxonomic and morphological literature, careful analysis on broad lines, and classification. Striking instances occur in the Spermatophyta, and very similar ones are known in Algae, Fungi, Bryophyta, and other groups.

The difficulties and perhaps impossibility of tracing the phylogeny in many plant groups have caused a reaction which appears to be gathering strength. In 1924 Scott (p. 18) wrote “ I have become sceptical of late as to most phylogenetic reconstructions ”, though he adds “ in ‘dim outline ’, at all events, we may still hope to catch glimpses of the course of evolution”. Such a statement suggests little hope of founding a classification on a certain basis of phylogeny, and, since it is made after many years’ study more especially of vascular cryptogams and gymnosperms where the fossil data are relatively satisfactory and abundant, must apply with still greater force to angiosperms. Rendle (1925) says “various attempts have been made to construct a phylogenetic system of Angiosperms, but the results are not convincing, bear no suggestion of permanence, and bristle with difficulties for the student ”. Some authors have stated even more extreme conclusions (e.g. Lotsy, 1916; Gilbert-Carter, 1936).

It might be thought that the value of a phylogenetic system, if such were possible, would be very great, since it would give an objective basis independent of mere scientific convenience and of personal idiosyncrasies. Realization, however, of what is involved in biparental generation, with its resultant widespread recom- bination of characters by gene inheritance, and the intimate network and repetition of characters actually shown by accepted taxonomic units, makes it most probable that such a scheme could only be represented either by complicated models in three dimensions or by mathematical devices. Certainly there remains a very considerable doubt whether a real phylogenetic scheme of the plant kingdom would be workable for most of the functions taxonomy has to fulfil. On the other hand, further advances in genetics are bound to increase our knowledge of the phylogeny of those species which can be genetically investigated. Cytological data can also throw some light on the origin and history of taxonomic groups at, below, and above the species level. Lastly, palaeontological research may one day enable the “ dim outlines ” of evolution to be traced up to and within the Angiospermae. All this, however, is far removed from phylogeny as the basis of a general classification.

IX. BIOMETRICS A N D TAXONOMY

The development of statistics in relation to biology has not yet affected taxonomy to the extent it has some other branches of botany. This is partly because no botanist with a good working knowledge of both taxonomy and statistics has yet abstracted from the former the problems which could with advantage be dealt with statistically, and sorted out from the mass of statistical methods those that could be used with a reasonable expenditure of time, with a strong probability of obtaining valuable results, and without the need of too high a standard of theoretical knowledge in a specialized branch of applied mathematics. Text-books of statistics (Fisher, I 928 ;

The expansion of taxonomy 367 Tippett, 193 I) are available but they contain much that is not relevant to taxonomy and are usually not written in a readily understandable form. Fisher (1928) says " Statistics may be regarded as (i) the study of populations, (ii) as the study of variation, (iii) as the study of methods of the reduction of data." Since taxonomy is the classification of populations, mainly by a study of samples, is deeply involved in a consideration of variation, and attempts to reduce its data with precision and conciseness, one would expect much more reciprocal co-operation between statisticians and taxonomists than there is.

Relatively simple attempts have, of course, been made to apply statistical methods to taxonomic problems. MacLeod (1917) gave an interesting account of results obtained by a quantitative study of characters in British species of Mnium. Later (1919) he published a book on quantitative measurements applied to biology and in it gives numerous botanical examples. Anderson (1936) has devised methods for representing both graphically and quantitatively the results of hybridization. Anderson & Turrill (1935) have urged that more biometrical studies should be made on herbarium material and have experimented with samples of FraXinw from the Balkan Peninsula (results not yet published). Gain (1937) has worked out, for the study of Hewea seeds, formulae whose use could easily be extended, with modifications, to a wide range of taxonomic problems.

Taxonomists recognize the value of giving measurements whenever possible but the values frequently given are usually unaccompanied by any statements as to the number of variates used, and when mean and extreme values are stated the standard deviation is usually neglected. The correlation of characters is often only mentioned when it is unity or is merely assumed and statistical values are rarely given for it when it is incomplete. Sometimes, indeed, the use of measurement may give a false impression of precision. Again, many descriptive terms, such as those for shape, are used in a vague qualitative manner and stand in need of definition in quantitative terms.

While it is evident that there is a wide scope for the application of statistical methods in taxonomy, it should be noted that such an introduction could, by itself, only lead to a refinement of alpha taxonomy and would not necessitate an entirely new outlook. On the other hand, it is impossible to conceive of an omega taxonomy that would not involve the constant application of biometrics.

X. PHYSIOLOGY AND TAXONOMY

A priori there is no legitimate theoretical reason why an entirely physiological classification of plants should not be made. Functions are as varied as organs, and show comparable stability and plasticity. In practice they are usually less easy to observe and sometimes less easy to define in concise terms. The close linkage so often obvious between form and function and the fact that subjects such as cytology, ecology, and genetics, have fundamentally important, though often little understood, physiological aspects, must eventually lead the taxonomist to give mote consideration to physiological attributes. In what might be termed pure physiolou

25 B R XI11

368 W. B. TURRILL there are at present few signs of co-ordinated research on taxonomic-physiological lines. There are, however, a number of subjects which should probably be placed, in a broad classification, under physiology and whose results either have been used by taxonomists or have actually been obtained with a taxonomic end in view. A few remarks may for these reasons be made about biochemistry and serology.

A good deal is known regarding the taxonomic distribution of certain products of metabolism and chemically the same or similar products are frequently found to occur in organisms grouped together by the taxonomist oh the basis of other (usually morphological) characters. Essential oils, resins, latex, glucosides, and alkaloids show in their occurrence some degree of correlation with other characters, and their presence or absence has been used in classification. Detailed work on certain plant products has yielded even more interesting results. It is known, for instance, that it is possible to identify at least many plants by their starch grains, or alternatively to name the plant from which a given sample of starch grains was obtained, by utilizing a combination of physiological and morphological characters in an analysis of the shape, size, markings, and chemico-physical properties of the grains alone (Reichert, 1919).

A modern development is the study of serology. In its taxonomic application this is the injection into an animal of protein or other organic material of known specific origin. The blood serum reacts to the extraneous matter and after being cleared will cause a visible reaction when added to a solution or suspension of what is chemically the same as the original or closely related material. Several methods, varying in detail, have been used (Precipitation, agglutination, anaphylaxis, complement-fization). For taxonomic and phylogenetic studies in plants the precipita tion and agglutination methods have been chiefly employed, proteins obtained from seeds being the injected material. Serum from an animal into whose blood protein from a given plant A has been injected is added to a solution or suspension of material from a plant B. The degree of precipitation or agglutination is taken as a degree of relationship between A and B . An enormous number of experiments along these lines have been made especially by Mez and his colleagues. It is not proposed here to summarize the very considerable literature on the subject. This can mostly be traced by consulting the papers in Botanisches Archiv and Cohn’s Beitrage.

That serological characters must be considered in future taxonomy is certain and improvements in methods and in presentation of results are likely to make serum-diagnosis more useful than it has been up to the present. Chester et al. (1933), in a critical paper in which it is shown that positive tests may be obtained with widely different families, say, “in pointing out this limitation of the applicability of the precipitation reaction in plant systematics let us compare it with the use of chromosome number in taxonomy. Within limited groups of plants chromosome number may aid in classifying species ; the fact that the same chromosome number may be found in very distantly related plant groups does not detract from the use of chromosome number in plant classification. The same is true of the precipitation reaction; its applicability in closely related groups of plants is not to be belittled

The expansion of taxonomy 369 by the fact that the same reactive substance may occur in very distantly related plant families.” Recognizing-that serum-diagnosis can yield characters which may be used in classification, there remains to consider whether such characters are of any greater value for phylogenetic theories than other, for example morphological, characters. Moritz (1929) has urged that a high diagnostic value must be given to proteins on account of their occurrence in all organisms and of the structural complication of the protein molecule. This last, he thinks, makes convergence improbable, and, therefore, the theory of parallelism of serological activity and physiological significance cannot be assumed. Whether improved biochemical technique and direct determination of the proteins will not eventually replace the indirect determination of protein relationship it is difficult to say. Logically, in spite of Moritz’s contentions, it seems difficult to give protein characters any higher taxonomic value than other characters which may, after all, be considered as manifestations of -protoplasmic (genic) activity.

XI. DEVELOPMENT TOWARDS AN OMEGA TAXONOMY

In the preceding pages an attempt has been made to indicate some of the reactions already evident between plant taxonomy and other branches of biology. It is evident that taxonomy is both a basic tool to these other, and younger, divisions and also a natural focus where they can meet. It is difficult, and from some standpoints even undesirable, to attempt to foretell the future, but it is desirable to consider in general terms in what directions we wish taxonomy to expand. I t is obvious that increased guidance by co-operation will alone direct research towards the realization of a system or systems of plant and animal classification not merely most in accord with all known facts but most useful to all research workers. So long as knowledge increases classification can never be completed. Identification is the recognition that given material belongs to such and such a taxonomic group, and to this extent, and to this extent only, is “ the same as ” other material belonging to that taxonomic group. Since taxonomy, and especially identification and its associated nomenclature, is basic to cytology, ecology, genetics, etc., it should, if for no other reason, be as precise and stable as possible. Taxonomists are, therefore, left with the difficulty of maintaining an unchanging (or relatively unchanging) system and at the same time incorporating new data largely derived from investigations which have used taxonomy as a tool.

The critical but very brief analysis of the situation given in the previous pages appears to lead to the following conclusions :

( I ) It is essential that alpha taxonomy (based entirely or essentially on morphology) should be maintained. Its methods can, of course, be extended and improved without alteration of fundamental principles-precision of descriptive terms, more easily workable rules of nomenclature, limiting of publications for new species, etc., are subjects needing attention. This alpha taxonomy gives and must continue to give the first approximation towards the wider complete knowledge regarding organisms which biologists seek.

25-2

370 W. B. TURRILL (2) Subsidiary classifications, for special purposes and often on a very limited

and deliberately abstracted number of attributes, should be prepared whenever they are thought desirable or likely to give valuable information of a particular kind. So far many of these still essentially retain the alpha morphological taxonomy and, in part, its nomenclature. Turesson (1922) says “the species problem is thus seen to be in large measure an ecological problem”. Danser (1929) uses genetical (sterility-fertility) criteria as the primary basis of classification, within a morphological framework. Rensch (1929) and Du Rietz (1930) stress phytogeographical aspects. All of these authors, however, retain at least a large part of orthodox taxonomic method, concept, and nomenclature. Apart from these limited, but useful, expansions (or “ patchings-up ”) of alpha taxonomy many kinds of more specialized classifications, usually on a smaller scale, have been made-for example, on the basis of chromosome number (diploids, triploids, etc.), of habit, of habitat, of cross- or self-sterility, etc.

(3) There should be continued experimentation as to how the new kinds of data can be incorporated in and used in taxonomy. This free “experimenting taxonomy” need not be bound by the traditions of alpha taxonomy of which it will represent an easily modifiable fringe, in the main advancing but always ready to evacuate positions no longer tenable. By trials and errors this “ experimenting taxonomy ” will enable, one hopes, orthodox relatively stabilized taxonomy to incorporate new data and so to advance gradually and cautiously, because of the need for conservation and stabilization, from an alpha position towards a far off omega perfection of the classification of all (biological) knowledge.

(4) The recently formed “ Association for the Study of Systematics in Relation to General Biology” should receive the active support of all biologists, whatever their special lines of work. It is only through such a central and co-ordinating association that the ideals outlined above can be approached. An account of the formation of this Association has been published in Nature of 24 July 1937, p. 163, and a leading article in the issue for 7 August 1937, pp. 211-12, outlines many of its aims.

Finally, it is hoped that this article will, in spite of its many failings and omissions, draw attention to some of the potentialities of taxonomy as a meeting ground for the different branches of biology. If it removes the common mis- conception that taxonomy is merely a dry museum or herbarium study, hidebound by tradition, and limited to the preparation of technical descriptions, identification of specimens, and discussion over names, it will have achieved an important function. If, further, it serves to attract some younger biologists to studies helping directly towards an “omega” taxonomy it will have fulfilled its main purpose.

XII. SUMMARY

An attempt is made, within a limited space, to indicate the importance of taxonomy, especially of the Spermatophyta, to the more recently developed branches of biology, and to show that these in turn are reacting on taxonomy. Certain basic

The expansion of taxonomy 371 logical problems are discussed and a via media between extreme lines of development is suggested. The interactions between taxonomy and cytology, genetics, ecology, palaeontology, phylogeny, biornetrics, and physiology are discussed briefly with a few selected examples. Practical suggestions for improving taxonomic methods and for increasing co-operation between different branches of biology are made in various places throughout the text. Lastly, proposals are made that in practice should lead gradually and some way towards an “omega” or perfected taxonomy, which, idealistically, should be a completed classification of all biological knowledge.

XIII. A C K N O W L E D G E M E N T

My best thanks are gratefully given to my colleagues Mr J. S. L. Gilmour and Dr T. A. Sprague for valuable constructive criticisms of the original manuscript of this paper. They are kind enough to express general approval of its practical aims and of most of the theoretical conclusions reached, but they are not in any way committed by statements made in this paper, except where quotations are given from their published works.

XIV. REFERENCES ANDERSON, E. (1936). “Hybridization in American Tradescantias.” Ann. Mo. bot. Gdn, 2 3 , s ~ 1-25. ANDERSON, E. & TURF~ILL, W. B. (1935). “Biometrical studies on herbarium material.” Nature,

ARBER, A. (1937). “The interpretation of the flower.” Biol. Rev. 12, 157-84. ARBER, E. A. N. & PARKIN, J. (1907). “On the origin of the Angiosperms.” J. linn. SOC. (Bot.), 38,

BABCOCK, E. B. & CAMERON, D. R. (1934). “Chromosomes and phylogeny in Crepis. 11.” Univ.

BARTON-WRIGHT, E. C. (1932). Recent Advances in Botany. London. BATHER, F. A. (1927). “Biological classification, past and future.” Quart. J. geol. SOC. Lond. 83,

BERRY, E. W. (1923). Tree ancestors. Baltimore. RLACKBURN, K. B. (1933). “On the relation between geographic races and polyploidy in Silene

BRWN, H. G. (1932). CHANDLER, M. E. J. (1923). “Geological history of the genus Stratiotes.” Quart. J. geol. Soc. Lond.

CHESTER, K. S., ARBE, E. C. & VESTAL, P. A. (1933). “Studies on the ‘precipitin reaction’ in plants.

CHITTENDEN, R. J. & TURRILL, W. B. (1926). “Taxonomic and genetical notes on some species of

CRANE, M. B. & LAWRENCE, W. J. C. (1929). “Genetical and cytological aspects of incompatibility

DANSER, B. H. (1929). “Uber die Begriffe Komparium, Kommiskuum und Konvivium und iiber

DARLINGTON, C. D. (1928). “Studies in Prunus. I and 11.” J. Genet. 19, 214-56.

Lond., 136,986.

29-80.

Gal$ Publ. agric. Sci. 6, No. XI, 287-324.

lxii-civ.

ciliata Pourr.” Genetica, 18, 49-66. “Cytological studies in Primula.” Symb. bot. upsaliens. 1.

79, pt. 2, 117-38.

V. Application to plant relationships.” J. Arnold Arbor. 14, 394-407.

Nemophila.” Kew Bull. 1926, pp. 1-12.

and sterility in cultivated fruits.” J. Pomol. 7, 276-301.

die Entstehungsweise der Konvivien.” Genetica, 11, 399-450.

- (1932). Recent Advances in Cytology. London. - (1937). Recent Advances in Cytology, ed. 2 . London.

Du RIETZ, G. E. (1930). “The fundamental units of biological taxonomy.” Svensk bot. Tidckr. 24,

FWGRI, K. (1937). “Some fundamental problems of taxonomy and phylogenetics.” Bot. Rev. 3,

FERNALD, M. L. (1918). FISHER, R. A. (1928). Statistical Methods for Research Workers. Edinburgh and London. FRANKEL, 0. H. & HAIR, J. B. (1937). “Studies on the cytology, genetics, and taxonomy of New

3 3 3-428.

40-23. “An intergeneric hybrid in the Cyperaceae.” Rhodora, 20, 189-91.

Zealand Hebe and Veronica (Part I).” N.Z. J. Sci. Tech. 18, 669-87.

372 W. B. TURRILL GAIN, E. (1937). “ Contribution B 1’6tude de la graine d’Hevea brmliensis des plantations malaises.”

GATPS, R. R. (1936). “Genetical and taxonomic investigations in the genus Omothera.” Philos.

GILBERT-CARTER, H. (1936). British Trees and Shrubs. Oxford. GILMOUR, J. S. L. (1937). “A taxonomic problem.” Nature, Lond., 139, 1040-2. GODWIN, H. (1934). “Pollen analysis. An outline of the problems and potentialities of the method.”

GREGOR, J. W., DAVEY, V. McM. & LANG, J. M. S. (1936). “Experimental taxonomy. I. Experi- mental narden techniaue in relation to the reconnition of the small taxonomic units.” New

Ann. Sci. nut. I& skr. 19, 233-45.

Trans. B, 228, 239-355.

New Phytol. 33, 278-304.

Phytol. 55, 323-50. -

HALL, A. D. (1937~). “Polyploidy in the genus Tulipa.” Proc. linn. Soc. 149th Session, pp. 28-9.

HARLAND. S. C. (1~36). “The genetical conception of the species.” Biol. Rev. 21, 83-112. - (1937b). “Polyploidy in Tulipa.” J. linn. SOC. (Bot.), 50, 481-9.

. _ _ . HAYATA, B. (1921). “The n a t d classification of plants acwrding to the dynamic system.” Icon.

- (1931). “uber das ‘Dynamische System’ der Pflanzen.” Ber. dtrch. bot. Ges. 49, 328-48. HEILBORN, 0. (1924). “Chromosome numbe rs... in the genus Carex.” Hereditas, Lund, 5, 129-216. HUSKINS, C. L. (1931). “The origin of Spartina Townsmdii.” Gmetica, 12, 531-38. HUXLEY, L. (1918). Life and Letters of Sir Joseph Dalton Hooker, 2. London. KRISTOFFERSON, K. B. (1926). “Species crosses in Malva.” Hereditas, Lund, 7, 233-354. LOTSY, J. P. (1916). Evolution by Means of Hybridization. The Hague. MACLEOD, J. (1917). “Quantitative description of ten British species of the genus Mnium.” J. linn.

Plant. formos. 10, 97-234.

SOC. (Bot.), 44, 1-58. .__ (1919). The Quantitative Method in Biology (ed. 2, 1927). Manchester and London.

MANTON, I. (1934). “The problem of Biscutella lamgata L.” 2. indukt. Abstamm.- u. VererbLehre,

The problem of Biscutella lamgata L. 11. The evidence from meiosis.” Ann. Bot., 67, 41-52 - (1937).

Land., N.S. 1,.439-462. MARSDEN-TONES. E. M. &TURRILL. W. B. (1928-37). “Researches on Silene maritima and S. vulgaris. - ,

parts I-XVIII.” Kew Bull. 1928, p. I, k n t k u e d to Kew Bull. 1937. p. 492, and further paits in‘ preparation. -- (1930). “Report of the transplant experiments.. .at Potterne.” J. Ecol. 18, 352-78. -- (1931). “Species studies in plants.” B.E.C. 1930 Report, pp. 416-20. - -- (1933). Second Report, op. k%. 21, 268-93. -- (1935). Third Report, op. k t . 23, 443-69. -- (1937). Fourth Report, op. cit. 28, 189-212. -- (1937~). “Genetical studies in Centaurea Scabiosa L. and Centwrea collina L.” J.

~~ARSDEN-JONES. E. M., SUMMERHAYES, V. S. & TURRILL, W. B. (1930). “Special herbaria as

MILL, J. S. (1919). A System of Logic, 8th ed., new impression. London. MORITZ, 0. (1929). I‘ Betrnchtungen zum ‘Ende’ der botanischen Serodiagnostik.” B k h . bot. Cbl.

48, 2, 114-18. M~NTZING, A. (1936). “ The evolutionary significance of autopolyploidy.” Hereditas, Lund, 21,

MURBECK, S . (1925). “Monographie der Gattung Celsia.” Acta Univ. Lund, N.F. Avd. 2, Bd. 22,

- (1933). “ Monographie der Gattung Verbascum.” Acta Univ. Lund, N.F. Avd.2, Bd. 29, Nr. 2. Orchid hybrids (1931). Sander’s Complete List, 2, 226. PAX, F. (1887). “Cyperaceae.” Engler und Prantl, Die Naturl. Pflanaenfam. 2, 2, 98. REICHERT, E. T. (1919). A Biochemic Basis for the Study of Problems of Taxonomy, Heredity, Evolution,

REID, E. M. & CHANDLER, M. E. J. (1926). The Bembridge Flora. London. __- (1933). The London Clay Flora. London.

RENDLE, A. B. (1925). The Classification of Flowering Plants. Cambridge. RENSCH, B. (1929). Dos Prinzip geographischer Rassenkreise und das Problem der Artbildung. Berlin. SAX, H . J. (1933). “Chiasma formation in Larix and Tsuga.” Genetics, 18, 121-8. SCOTT, D. H. (1924). Extinct Plants and Problems of Evolution. London. SEWARD, A. C. (1926) “The Cretaceous plant-bearing rocks of Western Greenland.” Philos. Trans.

Genet. 34, 487-95.

adjuncts to modern botanical research.” J. Ecol. 18, 379-83.

263-378.

Nr. I.

etc., with especial Reference to the Starches. 2 vols. Washington.

B, 215, 57-175. - (1931). Plant Life through the Ages. Cambridge.

SMITH, W. W. (1933). “Some aspects of the bearing of cytology on taxonomy.” Proc. linn. SOC. 145th Session, pp. 151-81.

The expansion of taxonomy 373 SPRAGUE, T. A. (1925). “The classification of dicotyledons. I. General principles.” 3. Bot., Land.,

STAPF, 0. (1927). “Spartina Townsendii.” Bot. Mag. t. 9125. STOJANOFF, N. (1936). “Am Wendepunkte der Systematischen Wissenschaft.” Spk. Blg. A M .

THOMAS, H. HAMSHAW (1925). “The Caytoniales, a new group of angiospermous plants from the

63, 9-13.

N. 63, 95-131.

Jurassic rocks of Yorkshire.” Philos. Tram. B, 213, 299-363. - (1931). “The early evolution of the Angiosperms.’’ Ann. Bot., Lond., 45, 647-72. - (1932). “The old morphology and the new.” Proc. linn. SOC. 145th Session, pp. 17-32. - (1934). “The nature and origin of the stigma.” New Phytol. 33, 173-98.

TIPPETT, L. H. C. (1931). The Methods of Statistics. London. TURESSON, G . (1922). “The genotypical response of the plant species to the habitat.” Hereditas,

TURRILL, W. B. (1925). “Species.” J. Bot., Lond., 63, 35966. - (1929). The Plant Life of the Balkan Peninsula. Oxford. - (1935). “The investigation of plant species.” Proc. linn. SOC. 147th Session, pp. 104-5. - (1936). “Natural selection and the distribution of plants.” Roc . roy. SOC. B, 121, 49-52. - (1936~). “Contacts between plant classification and experimental botany.” Nature, Land.,

L U n d , 3, 21 1-350.

137, 563-6: Various authors (1935). “Discussion on the origin and relationship of the British flora.” Proc. roy.

WIELAND, G. R. (1906). American Fossil Cycuds. Carnegie Institute of Washington, Publication SOC. B, 118, 197-241.

No. 34. - (1929). “Antiquity of the Angiosperms.” ,Roc . Int. Cow. P1. Sn’. Ithuua, I, 429-54.

- (1934). “Research on phylogeny of species and of single characters (Sippenphylogenetik und ZIMMERMA“, W. (1930). Dic Phylogenie der Ppanaen. Jena.

Merkmalsphylogenetik).” A M . Nut. 68, 381-4.

Documents

THE EXPANSION OF TAXONOMY WITH SPECIAL REFERENCE TO SPERMATOPHYTA