Morphological Classes as Natural Categories

8/3/2019 Morphological Classes as Natural Categories

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Linguistic Society of America

Morphological Classes as Natural CategoriesAuthor(s): Joan L. Bybee and Carol Lynn ModerSource: Language, Vol. 59, No. 2 (Jun., 1983), pp. 251-270Published by: Linguistic Society of AmericaStable URL: http://www.jstor.org/stable/413574 .

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MORPHOLOGICALCLASSES AS NATURAL CATEGORIESJOAN L. BYBEE and CAROLLYNN MODERState University of New York at Buffalo

The most productiveclass of strongverbs in English s thatexemplifiedby the verbstring/strung.Both historical and experimental vidence show that the class is phono-logicallydefined. However, the membersdo not share a singleset of phonological ea-tures.Rather, he class is organizedarounda prototypicalmember inthe sense of Rosch1978),to which the other membersstandin a family-resemblanceelation in the senseof Wittgenstein1953).The definingattributesof the class includethe final consonantsas well as the initialconsonantsand consonantclusters,andto a lesser extentthe vowelof the base verb. The organizationof the formalaspects of linguisticunitsis showntofollow the same principlesas the organization f the content of the linguisticunits. Thephonological shapes of verbs in morphological lasses can be organized nto naturalcategories, like other naturaland culturalobjects.*

Recent theories of phonology make a distinctionbetween alternationsthatare phonetically conditionedand those that occur in environments dentifiedin morphological, exical, or syntactic terms (Vennemann1971,Stampe 1973,Skousen 1975,Hooper 1976).The lattertype of alternationmay occur in con-junctionwith some morphological ategory,e.g. pluralor past tense; or it maybe restrictedto certainclasses of lexical items-either large meaningful lasses(e.g. noun andverb)or smaller,morearbitrary lasses (e.g. Englishverbs thatundergovowel changes in the past tense and past participle).The two typesof alternationsor rules have differentproperties; hey undergodifferenttypesof change; they are acquiredin differentways; and they have differentmoti-vations, and hence explanations.Phonetically conditioned processes are often called 'natural'because theyrecurin the languagesof the world, andbecause they seem to have a phonetic(usuallyarticulatory) eleology. It is not so obvious thatmorphologicallycon-ditioned rules are 'natural' n a comparablesense. They often seem arbitraryfrom a synchronicpointof view, because they areusuallythe residueof older,phoneticallyconditionedprocesses. Furthermore,such alternationsare oftenunproductiveand scheduled for leveling, since they disrupt the one-to-onecorrespondencebetween sound and meaning.However, if they were entirelyarbitraryand withoutfunction, it would be difficultto explainwhy such irreg-ularities are preserved so well and so long in many languages.Furthermore,the changes that occur in these alternationsare not random,but patterned.Itseems possible that a theoryof morphophonemics ouldbe developedin whichspeakers' treatmentof morphophonemicalternations n certaindynamicsitu-ations-e.g. in the acquisition process, or under conditions of diachronicchange, or underexperimentalsituations-could be explainedby reference topsychological principles, which would correspondto the phonetic principles

* We aregrateful o BrianMacWhinney ndDavidZager or discussingearlierversions of thispaper.JoanBybee was supportedby a stipendfromthe NationalEndowment or the Humanitiesduring he periodin which this paperwas written.251


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LANGUAGE, VOLUME59, NUMBER 2 (1983)which explain naturalphonological processes. Some principlesin a theory ofmorphophonemics,especially concerninghow the structure of paradigmsaf-fects morphophonemicalternations,have been discussed in Hooper 1979 andin Bybee & Brewer 1980. We propose to study here classes of lexical itemsthat exhibit the same morphophonemicbehavior, such as the English strong-verbclasses exemplifiedby sing/sang/sungandstring/strung. n particular,wewant to know how such classes are defined by speakers-i.e. what criteriathey use in decidingthat a set of verbs forms a class. We want to know whatdetermines the productivityof such a class-i.e. why some classes becomeproductiveandattract new members,while othersgraduallydisappearas theirmembers are regularized.Finally, we want to know what determines the di-rection in which any of these classes expands.

1. Two RELATEDCLASSESOF ENGLISHVERBS. The English verbs with whichwe will deal are listed in Table 1, divided into two classes and organizedac-CLASS CLASSIFINAL: FINAL:/m/ swim swam swum In/ spin spuncome came come win won/n/ begin began begun /r/ cling clungrun ran run fling flung*/r/ ring rang rung* sling slung*sing sang sung sting stung*spring sprang sprung string strung*/Ik/ drink drank drunk swing swungshrink shrank shrunk wring wrungsink sank sunk hang hung*stink stank stunk bring brung**/ok/ slink slunk/k/ stick stuck*strike struck*sneak snuck**shake shuck**

/g/ dig dug*drag drug**TABLE. English strongverbs with past or participle n /A/.

cording to the final consonant of the base. The distinction between the twoclasses is not entirelydiscrete, since the historicaltrend is for members of theclass with threeforms(sing/sang/sung)to lose theirseparatepast-tenseforms,and thusbecome membersof the string/strung lass. ManyAmericanspeakersdo not use the forms sprang, shrank,and stank. The forms marked with oneasterisk in these lists belong to verbs that were not in the strongclass in OldEnglish, but have been attractedinto it since the OE period.' We have alsoincludedcertaindialectal forms that also were not membersof the OE strongclass, such as shuck and drug;such forms are markedwith two asterisks.

Some of the additionsto the class were originallyweak, e.g. dig, the Old Norse borrowingfling, andthe denominative tring.A few othersbelongedto other OE strongclasses, e.g. strike,of whichthe originalpast form was stroke(cf. Jespersen1942).

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MORPHOLOGICAL LASSES AS NATURAL CATEGORIESThe OE strongverbs from which these two classes developed had a three-way alternation: I/ occurredin the infinitive and presenttense, /a/ in the lsg.and 3sg. preterit, and /u/ in the 2sg. and the pluralof the preteritand in thepast participle.When the person and numberdistinctionswere lost, the stem-vowel alternation nthe preteritwas resolvedinfavor of/a/ (which ater became/a/) in some verbs, and lul (which later became /A/)in others; thus the twoclasses of Table 1 were created. Subsequently,these verbs have also partici-pated in the general trend toward the eliminationof the distinction betweenthe past and participleforms. In the verbs of Class I, this is accomplishedbysubstituting he past-participle ormfor the past; the result is a verbfollowingthe patternof Class II: thus spin/span/spunbecomes spin/spun.Of all the strong-verbclasses that survive in ModernEnglish, Class II of

Table 1 is the most productive.Class I is also of interest, because it is closelyrelatedto the otherclass, and because it shows some limitedproductivity.Theevidence for the productivityof the string/strungclass is the large numberofnew members hat have been attracted o it over the centuries(Jespersen1942):twelve of the eighteen verbs in Class II were not members of the strongclassin Old English. This demonstrates relatively higher productivity than thesing/sang/sungclass, since only one memberof the latter class has been addedsince the OE period.2Evidence for the currentproductivityof these two classes is found in theexperimentsreported nBybee and Slobin1982, nwhichinnovations nEnglishpast-tenseforms wereelicitedfromadultsandfrom childrenaged8' to 10years.The experiment involving adults was designed to get them to produce inno-vative verb forms-especially regularizations, .e. -ed forms for irregularorstrongverbs. The subjects,who were all nativespeakersof English,were askedto listen to a list of verbs readby the experimenterat a fast pace. The subjectswere asked to supplythe past-tenseform of the verb they heard,and to do soas quickly as possible. There were forty subjects, and each one heardhalf ofa list consistingof 90 irregular r strongverbs, interspersedwithapproximately270 weak verbs. In additionto the regularizations licited in this way, someinnovations in vowel change also occurred. In all, 85 innovations involvedvowel change;of these, 41 involved verbs of the two classes under discussionhere. The experimentwith the fifteenchildren,who will be referred o as 'third-graders',was somewhatdifferent,in that no pressurewas put on them to per-form quickly; only the 90 irregularverbs were given to them, and these werepresented n a sentence-completion rame. Thistest was designedto elicit formsclose to those that the childrenactuallyused. Mostof the innovationsproducedwere regularizations;but in addition, the children produced 38 innovationsinvolvingvowel change,and32 of these involved the verbsof the classes underdiscussion here.

These innovationssuggest that something s goingon in these verb classes.2 Ofcoursethisproductivitys extremely imitedcompared o the weak-verbpatternof suffixing-ed, the productivityof which overshadowsthat of any strong-verbpatterns.However, the factthat so manymembersof the string/strung lass are new additionsneeds to be explained.

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LANGUAGE, VOLUME59, NUMBER2 (1983)An analysis of the particular nnovationsrecordedreveals the productivityofthe string/strung class.3 Three types of vowel change were found:

(a) The most frequentwas the use of a past form in /A/ for a verb in ClassI. Suchresponsesarecountedas innovations orthepurposesof ourdiscussion,even thoughthey mayregularlyoccurin the subjects' spontaneousspeech. Wetake the forms in Table I to representa conservative norm, and we want tofind out what changes are currentlyin progress among these verbs. The re-sponses of this type are listed in section A of Table2. The responses listed areADULTS THIRD-GRADERSA. swim swum 25% 7%

begin begun 10% 13%ring rung 25% 40%drink drunk 10% 40%shrink shrunk 25% 60%sink sunk 35% *

B. bring brung 5% 13%think thunk 0 7%clink clunk 5% *streak struck 20% *eat ut 0 7%shake shuck 0 13%

C. string strang 10% 7%sting stang 10% 7%slink slank 5% *clink clank 5% *clanked 15%

TABLE. Elicited past-tenseinnovations n two age groups:Percentageof totalresponses.* Not on third-gradersest.percentages,out of a possibletwenty responsesto each verbfor the adults,anda possible fifteen responses to each verb for the third-graders.These six verbsare the only ones of this class that were given on the tests. The experimentalresults reflect a clear historical trend toward substitutingthe past-participleform in /A/ for the past form in /a/.(b) A second type of innovationinvolved using a past formin /A! for a verbthat ordinarilyhas some other type of past tense. These responses are listedin section B of Table 2. Some of these forms (e.g. brung, thunk,and shuck)are establisheddialectalforms, while others are moreinnovative. But they allshow that the string/strungclass of verbs is relatively productive, especiallyconsideringthat the maximum number of innovationson the patternof anyother strong-verbclass was only four.(c) Examplesof the thirdtype of innovationwere somewhatsmaller n num-ber: they involved using a past tense in /a/ for verbs with some other pasttense-specifically, a past tense in 1A1or all the examples except clink (seesection C of Table 2). Innovationsof this type were less frequent, especiallyamongthe third-graders.Still, some productivity or this type of past tense isindicated.

3The productivityof this class is also evident in the work of Berko 1958andNewfield 1981.

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MORPHOLOGICAL CLASSES AS NATURAL CATEGORIESIf the strong-verbclass representedas Class II in Table 1is moreproductivethanany others, as the evidence indicates,thentwo questionsarise. First,whatare the defining properties that allow speakers to decide that certain verbsbelong in these groups, while others do not? Second, why is this class pro-ductive, while others are not?2. PRODUCT-ORIENTEDCHEMAS.t is apparent that the two classes of verbsin Table 1 are defined on phonologicalcriteria;syntactic or semanticcriteriaseem to play no role in definingthem. The most obviousphonologicalpropertysharedby all the verbs concerns the final consonantor consonantsof the base.The verbs of Class I all end in a nasal, or a nasal followed by a velar; thosein IIall end in a nasal or a velar.It seems reasonable o expect that this definitionwouldincludeinformationabout the vowel of the baseform;however, althoughthe vowel is usually I/i, it is not always, e.g. hang and strike from Class II. Itis possible, however, to make a valid generalizationabout this class of verbs,if this generalizationdescribes the past and participleforms, ratherthan thebase form. Such a generalization would be PRODUCT-ORIENTED.Zager 1980distinguishestwo types of morphologicalmodifications(where'modifications' are roughly equivalent to innovations). SOURCE-ORIENTEDod-ifications take a source word (a base), and modifyit in a very specific way toproducea new, derived form. Thus a source word X undergoessome processY to producea formZ, and the process Y is well-defined.The regularEnglish

past-tense suffixation process is an example: a verb walk has the suffix /d/addedto it, and the result is a form walked.The relevantrelationin this caseis between walk and walked, and it is 'basic-derived' (cf. Bybee & Brewer).In a 'product-oriented'modification,the process Y by which the new word isformed is not well-defined,nor is the shape of the source word X necessarilywell-defined.However, theproduct,Z, is well-defined.In ourpresentexample,the past form for a verb of the string/strung ype must end in /A/followed bya nasal or a velar; but the vowel of the base does not necessarily have to be/i/. In this case, the relevantrelationsare amongthe past formsof the differentverbs (strung, slung, swung, wrung, hung etc.), rather than those between baseandderived forms.This is not to deny the existence of relationsbetween stringand strung, hang and hung-but only to assert that, in addingnew membersto the class, the focus is on the propertiesof the product(orpast-tenseforms),and the relevantrelations are amongthese productforms.Note that the postulationof product-orientedmodificationsallows for mor-phological innovations that would be impossible if proportionalanalogy weretheonly meansof creatingnewforms,because innovationssuch as strike/struckand sneak/snuckhave no pre-existingmodel with appropriatevowels in thebase form.

Bybee & Slobin propose that the synchroniccounterpartof a product-ori-ented modification is a 'schema', i.e. a generalizationabout the shape of alexical item of a certain category. Schemas are not used in generatingformsby combination, but rather are lexical associations among existing forms.Bybee & Slobin reportevidence that English irregularverbs have their past-tense forms listed in the lexicon, and that any associations among them are

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LANGUAGE, VOLUME 59, NUMBER 2 (1983)handledby schemas. In the experimentaltask for adultsdescribedabove, 44vocalic innovations were found in addition to the ones we have alreadyde-scribed. Of these, only ONE response was not a real Englishword; only FIVEwere not real Englishverbs;andthirty-fivewere realpast-tenseformsof otherverbs. This indicates that producinga past form of an irregularverb is not amatter of applyinga feature-changing ule to an underlying orm, but is rathera matter of lexical selection. If the innovationswhich the subjects producedwere the result of the misapplicationof vowel-changerules, then the resultswouldcontainmanynonsensewords, such as the one example heaplhept.Sincethe responses were not nonsense, but realpast-tenseforms of otherverbs (forthe most part),it appearsthatproducingan irregular r strong past-tenseformin English is a lexical selection procedure. Since Bybee & Slobin also foundevidence of classhood in a few cases-especially in the verb classes underdiscussion here-they propose that schemas define these classes, and aid inthe lexical selection process.

3. DEFINING HECLASSES.The schema for Class I says that the past tensehas the vowel /a/ and ends in a nasal, followed by an optional/k/:(1) ... aenasal (k)]verbpast

The schema for the verbs in Class II states that the vowel of the past tense is/A/, and that the final consonant is either a nasal or a velar:(2) ... A nasal or velar]verbpast

This latterdescriptionis unsatisfactory or several reasons. First, the disjunc-tion 'nasalor velar' is merelya list of the possible finals. Second, the list doesnot provide a clue as to the reason for this particularselection of final con-sonants.Third, t neitherdescribes norexplainsthefact thatthe largestnumberof verbs in this class actuallyhave final consonants with BOTHproperties,i.e.velar nasals. Finally, naturalclasses in phonologyare not usuallyof this type;i.e., they are not usuallydescribableas disjunctionsof two features where theusual case is for the two features to occur together.However, there are other categories that work in precisely this way-namely, the 'naturalcategories' that people use when they label natural orculturalobjects. Wittgenstein1953proposedthatthe categorieswhich we cre-ate in construingouruniverse do not consist of sets of items sharinga commonset of attributesnot sharedby otheritems,but rather hatmembersof categoriesform chains of family resemblances. That is, all that is necessary is that eachmember of a category share one attributewith some other member.Rosch &Mervis 1975 and Rosch 1978develop this idea furtherby proposingthat eachcategory has a PROTOTYPEr best exemplar,which has a cluster of attributescharacteristic of the category. Thus there can be both PROTOTYPICALembersof categories, with a numberof relevantattributes,and more marginalmem-bers, with perhaps only one attributethat puts them in the category. For ex-ample, a wooden, straight-backedchair is a good exemplar of the category'chair', since it has an identifiableseat, back, and four legs, and is an object

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MORPHOLOGICAL CLASSES AS NATURAL CATEGORIES

designedto be sat upon;but a bean-bagchairis a poor or marginalexemplar,since the structuralattributesof chair-hoodare lacking. Nonetheless it is achair, since its functionis to be sat upon, an attributesharedwith other mem-bers of the category. Zager suggests that morphologicalclasses may have thestructureof naturalcategories in this sense.Indeed, the verbs of the string/strung class seem to form a class with thestructureof a naturalcategory. The prototypicalverb in this class has a velarnasal as a final consonant. The other verbs share at least one propertywiththis prototype, so far as the final consonantis concerned; i.e., they either endin a velaror in a nasal. Thus even thoughthe finalconsonants of won anddughave no attributes n common,they each shareone attributewith the prototype,which is enough to determine class membership.The evidence that the structureof this class is notjust anhistoricalaccident,but is indeed a real, psychologicallyvalid structure, s quite strong.In lookingat Class II again, it can be seen that all the originalmembers of the classcontained a nasal consonant, and that all the members with a non-nasal velarare 'new' membersof the class. This means that, as the class has expanded,it has created the particular tructurewe see today. It has expandedoutward,allowingverbs that shareonly the propertyof velarness tojoin the class. Thisis strong evidence that its family-resemblancestructure is psychologicallyvalid.Furtherevidence for the psychological validityof this type of structuringcan be obtainedexperimentally.Wedescribe belowthe resultsof anexperimentdesignedto test our hypothesis about the structureof this verb class.

4. THEEXPERIMENT.he purpose of the experiment was twofold: (a) to de-termine the definingattributesof the most productive strong-verbclass (ClassII), and (b) to test the hypothesis that this class has the structureof a naturalcategory.We hypothesized that this class of verbs is organizedarounda prototype,such that each member of the class shares one or more attributes with theprototype.4We arrived at a working hypothesis about the prototype by ex-amining he various membersof the class. As mentionedabove, the final con-sonant(s)of the prototypewould containboththe features nasalandvelar, andthus would be either /r/ or /rk/. Since there are manymore verbs with a final/rj/, we might furthermoresurmise that /r/ would make a verb closer to theprototypethan/rk/. The vowel of the prototypebase-form s hypothesizedtobe /I/. Recall, however, that we are suggestingthat the prototypeis product-oriented;it defines the past form, not a base form. Thus we are interested inthe importanceof the vowel /I/ in determining loseness to the prototype.Thelist of Class II verbs in Table 1 also suggests that the initial consonantsof theverb may serve as attributesthat define the prototype:it is strikingthat nineof the eighteen verbs begin with /s/ followed by one or two consonants. We

4 In the following, the term 'feature'is used in its usual sense of 'phonological eature',while'attribute'refers to a particular eature that characterizes he prototype. 'Parameter' s used torefer o a dimensionof whichtheattributes a part.Forexample,theattribute velarnasal'belongsto the parameterof final consonants.

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LANGUAGE,VOLUME59, NUMBER 2 (1983)hypothesized, then, that the prototypewould begin with /s/ plus one or moreconsonants, so that a form like stringwould be a likely prototypefor the pasttense of this class of verbs.

In the experiment,we asked subjectsto form the past tense of nonce verbsthat were constructedphonologicallyso as to stand in varyingrelationsto theprototype. We expected the subjects to give more past-tense responses with/A/ for nonce verbs that were closer to the prototypethan for nonce verbs thatwere farther from it. We turn now to an outline of this experiment,and theresults we obtainedfrom it.SUBJECTS: he subjects of the experimentwere 34 undergraduate tudentsenrolled in an introductory inguisticscourse at SUNY Buffalo. All subjectswere native speakersof English.Neither the hypothesisnor the verbclass had

been discussed in the course.METHOD:A list of 93 nonce verbs interspersedwith 16 real verbs was pre-sented orally. These verbs were all monosyllabic, and varied phonologicallyalong three dimensions:the initialconsonants, the vowel, and the final con-sonants. Initial-consonant egments were of four types: the consonant /s/ fol-lowed by a consonant cluster (sCC); the consonant /s/ followed by a singleconsonant (sC); a two-consonantcluster not containing s/ (CC); and a singleconsonant (C). A variety of vowels were represented,including I a A ey iyay uw ow/. Final-consonantsegmentsfell into four categories:velar and nasal(/ik q/); velar (/k g/); nasal (/m n/); or other single consonants (C). A list ofverbs appearsin the appendixto this paper.Subjects were asked to write the past-tense forms of the verbs presented.At the beginningof the experiment,the following sentence-completion ramewas given: Sam likes to ... Yesterday he ... Subjects were asked to refer to thisframe as they felt necessary duringthe course of the experiment.RESULTS: The threephonologicalparameters-the initial,the vowel, and thefinal segments-were all significant actorsaffectinga subject'sresponse. Theresponsescanbe divided into twotypes:(a)regularpasttense formedby adding-ed, and (b) past tense formed by vowel change. In a very small number of

responses, the subject changedthe vowel and added -ed. These did not figurein our analysis. In formingthe past by vowel changealone, there was an over-whelming tendency for subjects to use /A/ and /e/; we will thereforegive thepercentage of responses with -ed and with these two vowels.5 The over-allpercentagesof vowels which occurred npast-tenseresponsesformedby vowelchange was as follows:(3) /A/ 49%/aI/ 34%other 17%

To test the effect of the vowel of the base in elicitingan /A/ vowel in nonceverbs, we constructed such verbs with a varietyof vowels. However, ourinputitems were limitedby the fact thatthe numberof vowels that can occurbeforea velarnasal is severely restricted n English.Thus, for the nonce verbsending5 All statisticaldifferencesdiscussed in the text are significantat the .05 level.

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MORPHOLOGICAL LASSES AS NATURALCATEGORIESin a velar nasal, we present the results for base verbs with the vowels I aeA/in Table 3.

-ed /a/ /A/Inputvowel: /I1 18% 34% 44%/A/ 84% 8% 4%

/a/ 67% 8% 25%TABLE. Past-tenseresponsesfor nonce verbsof the shapesCC velar nasal.

InTable4, we presentthe results for nonce verbsending n a velar, includingverbs containing diphthongs. It can be seen that the highest numberof /A/responseswas obtainedfor nonce verbswhichcontained I/ butwereotherwise-ed /a/ /A/Inputvowel: 1/1 56% 13% 25%/A/ 74% 3% 0%1/a/ 100% 0% 0%

diphthong 53% 3% 15%TABLE. Past-tenseresponsesfor nonce verbs of the shapesCC velar.

close to the prototype. All the diphthongs ested showed a strong tendency toelicit regularpast tenses, anda weakertendencyto make vowel changesotherthan /ae/or /A/ in the past tense. Since their response patternsdid not differsignificantly, they are grouped togetherin the analysis.Thetendency(mentionedabove)forsubjectsto use either a/ or//1 informingthe past by vowel changehadan effect on the responsepatterns or verbs with/ae/or /A/ as input vowels. In general, subjects avoided no-change responseswhich would make the past tense identicalto the present.Thus, for inputverbslike /straejk/, which contains one of the favored past-tense vowels, subjectsrarelygave /straerk/as the past tense. In most cases, this avoidance of a same-vowel response led to an increase in responses with the otherfavored vowel.Forverbswith/ae/, he percentageof/A/ often increasedrelative to otherverbs;for verbs with /A/, the percentageof /ae/responses was often greater,as wasthe percentageof regularizations. n spite of this elevation, the percentageof/A/responses remainedgreatest for verbs with /I/ as input vowel. The voweleffects maybe summarized n the followingway: /I! elicitsa significantlyhigherpercentageof /A/responses, over-all, thanother vowels; it is followed by /a/,the diphthongs,and /A/.As regards nitialconsonants, nonce verbs of the form#sCC and #sC weresignificantlybetter at eliciting/A/ n the past tense than those with #CC or #C,as shown in Table 5.

-ed /e/ /A/Input nitials: sCC 42% 23% 23%sC 48% 21% 23%CC 63% 14% 12%C 56% 23% 14%TABLE5. Past-tenseresponsesfor nonce verbsby initialsegments.

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LANGUAGE,VOLUME59, NUMBER 2 (1983)There was no over-all significantdifference between #sCC and #sC; butamong the verbs close to the prototype, #sCC was significantlybetter, asshown in Table 6. No differencewas found between consonant clusters within

a group.-ed /a/ /A/

Input nitials: sCC 18% 34% 44%sC 34% 26% 37%CC 42% 26% 27%C 40% 32% 22%TABLE 6. Responses for verbs of the form i + velar nasal by initial segment.

Amongthe finalsegments, the ones that hadboth thefeatures velar and nasalelicited the highest percentageof/A/ responses, as shown in Table 7.

-ed /a/ /A/Inputfinals: /0 nk/ 33% 29% 32%/k g/ 67% 10% 14%/n m/ 50% 21% 15%C 61% 19% 10%

TABLE. Over-allresponsesby finalsegment.Withinthis group,/g/ was significantlybetterthan/Ikl, as shown in Table8.

-ed /e/ /A/Input inals: /0/ 29% 29% 37%/nk/ 38% 29% 27%

TABLE . Over-all responses of //1 vs. /!k/.

Velars and nasals elicited approximatelyequal percentagesof /A/ vowels.However, verbs withfinal nasalshad a higherpercentageof /a/ responses than/A/responses, while the reverse was true for velars. Single consonants werethe poorest at eliciting/A/responses, but like the nasals, they had a high per-centageof /a/ responses. Withinthis group,final /t/ or Id/elicited morevowel-changeresponses than other finals, as shown in Table 9. No other differenceswithingroups were found.-ed /a/ /A/

Inputfinals: /t d/ 40% 31% 18%/p bv/ 82% 8% 2%TABLE . Over-allpercentages or final/t d/ vs. /p b v/.

Of the three parameters,the final segment had the greatest effect on thenumberof /A/responses. When no otherattributesof the prototypewere pres-ent, the presence of the best final (a velar nasal) elicited 34%;the best initial(#sCC) elicited 17%,and the best vowel (/I) elicited 7%.Combining he bestfinal and initial features with a non-prototypicvowel (ae/) results in 50%/A/

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MORPHOLOGICAL LASSESAS NATURAL CATEGORIESvowels. This percentagedoes not differsignificantly romthe percentageof/A/responses at the prototype, which is 44%.(Thepercentageat the prototypeisslightlylower in this case, because of the uniqueresponsefeaturesof the verbswith /a/ as an inputverb; see above.) This is furtherevidence for the minimaleffect of the vowel alone. The absence of the prototypic nitialsegmentcausesa significantdecreasein/A/responses:the best final andvowel, with thepoorestinitial (C + I + velar nasal) elicited only 24%. A still greaterdecrease wascausedby the absence of the final: the best initialand vowel (sCC + i) received4% /A/responses; see Tables 10-11. We conclude that the final velar nasal isthe strongestattribute,followed by the initial. The vowel makes a significantdifferenceonly when other features of the prototypeare present.

RESPONSES IN /A/Prototype:sCCi velarnasal 44%Two prototype eaturespresent:Final & initial: sCC e velar nasal 50%Final& vowel: C i velar nasal 24%Initial& vowel: sCC i C 4%One prototype eaturepresent:Final: C x velar nasal 34%Initial: sCC e C 17%Vowel: C iC 7%

TABLE10. Effect of the three parameters.

I + velar nasal: RESPONSESWITH A/Initials: sCC 44%

sC 37%CC 27%C 22%

sCC I :Finals: y, nk 44%k, g 25%n, m 21%C 4%

TABLE11. Progressionaway fromprototypeof initialand finalsegments;otherparametersheldconstant.On the basis of the experiment,the attributesof the prototypeof this classof verbs are:

(4) a. a final velar nasal (/i/ is betterthan/ok/).b. an initialconsonant cluster that begins with /s/.c. a vowel /i/, which has an effect only in conjunctionwith the pre-ceding elements.The strengthof the finaland initialparametersmaybe classified on the basisof their similarity o the prototype.The best final has the features of velarandnasal. The verbs with velars and those with nasals, each of which contain oneof these features, can both be consideredone step away from the prototype.The single-consonantset bears neitherof these features,and so is furtheraway

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LANGUAGE, VOLUME 59, NUMBER 2 (1983)from the prototype. The prototypic initial is an /s/ followed by a consonant orcluster. If /s/ + cluster is the prototype, we can say that the #sC initial andthe #CC initial are each only one step away from the prototype. The data makeit clear, however, that the /s/ is a more important attribute than the consonantcluster. The single consonant lacks two elements, the /s/ and the cluster; there-fore it is farthest from the prototype. Within this ranking, we can comparedistance from the prototype with the percentage of /A/ responses elicited, asin Table 11.Within each parameter, we find a linear decrease in /A/ responses as theverb's phonological properties are removed from the prototype. We find asimilar progression across parameters. As the number of prototypic attributespresent decreases, so does the percentage of A/ responses. The more a set offeatures within or across parameters diverges from the prototype, the fewer/A/ responses it elicits. This progression is evidence for the operation of amorphological prototype in this verb class in English.Table 12 shows the responses for the real English verbs that were included

REGULAR VERBS IRREGULAR VERBS-ed /a/ /A/ -ed /a/ /A/

streak 69% 7% 14% slink 39% 16% 42%stroke 77% 0% 19% bid 70% 7% 7%link 71% 18% 7% sting 6% 31% 63%stack 92% 0 4% stick 6% 13% 72%skim 88% 6% 3% fling 12.5% 12.5% 75%skid 90% 0 3% dig 12% 6% 82%trick 97% 3% 0trip 94% 3% 0clip 96% 4% 0tip 100% 0 0

TABLE12. Past-tense responses on real verbs.

in the experiment. It can be seen that the subjects gave standard past-tenseforms for the large majority. The over-all percentage of correct forms was 83%:this indicates that the subjects took the task seriously, and were attending toinput material sufficiently to determine whether or not the test item was a realverb. The subjects performed better on the ten regular verbs tested (87% cor-rect). Of these verbs, seven verbs received 88% or better correct responses(94%on the average); and the remaining three averaged 72%correct. The verbsstreak and stroke elicited a greater percentage of vowel-change responses thanother regular verbs, favoring /A/ (17%). These verbs are also those which sharethe greatest number of attributes with the prototype of the class we have beendiscussing. Thus, even for regular verbs, the prototype of this productive classmakes its influence felt. The irregular verbs with /A/ in the standard past tenseelicited an average of 73% correct responses. The remaining responses weredivided between -ed regularizations and other vowel changes.

5. DIscussION. The experimental data clearly point to a morphological classconsisting of verbs bearing a family resemblance to a prototype. On the one

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MORPHOLOGICAL LASSESAS NATURAL CATEGORIEShand, the data show that the verbs that can form a past tense by using thevowel /A/ are not a randomset, but that the likelihood of a verbforminga pasttense in this way correlateswith its phonologicalsimilarity o a prototype.Onthe other hand, we have also demonstrated hat this class of verbs does nothave discrete boundaries, or phonologicalfeatures whose presence is abso-lutely required,but ratherconsists of both central andmarginalmembers.Thelatter point is importantbecause it helps us understandhow such a class canexpand and grow. It explains why a class of verbs endingonly in nasals canexpand to include items such as strikelstruck,and dig/dug. It explains occa-sional errors such as eat/ut, and usage such as slide/slud (this latter form isattributed o Dizzy Dean, a baseballcelebrity;MadeleineNewfield, p.c.) Mem-bershipin morphologicalclasses is not a matter of strictpresence or absenceof features, but ratherof similarityto a prototype, which may be defined ona numberof features. Given the findings reported n ?4, we can predictthat aparticularverb will join the string/strungclass on the basis of the numberoffeatures it shares with the prototype, and the rankingof these features. Wecan also predictthat certain verbs are unlikelyto join the class (i.e. those withno features of the prototype,such as rob or type), and that others have a verylow probabilityofjoiningthe class, since they lack a finalconsonantthat sharesfeatureswith the prototype(e.g. stop, scratch, or strip).The experimentdemonstratesthat, at least to some degree, the initialcon-sonants, the vowel, andthe finalconsonantsdetermine he likelihood of mem-bership n this verb class. The qualityof the vowel is the weakest determinant,as we expected, since the class is actuallydefinedby a product-oriented en-eralization that refers to the past form, not to the base. The existence of newmembersof the class (e.g. strike,hang, drag, sneak) led us to thatconclusion,which is confirmedby the experiment. The minor importanceof the vowelargues againsta descriptionof past-tenseformation or these verbsas a processby which a particularvowel /i/ is changedinto anothervowel, as proposed inChomsky& Halle 1968and in Hoard & Sloat 1973.It also argues againsttheview that new verbs enteringthe class do so on the basis of strictproportionalanalogy:thus, before strike oined the class, no verb with/ay/ in the base formwas available to provideforms for the first two members of the proportion.Our experimentyields furtherevidence in supportof the product-orientedschema. In some cases, subjectssubstituted inalconsonantsin the test wordsthat were different from the consonants in the inputitems. These errors mostfrequentlyinvolved the productionof a past-tenseform with more attributesof the prototype than the originaltest word. For example, some subjects re-sponded to the nonce form spriv by producinga past tense sprung or sprug.This patternof producinga past-tense form with the vowels /A/ or /a/, and afinal consonant closer to the prototype, occurred as the most frequenterrorpatterninvolving consonants for all the nonce verbs presented(see Table 13,overleaf). In these cases, subjectsareclearly operatingwith a product-orientedstrategy: they are changingthe final consonant as well as the vowel, in orderto make the inputverb conform to the past-tense schema.The strongestdeterminantof class membership,as we predicted,is the final

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LANGUAGE,VOLUME59, NUMBER 2 (1983)INPUT REPLACED BY ERRORS EXAMPLE/-rok/ // 58% spink:sprung/-g/ /1/ 65% trig:trung/-k/ /lk/ 60% strick: strunk/-m/ /n/ 82% spim:spun/-n/ U// 76% vin: vung/-C/ /k g/ 43% smip:smukstid:stug

TABLE13. Most frequenterrorpatterns n finalconsonants.consonant of the stem. This was to be expected, because the existing class ofverbs shows more coherence on this point thanon any other. Why should thefinal consonant(s)of the verb define the class more stronglythan the initials?Thefinals with the characteristicAl vowel could makeupa unit,whichin itselfis a markerof past tense. Phonologically,the vowel of a syllableforms a morecohesive unit with the consonants of the coda than with the consonants of theonset (cf. MacKay1972).Furthermore,n English,the inflectionalmorphologyis suffixing-suggesting that, in general,inflectionalmeaningwill be expressedatthe end of theword.Indeed,Slobin 1973has suggesteda language-acquisitionstrategythatdirectschildrento the ends of words to findgrammaticalmaterial;this strategy may be related to the universalpreferencefor suffixingof inflec-tionalmaterial. It would be interestingto determinewhether, in prefixing an-guages, morphologicalclasses are defined on the beginningsof words.Giventhese considerations,the influenceof initialconsonantson class mem-bership is somewhat surprising.However, it is paralleledsomewhat by thefindingsof Zubin & Koepcke 1981on genderin German.These authorsreportthat initial consonant clusters in monosyllabicnouns show a significantcor-relationwith the assignmentof nouns to the masculinegender. Thus, takingnative nouns whose genderis not semanticallydetermined,73%of those witha two-consonantinitial have masculinegender,and82%of those with a three-consonant initialhave masculinegender.Whatourfindings regarding he attributesof this verbclass indicate, in gen-eral, is that English morphologymay be a kind of 'whole word' morphology,in which both meaningfuland classificatory information s distributed overthe whole word. The decisions which our subjects made about how to formthe past tense of a particularnonce verb may have been based on how thewhole resultingword would sound. Evaluationsof how the word sounds maybe based on comparisonswith whole existing words.We find in general, then, that the experimentalresults corroboratethe hy-potheses which we reached after examinationof the synchronicanddiachronicdata.On one issue, however, the experimentaldata afforda somewhatdifferentview than the diachronic, namely the relative productivityof the class withregardto verbs ending in /n/ vs. verbs ending in /k g/. While each of theseconsonants shares a feature with the prototypicfinal, the diachronicevidenceshows that the class is expanding n the directionof /k g/, and not toward/n/.That is, all the verbs ending in /k g/ are new members of the class, while thetwo verbs ending in /n/ (win and spin) are originalmembers. However, our

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MORPHOLOGICAL LASSESAS NATURAL CATEGORIESexperimentalresults show thatsubjectsarejust as likely to assigna nonce verbending in /n/ to this class are as they are to assign a verb endingin a velar toit. Thus we must ask why the class has not added new memberswith final /n/over the last few centuries. A possible factor is the low availabilityof verbsending n/-in/as possible candidates or inclusion.A search of the reverse wordlist in Dolby &Resnikoff 1967 urnsuponly spinand win as monosyllabicverbsending in /-in/, plus a few denominative verbs: skin, grin, and pin. Perhapsthere simply have not been many opportunities or verbs in /n/ to be addedtothe class.Another area in which the experimentalresults diverge somewhatfrom thediachronicpatterns is in the use of /a/ as a vowel in the past tense. An ex-aminationof Class I, above, shows thatonly one non-original erb(ring)existsin this class. We took this to mean that this class was not productive.In viewof this, it is somewhat surprisingthat our subjects used /a/ in 34% of theresponsesin whichthey madea vowel change.Table14displaysthe percentage

/0/ 29%/Ok/ 29%/n m/ 21%/k g/ 10%/p b v/ 8%/t d/ 31%TABLE14. Percentageof /a/ responsesout of total responsesfor each final consonanttype.

of /a/ responses for nonce verbs endingin each of the finalconsonantstested.These figures indicate some productivityfor Class I verbs, since the /ae/ re-sponses occur in nonce items endingin nasals and velarnasals, which are theconsonants occurringon the real verbs of this class.Note also that the largest percentageof /e/ responses occurs in nonce itemsending in a dental consonant. This is an even more surprisingresult, sinceEnglishhas so few verbs in Englishwhich end in /t d/ and whose past tense isformedwith /ae/.Sit, sat is the only fully standardexample, since spit andshitboth have unmarkedpast forms in additionto formsspat and shat. We do notthink that sit forms a productiveclass by itself, but ratherthat anotherphe-nomenon accounts for the results. Looking againat Table 9, we see that thepercentageof regularizations -ed responses) for verbs endingin /t d/ is quitelow. In fact, if we comparethe percentageof -ed responses on Tables 7 and9, we see that only items endingin the velar nasal have a lower percentageof-ed responses. This results, we hypothesize, from the avoidanceof addingthedental suffix -ed to a word that ends in a dental. Such an avoidance has longbeen noted amongchildren,who regularize rregular erbsending n /t d/ muchless often than verbs ending in other segments (Slobin 1971, Kuczaj 1978).Furthermore, inal consonants in Englishvowel-changeverbs show an asym-metricaldistributionsuch that approximately31%end in /t d/, 28%belong toeither Classes I or II, 13%end in diphthongs,and the remaining28%are dis-tributed over eight differentconsonants, /r 1n c z v k p/. In other words, astrikingconcentrationof vowel-change verbs occurs among those ending in

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LANGUAGE, VOLUME59, NUMBER 2 (1983)/t d/. Wehypothesizethatspeakers endto take verbs that end in /t d/ as irregularin some respect; thus our subjects suppliednonce verbs endingin these con-sonants with vowel changes. If a general prototypeexists for a vowel-changeverb in English, then it might end in /t d/. We do not know why so manyresponses involved /a/; but this could result from the input vowel //, whichsuggests a reference to sit, or from the fact that most of the items on the testresembledverbs that could have either/a/ or /A/ in theirpast tenses.Turningagain to the historicaldevelopmentof strongverbs in English, wemay ask why this particular lass of verbs becameproductive-while the otherclasses have tended to remainunproductive,andto shrink n size. The answerhas to do with thefact that thisclass hadcertaincoherentdefiningphonologicalfeatures, while the others did not. Verbs of Classes I and II that were strongin Old Englishbelongedto a groupwhose stem vowel was followed by a nasalor a liquid, plus anotherconsonant(the Class III of Sweet 1882).Manyof theverbs inthis class which containeda liquidplusanotherconsonant have becomeweak or obsolete; but manyof those with a nasalplus anotherconsonanthavesurvived. The class included bindan 'to bind', findan 'to find', grindan 'togrind', windan 'to wind', drincan 'to drink', scrincan 'to shrink', swincan 'totoil', ge-limpan 'to happen', swimman 'to swim', winnan 'to fight', springan'to spring'.No otherOEverb class couldbe so narrowlydefinedphonologicallyby features of the stem. Most of them were like the class that included rideand drive, in that any consonantcould follow the stem vowel.Regularsoundchangesnarrowed hephonologicaldefinitionof theclass evenmore. OE short vowels were lengthenedwhen followed by voiced homorganicconsonant clusters such as /nd/ (Moore 1968).This changeaffectedverbs suchasfindan and bindan,while leavingother membersof the class untouched. InMiddleEnglish,these longvowels were diphthongized,giving/faynd/,/fawnd/,/baynd/, bawnd/,in effect removingverbsending n/nd/fromthe class of verbswith shortl/u. This left an overwhelmingpredominanceof verbs endingin /g/or /jk/. There simply is no other verb class in English with such a coherentset of phonologicaldefiningfeatures, and this was a majorfactor that allowedthe class to become productive.Another actormustalsobeconsidered,however-namely, what Rosch termsCUE VALIDITY.A category has high cue validity if the features associated withit frequentlyoccur with membersof the category, andrarelyoccur with mem-bers of othercategories. The sequences /A0/ and /A^k/can serve effectively assignals of the past tense so long as these sequences do not, in other items,signal a variety of other meanings.The class of verbs underconsiderationisfavored in this respect also. There are no verbs which, in theirpresent tenseor base form, end in /Af/, and there are very few nouns or adjectives:tongue,lung, andyoung are the most commonexamples. Thus /Alr/ is almostuniquelya signalof past tense. The sequence /A0k/ is also highlydistinguishable;onlytwo verbs have this sequence in their bases, flunk and dunk. It seems, then,that this class of verbs has been especially well suited for productivity,havingboth a coherent phonologicaldefinition and a past-tense shape with high cuevalidity.

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MORPHOLOGICAL LASSES AS NATURALCATEGORIESNote furtherthat the attributesof the finalsfoundin this class of verbs havehighercue validitythantheattributesof initials,since #sC(C) frequentlyoccursin wordsof all types in English.This may account for the fact that the featuresof the final provide the strongestdeterminantof class membership.Since ourexperimentdealtprimarilywithnonce verbs, it does not yield datato bear on the issue of how the product-oriented chema, incorporating heprototype, is used in ordinaryprocessing. We can only presenta speculativesketch-which, without details, outlines the role we see for schemas in lan-guage use.Schemas should be thoughtof as associations among lexical items. Thereare associations on many levels: on the phonological level, words may beassociatedby initialsegment,by rhyme,by stresspattern,or by numberof syl-

lables; on the syntactic level, they may be associated by membership n cat-egories such as noun or verb; on the semantic level, they are associated bybeing similaror opposite in meaning, or by belongingin the same semanticfield. The morphologicalschema which we propose associates words on twolevels simultaneously:those with a particularphonologicalshape, and thosewith a particularmorphological unction. We assume that these lexical asso-ciations function in languageuse to facilitate lexical accessing. We have noevidence thatbearson the questionof how a schema interactswiththe regularsuffixationrule for the Englishpast tense; but we assume that, because of theidentity of function, the schema and the regular past-tense rule are closelyassociated.This close associationprobablyaccounts for the new words thatare formedon the basis of similarityto the prototype expressed in the schema. When adiscourseis framed n a past-tensecontext, the morphological unctionof pasttense is constantly being accessed. We suggest that, even if regularverbs areused, the schemas associated with past tense are made available. This avail-ability makes it possible for innovations to be based on the prototype. Theseinnovations, which form new members of the verb class, are like other inno-vations that result in new lexical items (such as blends), in that they are notthe result of combinationby rule, but are nonetheless made likely by stronglexical associations.

Perhaps the most importantfinding from this combination of synchronic,diachronic, and experimentaldata is that speakers of naturallanguageformcategorizationsof linguistic objects in the same way that they form categori-zations of naturaland culturalobjects. These verbs are not categorized bypropertiesof theirmeaning,or by propertiesthatassociate them with externalreality;rather,they are categorizedby theirform, i.e. theirphonologicalprop-erties. We do not know how pervasive this type of categorization s in mor-phologicalclasses. The present example is, however, not unique, since Zagermakes a quite convincingcase that Old Spanishstrongpreteritsare organizedin this same way. These cases suggest that the psychological principleswhichgovern linguisticbehaviorare the same as those which govern other types ofhumanbehavior, and that explanationsof linguisticstructurescan be soughtoutside the linguistic system itself.

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268~LANGUAGE,OLUME59, NUMBER2(1983)APPENDIX

NONCE ERBSCLASSIsplingskringsprinkstrinkstrinkstrickstrig

skrimsprinsprivskritsmingskingsminkspinksmickskig

spimstinstidsmip

plingtringkrinkglinktrigglick

krinplimtribclidshinktinkpingving

OTHERstrankscronk

spraykspreyksprocksprookstreykstrugskreekscragsplamspreenspratspang

steegskeykstaykspugspockspaksianskeenstomstapspeebskeypskeepglank

kragtreekplaykgloktrooktreykkreeggreemtrantreeptradtang

REALVERBSCLASSI OTHER

streakstroke

stingslink

stick stack

skim

skid

fling

trick

tripcliplink

1. sCCiy/yk

2. sCCiklg

3. sCCin/rn4. sCC iC5. sCiy/lk

6. sC ikig

7. sC i/rn

8. sC iC

9. CCim

10. CCikIg

11. CC In/rn12. CCiC

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MORPHOLOGICAL LASSES AS NATURAL CATEGORIESAPPENDIX(Continued)

NONCE VERBS REAL VERBSCLASS I OTHER CLASS I OTHER

14. C i klg gick pook digsig deykgackgokseeg15. C i nlm tim tamvin veem16. C i C sid toop bidkib bive tippeetseybdoptad

REFERENCESBERKO, JEAN. 1958. The child's learning of English morphology. Word 14.150-77.BYBEE,JOANL., and MARYALEXANDRIAREWER. 980. Explanation in morphopho-nemics: Changes in Provencal and Spanish preterit forms. Lingua 52.271-312.BYBEE, JOAN L., and DAN I. SLOBIN. 1982. Rules and schemas in the development anduse of the English past tense. Lg. 58.265-89.CHOMSKY, NOAM,and MORRISHALLE. 1968. The sound pattern of English. New York:

Harper & Row.DOLBY,J. L., and H. L. RESNIKOFF. 1967. The English word speculum. The Hague:Mouton.HOARD, AMES,and CLARENCESLOAT. 1973. English irregular verbs. Lg. 49.107-20.HOOPER,JOAN BYBEE. 1976. Introduction to Natural Generative Phonology. New York:Academic Press.-- . 1979. Child morphology and morphophonemic change. Linguistics 17.21-50. [Alsoin Historical morphology, ed. by Jacek Fisiak, 157-87. The Hague: Mouton, 1980.]JESPERSEN, OTTO. 1942. A modern English grammar on historical principles, IV: Mor-phology. Copenhagen: Munksgaard.KUCZAJ,STANA. 1978. Children's judgments of grammatical and ungrammatical irreg-ular past tense verbs. Child Development 49.319-26.MACKAY,DONALD. 1972. The structure of words and syllables: Evidence from errorsin speech. Cognitive Psychology 3.210-27.MOORE, AMUEL. 968. Historical outlines of English sounds and inflections. Revisedby Albert H. Marckwardt. Ann Arbor: Wahr.NEWFIELD, MADELEINE. 981. Options in language: The productive formation of theEnglish verb preterit. (Katholieke Universiteit Leuven, Voorlopige Publikatie 78.)Leuven, Belgium.ROSCH,ELEANOR. 978. Principles of categorization. Cognition and categorization, ed.by E. Rosch and B. Lloyd, 27-48. Hillsdale, NJ: Erlbaum.- , and CAROLYN. MERVIS. 975. Family resemblances: Studies in the internal struc-ture of categories. Cognitive Psychology 7.573-605.SKOUSEN, ROYAL. 1975. Substantive evidence in phonology. The Hague: Mouton.SLOBIN,DAN I. 1971. On the learning of morphological rules: A reply to Palermo andEberhart. The ontogenesis of language, ed. by Dan I. Slobin, 215-23. New York:Academic Press.. 1973. Cognitive prerequisites for the development of grammar. Studies of childlanguage development, ed. by Charles Ferguson & Dan Slobin, 175-208. NewYork: Holt, Rinehart & Winston.

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270 LANGUAGE,VOLUME59, NUMBER2 (1983)STAMPE,DAVID.1973.A dissertationon naturalphonology. Universityof Chicagodis-sertation.SWEET,HENRY.1882. Anglo-Saxon primer. Oxford: Clarendon Press. [Reprinted, 1965.]VENNEMANN,THEO. 1971. Natural Generative Phonology. Paper read at the LSA AnnualMeeting, St. Louis.WITTGENSTEIN,LUDWIG. 1953. Philosophical investigations. New York: Macmillan.ZAGER, DAVID. 1980. A real-time process model of morphological change. Buffalo:SUNY dissertation.ZUBIN, DAVID A., and KLAUS-MICHAELKOEPCKE. 1981.Gender: A less than arbitrarycategory. CLS 17:439-49.

[Received7 December 1981;revision received 16March1982;accepted28 April1982.]

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Morphological Classes as Natural Categories