PARASITES AND DARWIN'S INTELLECTUAL TRIUMPH

-Jack R. Holt

A BOIL? The study of parasitic adaptation is one of the most important buttresses of evolutionary theory.

-Hans Zinsser (1934)

Several months ago a colleague from the Mathematics department asked me to speculate about the most highly evolved species on the planet. I tried to convince him that the concept of "most highly evolved" had little meaning in modern biology. He then changed the question to be the most complex species. Without hesitation, I responded that any number of organisms that we call parasites would qualify for that position. Parasites include all manner of living things from every kingdom of life. They are marked by tuning their life histories and physiological processes to those of their host organisms. Some of them have more than three different hosts. The concomitant complexity in life cycle is usually balanced by a simplification in body form as the host(s) becomes the habitat for the parasite. I have been the host of a myriad of parasites. I have suffered infectious diseases, the annual flu, colds, fungal diseases, etc. However, my most memorable parasitic visitation was that of a type of fly called a Botfly (Dermatobia hominis). I began to notice the organism two years ago within a week of my return from Costa Rica. A boil began to rise on my wrist where I had noticed a particularly itchy mosquito bite. I tried to lance the boil myself but found only swollen tissue (See Figure 1). I went to my local physician who tried twice to lance the swollen area. Out of desperation, she sent me to a regional hospital where they took biopsies to determine the nature of the infection. In the mean time, the boil would occasionally cause acute pain and a large flow of blood. The odd nexus of symptoms caused almost everyone to scratch their heads especially when the report came back from the dermatological biopsy that neither a bacterium nor a fungus caused the "boil". On an off chance I asked the university physician what it might be. He looked at the wrist and said that it appeared to be a parasite of some sort. I went straight back to my lab and put my wrist under a dissecting microscope. There, I saw the movement of a black and tan striped animal under my skin. Periodically, it stuck its breathing tube out through a hole in the raised area. I realized in a moment that I was the host of a parasitic insect. Within minutes I had found the culprit in my parasitology books and called the physician who had already come to the same conclusion. Within the hour, he removed a 1 cm long maggot from my wrist (See Figure 2). The month of uncertainty finally had an explanation. All of the symptoms began to make sense in retrospect. The lifecycle of the human botfly is tuned to two different kinds of organisms. First, the female botfly locates an appropriate blood-feeding insect, a mosquito or a biting fly. Somehow the female lays an egg on or near the head of the other insect while it is resting. The blood-feeding insect then seeks out an appropriate mammalian host (humans are just one of several possible hosts). While the insect is taking a blood meal, the botfly egg drops off and secures itself to the skin of the host. It quickly hatches and burrows into the skin. There, it causes a cellulitis-like response where it grows embedded in a plentiful food supply. The maggot

maintains an opening to the outside through which it extends its air tube. When the maggot is ready to pupate, it backs out of the skin of the host and drops to the ground. After a brief pupal stage underground, the mature fly emerges to continue the life cycle, which is elaborate but effective. Although it is uncomfortable, the botfly is rarely fatal to its host.

FIGURE 1. My wrist with the raised "boil" and the open hole at the apex of the swelling.

FIGURE 2. The botfly maggot that came from my wrist in December 1999. The total length of the animal is about 1 cm. Its head is on the right and the abdominal end with the air tube is on the left. Much of the world is controlled by the occurrences of a broad range of parasites such as malaria, sleeping sickness, ascariasis as well as infectious diseases and spore-disseminated bacteria (like anthrax). That is particularly true of those diseases that can exist in a debilitating chronic form. Such an extended sublethal disease erodes the human potential of a population at the same time that it consumes the financial support of the society in which the disease occurs. In some cases, parasitic diseases are so pervasive that their incidences might be nearly 100%. That was the case of Chagas' disease in certain areas of Central and South America through the

19th Century. While on his voyage of discovery, Charles Robert Darwin (1809-1882; See Figure 4) visited the areas of South America in which malaria and Chagas' disease were endemic. Indeed, Darwin describes the Benchuca bug (Triatoma infestans), a large South American bed bug that is the intermediate host for the causative agent of Chagas' disease, and its ability to suck a large volume of blood in a very short time. Almost certainly Darwin became infected with the trypanosome (Trypanosoma cruzi) that produces the disease somewhere in Chili (See Figure 3). He became acutely ill from September 20 to the end of October, 1834. DARWIN EXPLAINED? But it is…a principle feature of his life, that for nearly forty years he never knew one day of the health of ordinary men, and thus his life was one long struggle against the weariness and strain of sickness. -Francis Darwin When an infected bug feeds on a victim, it quickly engorges itself with blood and more than doubles its size. As it bloats, the bug typically defecates. Trypanosomes in the feces can be moved into the wound created by the bug when the sleeping person scratches the wound. At first, the infective cells seek out tissues of the lymphatic system and multiply. Then, they break out into the circulatory system as swimming trypanosome cells as in Figure 3. This is the acute phase and lasts a relatively short period. Following that, the trypanosome takes up residence in tissues like the heart and intestines. There, they cause chronic malaise and digestive problems, both of which were Darwin's complaints in later years.

FIGURE 3. A human blood smear from a person infected with Trypanosoma cruzi, the causative agent of Chagas' disease. Note the C-shaped flagellated cells among the circular red blood cells. Darwin was not always sickly as his son described him. Indeed, as a young man Darwin routinely went on hunting trips. Throughout the time that he was the Ship's Naturalist aboard the H.M.S. Beagle (1831-1836), Darwin showed enormous strength and endurance as he explored rainforests, mountains, rivers, plains, etc. His energy also poured through his writings and were

manifest in his many interests and attention to detail in his journals as presented in his publication of the Voyage of the Beagle. Upon his return to England, Darwin found that he was something of a celebrity among the scientists of Britain. His collections and preliminary notes on the voyage had excited the scientific community as much as those of Von Humboldt (1769-1859), a generation earlier. Darwin dove into organizing his field notes and collections for publication.

FIGURE 4. Darwin as he appeared after his return from the voyage of the Beagle.

His initial problem was how to organize the collections. Only Richard Owen (1804-1892) and Charles Lyell (1797-1875) came forward to offer help with them. Darwin had to cajole and attempt to enlist experts in botany, zoology, and geology to help him in the organization. Darwin soon lamented that "the collectors so much outnumber the real naturalists that the latter have no time to spare." Nevertheless, he gradually recruited naturalists to begin working on his collections. He was particularly successful in finding zoologists to take "whole tribes of animals." Darwin married his cousin, Emma Wedgwood in 1839 and moved into a small house in London. Soon thereafter Darwin began to experience the chronic ill health that would plague him for the rest of his life. By 1842, Darwin and his wife moved to an estate in the village of Down, south of London. They chose it because it was quiet and in the country. Darwin lived there for the rest of his life and conducted most of his observations and experiments there. They lived on money that they had inherited from their parents. Furthermore, Darwin made shrewd investments and later had some income from his many books. He became completely absorbed in the science that he had begun during the voyage and spent the rest of his life as a gentleman scientist with a fairly rigid work schedule. He vowed to return to London about twice each month so that he could remain active in the scientific establishment there and kept up that routine as long as his health permitted. A Darwin afflicted with Chagas' disease was still very active as a scientist. He wrote extensively on all areas of natural history: geology, zoology, and botany. He experimented with

anatomy and physiology. In 1846 he tried to organize his collection of barnacles and noticed that the taxonomy of the group was in such disarray that the task was impossible. Furthermore, a quick survey of the group indicated that he had collected entirely new groups of barnacles. His barnacle project grew in scale and occupied him until 1854 when he produced a massive three-volume treatise (2 volumes on living and 1 volume on fossil barnacles published between 1851 and 1854). Thus, Darwin became one of the most skilled natural historians in Britain and almost all self-taught. THE ORIGIN OF THE ORIGIN To Charles Lyell, Esq., F.R.S. this…is dedicated with grateful pleasure - as an acknowledgement that the chief part of whatever scientific merit this journal and the other work of the author may possess, has been derived from studying the well-known and admirable Principles of Geology.

- Charles Darwin (1845) -

Darwin took a copy of Lyell's Principles of Geology (Vol 1) when he left on the voyage of the Beagle. He read the book and assimilated its message of uniformitarianism1. Later, he had copies of volumes 2 and 3 shipped to him during the voyage. His observations showed him first hand that the earth was tremendously old and that much of what he saw could be explained more easily given large expanses of time. Darwin was delighted to make Lyell's acquaintance after his return. The friendship that developed between the two men was genuine and lasted throughout their lives. Lyell was one of the few allowed into the estate at Down and had a great influence on the science of Darwin. Volume 2 of Principles of Geology treated the possibility of "the transmutation of species" (a phrase that means evolution in today's usage). Lyell began the second volume by considering the views of Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (1744-1829, France; see Figure 5) who worked as a naturalist at the National Museum of Natural History (Musee National d'Histoire Naturelle) in Paris. First, he worked at the poverty level as an assistant with the plants in the Jardin des Plantes. Then, in the wake of the French Revolution, Lamarck was appointed professor of Invertebrates (Insects and Worms in the language of the day). At the time few, including Lamarck, knew anything about the "lower" animals. However, the numbers and diversity of invertebrate species also demonstrated to him that species could change. He published his view of evolution in 1801 in which he proposed that organisms were constantly climbing the ladder of life. They changed by passing on acquired characteristics to the next generation driven by an urge for perfection in form. Lamarck suffered a deafening silence from the scientific community and the scorn of his colleague, Georges Cuvier (1769-1832; See Figure 6) who believed that species were immutable. He believed that species could not change because all parts of an individual were beautifully integrated into a whole that was ideally suited to its environment. Nevertheless, he did recognize that animals occurred in related groups, some of the species were extinct. For example, the woolly mammoth was extinct but its parts were homologous (equal to) those of living elephants. Thus, he could take a single bone from an animal like a woolly rhinoceros and reconstruct the whole animal. Cuvier's encyclopedic knowledge of the vertebrates allowed him to compare anatomical structures of animals living and extinct. Thus, he created the discipline of Comparative Anatomy.

1 See Geology Through Lyell's Eyes The Saturday Scientist Vol 8, No 10

FIGURE 5. Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck.

Cuvier reconciled the immutability of species with comparative anatomy and extinction by supposing that the earth had suffered a succession of catastrophes, each followed by a separate creation. Similar animals in each creation must have occupied similar habitats and, therefore the necessity for similarity in form. Lyell, himself, adopted the view of Cuvier in the first editions of Principles. Thus, Darwin was well-acquainted with the philosophies of Lamarck, Cuvier and Lyell with regard to species and their mutability. Still, fresh from his voyage in 1836, when Darwin began to organize his journals, he noticed how many of his observations could be explained if species were mutable. He began to keep a series of notebooks in which he recorded any facts relevant to the species question. Later, Darwin claimed that even while in South America he noticed that certain observations were explainable only with the "supposition that species gradually became modified." Still, Darwin claimed that he did not become convinced of the mutability of species "until two or three years had elapsed."

FIGURE 6. Georges Cuvier.

Curiously, Darwin's own grandfather, Erasmus Darwin (1731-1802) explored the mutability of species in a major work called Zoonomia, or the Laws of Organic Life (1794-1796). Charles never met his grandfather who died seven years before he was born. Later, Darwin said that he did read his grandfather's work, but it made little impression on him when he was young. He claimed to have reread the book after his return from his voyage and commented, "I was much disappointed; the proportion of speculation being so large to the facts given." On Darwin's death, Thomas Henry Huxley claimed that "Erasmus was an anticipator of Lamarck, and not of Charles Darwin." Thus, Charles' belief in the mutability of species grew from his struggle to make sense of nature, not from a particular preconception about species. DARWIN AS PARASITE? The sick man is a parasite of society. -Friedrich Nietzsche (1888) Curiously, no one questioned the notion of a species. Lyell's, Lamarck's, and Darwin's treatments of species were quite modern in their outlooks. Indeed, a good working definition of species was independent of a belief in the fixity of a species. For example, Carolus Linnaeus (1707-1778) who defined the modern system of classification was a staunch believer in the inerrant unchanging character of species. Nevertheless, the general definition of a species was (and is) a group of organisms that can breed and produce viable offspring. That is, they must breed true. The real question was whether some individuals within a species might breed almost true and, therefore, allowed for a change. Darwin's study of his samples and notebooks demonstrated how much individuals might vary and still remain within the same species. Could variability alone lead to new species? Darwin said that he struggled with this issue until 1838 when he happened to read an essay by Thomas Malthus (1766-1834; See Figure 7), a political economist, called Essay on the Principle of Population (1798). Malthus asserted that the standard of living in England must decline because the lower classes produce too many offspring and use up finite resources. Unless, the lower classes were regulated, then famine and poverty were in the future. (He speculated that such were natural outcomes visited upon human populations by God to keep them from becoming indolent.)

FIGURE7. Thomas Malthus

Darwin wrote that he was "well-prepared to appreciate the struggle for existence" after reading Malthus. He realized that species would tend to produce offspring in excess of the resources available. Thus, those individuals with more favorable variations would tend to survive. An accumulation of such differential survival would lead to a new species. Darwin wrote, "Here, then I had at last got a theory by which to work." He called the new theory natural selection and began to seek evidence of its operation in nature. His notebooks began to fill with observations and later in 1842, Darwin wrote a sketch of the theory that he expanded into a 230 page essay in 1844. He set aside the essay with a letter to his wife and funds for its publication in the event of his sudden death. He named Charles Lyell as his preferred editor of the work. Why did Darwin not publish his book in its 1844 form? The essay generally laid out the arguments that he would expand on in The Origin of Species. He knew that the theory was important, but that it would generate much controversy. So, Darwin waited and gathered information. Loren Eiseley suggested that Darwin actually received his inspiration from a Calcutta apothecary named Edward Blyth (1810-1873; see Figure 8) who had published on "the struggle for existence" in 1835 and 1837 in The Magazine of Natural History, a journal that Darwin often read and referenced. Later, Blyth entered into an extended dialogue with Darwin about the mammals and birds of the Indian subcontinent and Southeast Asia. Indeed, Blyth may have introduced Darwin to the branching tree metaphor of life (although it had been used 30 years earlier by Lamarck). Eiseley claims that the formulation of the theory of natural selection actually came from Blyth and that Darwin invented the Malthus explanation years later.

FIGURE 8. Edward Blyth (left) and Robert Chambers (right)

Curiously, Darwin wrote his expanded essay in the same year that Robert Chambers (1802-1871; see Figure 8) anonymously published Vestiges of the Natural History of Creation. Chambers was a publisher and popular writer from Scotland with a particular interest in geology. His small but ambitious book attempted to explain the origins of the solar system and of life, the known geological periods, the development of life (evolution), and the origin of our species. He claimed that inanimate nature obeyed the Law of Gravitation while animate nature (life) obeyed the Law of Development. The book generated such controversy in the English-speaking world that Chambers retained his anonymity until years after his death. Although controversial, Vestiges was successful and was published through more that 12 editions (1844-1884). Later Darwin claimed that the book helped to introduce and therefore prepare people to accept the idea of evolution. However, Vestiges, like his grandfather's Zoonomia, was long on speculation and short on facts. Darwin bided his time particularly with his work on barnacles. Through that eight-year exercise, he learned first hand the challenges of taxonomy (classification) and the remarkable

variation that can occur within a group and within a species. Still, later he mused, "I doubt whether the work was worth the consumption of so much time." Meanwhile, Darwin was in communication with naturalists all over the world. He plied them with questions about species of all types. He visited the barnyards and became a member of a local pigeon fancier's club. There, he saw first-hand the power of selective breeding from the common rock dove to the myriad forms available in his day. His notebooks continued to fill with information. Lyell advised Darwin to write a more complete manuscript of his ideas in 1856. Darwin began the project of his species book, a book that was to have been much more massive than the Origin. He communicated his views openly with Lyell and other members of his inner circle like the botanist Joseph Dalton Hooker (1817-1911; See Figure 9). He even communicated the essence of his theory of natural selection to another botanist in the U.S. named Asa Gray (1810-1888; see Figure 9). These "bold" moves saved Darwin and allowed the priority of the discovery to go to him. In the summer of 1858 Darwin opened his mail and read a communication from Alfred Russel Wallace (1823-1913; See Figure 10), a collector and naturalist working in the Malay Archipelago (what is now Indonesia). The enclosed essay, On the Tendency of Varieties to Depart Indefinitely from the Original Type, was, in Darwin's words, "exactly the same theory as mine." His first inclination was to relinquish priority to Wallace and just see to the publication of his essay. However, Lyell and Hooker convinced Darwin to write a short paper and publish jointly with Wallace. Darwin did that. They published in The Journal of the Proceedings of the Linnean Society in 1858 with a copy of a letter that Darwin had sent to Asa Gray in 1857. Professor Haughton of Dublin retorted, "that all that was new in them was false, and what was true was old." Was Darwin a parasite on other's ideas? Eiseley claimed that Haughton's comment referred to the real contributions of men such as Blyth, Chambers, and Lamarck. Did Darwin really get his inspiration for natural selection from Malthus or Blyth or Chambers or Wallace? Ernst Mayr said that Darwin's theory was inspired by the writings of Charles Lyell. Curiously, Wallace also claimed to have been inspired to conceive natural selection by reading Mathus' essay.

FIGURE 9. Joseph Dalton Hooker (left) and Asa Gray (right).

Where did the theory come from? Does it really matter? Of these men only Wallace articulated the theory as clearly as Darwin did, but Darwin had much, much more evidence. The manuscript from Wallace forced Darwin's hand to quickly prepare a manuscript, a 300+ page book that Darwin referred to as an abstract.

FIGURE 10. Alfred Russel Wallace.

THE SPECIES BOOK To anyone who studies the signs of the times, the emergence of the philosophy of Evolution…is the most portentous event of the nineteenth century. -Thomas H. Huxley Darwin said that the book "cost me thirteen months and ten days' hard labour" and was published in November 1859 under the title, Origin of Species. He called his book "one long argument" in which he attempted to convince the reader that species changed over time and the mechanism of that change was natural selection. He began the argument as had other authors (e.g. Lamarck, Blyth, Chambers, and Wallace, etc.) with an examination of the power of selective breeding in shaping organisms. He considered the breeds of dogs, plants, and pigeons and summed up his conclusions with, "Over all these causes of change I am convinced that the accumulative action of selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant power." Darwin noted that house cats vary little because of their nocturnal (and therefore uncontrollable) breeding habits. Still, many cat breeds have been generated since Darwin's day. Consider the differences between my two cats (see Figure 11) and the relatively few generations required to produce them. This is a striking example of Darwin's argument about variation under domestication. He then considered variation under nature. In this chapter, he gave example after example (primarily of plants) of how large, widespread, dominant groups show the greatest variability. He argued that the subgroups thus generated could give rise to new species through the struggle for existence by a parade of examples primarily from the animal kingdom. He argued that in the struggle of life any form with a slight advantage over another will "survive and multiply."

FIGURE 11. My two cats, Lyuba (left) and Benny (right).

Darwin then deftly turned the argument by uniting the remarkable variability of organisms within a species with the struggle for existence. Just as organisms under domestication change in response to selective breeding, so do they change in nature as a consequence of the struggle of life. Thus nature tends to select for traits that are beneficial and cause a change from one species to the next. Similarly, the rise of new successful species must lead to the extinction of those that are displaced. In this way, natural selection drove the changes of life throughout its history. Here, Darwin employed the metaphor of the branching tree to describe the theory of descent with modification to explain life and its history. Darwin explored problems with the theory of natural selection. Namely, how were variable traits inherited (this was long before a good working theory of genetics)? How could complex organs like the eye evolve without large leaps in structure? He finally settled on the solution that however traits are inherited they include all of the former adaptations in the history of the organism. Thus, by small steps through the agency of natural selection even a structure as marvelous as the eye could evolve. Similarly, he did consider the imperfection of the fossil record and concluded that the gaps in the record just represent gaps in time during which large changes could occur in living things. The final chapters of the Origin carried the strongest weight of evidence. In these, he used natural selection and descent with modification to explain the distributions of livings over the globe. For example, the finches of the Galapagos Islands are all rather similar in form and resemble a kind of finch on the mainland of South America. However, the island finches vary considerably with regard to their beaks (See Figure 12). Some of them are adapted to eating seeds, some have changed to a beak suited to catching insects. One evolved a woodpecker-like beak with an elaborate behavior to pull insects out of wood.

FIGURE 12. Two species of finches from the Galapagos Islands. Note the similarity in body form and difference in bill type. Clearly, the Galapagos finches happened to find themselves on the islands as a consequence of storms or other means. Natural selection allowed them to diversify and occupy a variety of habitats excluded from them on the mainland. Nevertheless, the woodpecker finch would easily be displaced by a real woodpecker if it were to make it to the islands.

Just as organisms tend to be more similar to each other the closer that they are in time, they also tend to be more similar the closer that they are to each other on the earth. Darwin put it this way:

…the more nearly any two forms are related in blood, the nearer they will generally stand to each other in time and space; in both cases the laws of variation have been the same, and modifications have been accumulated by the same power of natural selection.

Finally, Darwin argued that the system of classification itself was a manifestation of the relationships between living things. For example, consider the orchids pictured in Figure 13. Phalanopsis is more closely related to other species of Phalanopsis that it is to the Dendrobium. In a Darwinian sense, the orchids are related through a common ancestor. They are related more distantly to oats (Avena sativa) through a common ancestor and they are related to common cats (Felis domesticus) through an even more distant ancestor. The relationships would branch like a tree with some lines becoming extinct while others might branch many times. Thus, the similarities that are used in a classification system (similarities in anatomy, development, etc.) simply show the characters or sets of characters that are inherited from a common ancestor.

FIGURE 13. Two orchids, Phalanopsis (left) and Dendrobium (right). Although different, they posses the characteristic structures of orchids. At the end of the book Darwin tries to present how exalted he views this explanation of life. He challenged directly the view of special creation by the following:

When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Silurian system was deposited, they seem to me to become enobled.

Thus Darwin presented his grand view, and in the last paragraph invoked the Law of Gravity to connect natural selection to physical law. The first edition of Origin was published by subscription; so, it sold out within hours of its release in November of 1859. The second edition that followed around the beginning of 1860 likewise sold out. Then, Darwin waited for the storm while he occupied himself with other projects. THE DARWIN CREW I finished your book yesterday…Since I read von Baer's Essays nine years ago no work on Natural History Science I have met with has made so great an impression on me.

-Thomas Henry Huxley (1859) to Darwin

Thomas Henry Huxley (1825-1895; see Figure 14) was one of the rising stars of British natural history, particularly comparative anatomy, when he became one of the first converts to Darwin's vision. He published a very favorable review of the Origin, and set out to defend the view at every turn. His tenacity earned him the name of Darwin's Bulldog.

FIGURE 14. Thomas Henry Huxley

Perhaps, Huxley's most famous defense of Darwinism came in 1860 when a great debate on evolution had been staged at Oxford. Huxley was one of the speakers for the Darwinian view while Archbishop Samuel Wilberforce spoke for the opposition. Huxley almost left the debate but was persuaded to stay by Robert Chambers. Richard Owen, who had developed a great dislike for Huxley, instructed Wilberforce on the appropriate arguments, particularly the scientific weaknesses of Darwinism. Huxley and others spoke for and against Darwin. Wilberforce took the podium and delivered an oration that ended with a question to Huxley, "Was I descended from an ape on my grandmother's or my grandfather's side?" Huxley is supposed to have said to those near him, "The Lord hath delivered him into my hands." Later Huxley claimed to have made this reply:

If then the question is put to me would I rather have a miserable ape for a grandfather or a man highly endowed by nature and possessed of great means of influence and yet who employs these faculties and that influence for the mere purpose of introducing ridicule into a grave scientific discussion, I unhesitatingly affirm my preference for the ape.

Thus, the debate ended. Other debates continued throughout science and western society. Huxley, Gray, and Hooker almost immediately became advocates for Darwinian science. Much later Lyell finally sided publicly with the Darwinists. Wallace, who did not quite like the term, natural selection, coined the term, Darwinism. Darwin himself stayed away from the fray and remained content to let others come to his defense. The concept of evolution swept through the scientific world with the same kind of fury that Newton's view of the universe had nearly 200 years earlier. Quickly, evolution, and less so the theory of natural selection, became knit throughout the life sciences. The integration of biology and evolution was nearly complete by 1870. Indeed, that all living things share a history of common descent became the unifying principle of biology and, therefore, made it a coherent discipline. Western society accepted evolution because it appeared to provide some support for progress of the human species. For that reason, society never quite accepted the full implication of natural selection. Unfortunately, evolution also manifest itself in all types of perversions, all of which sought to find a scientific support for the notion that might makes right.

ORIGIN’S THEORIES Darwin’s views on evolution are often referred to as The Darwinian Theory. Actually, they consist of a number of different theories that are best understood when clearly distinguished from each other. -Ernst Mayr Plasticity of understanding and acceptance of evolution occurred within science, particularly during the first fifty years following the publication of The Origin. In fact, many who said that they accepted evolution rejected natural selection. That was because The Origin presented five different concepts according to Ernst Mayr, one of the leading evolutionary theorists of the 20th Century (see Table 1). Mayr presented them as five theories. I interpret them slightly differently. I consider the nonconstancy of species to be a fact. That species changed over time was clearly demonstrated, even before 1859. However, no one said it more clearly or authoritatively before The Origin. This nonconstancy of species is the fact of evolution. TABLE 1. The concepts of evolution presented in The Origin according to Ernst Mayr (2001).

The nonconstancy of species Fact The descent of all organisms from common ancestors. Principle The gradualness of evolution Theory The multiplication of species Theory Natural selection Theory

That all organisms are the products of descent from common ancestors is the principle that unifies the science of Biology. It is this concept that makes clear the underlying similarities of living things. Thus, we can explain why all living things are cellular in their fundamental structures, why they use proteins made of L-amino acids, why they use carbohydrates made of D-sugars, and why they use the same set of nucleic acids to store and transmit information. The list of underlying similarities is quite long, but the point is clear. The study of life, at least life on planet Earth, is fundamentally the same. The gradualness of evolution grew from the study of paleontology and the application of the Principle of Uniformatarianism. Darwin’s application of Lyell’s book with this as its fundamental principle to observations that he made on the voyage of the Beagle was his earliest clue that life changed over time, because there was so much time for it to happen. The strict adherence to this theory caused Darwin to reject the notion that life could evolve in leaps and jumps. Thus, life could have no discontinuities (also a strict application of the principle of common descent). Evolution is the source of biological diversity or the multiplication of species. This theory, also a consequence of the principle of common descent, assumes that the extravagance of life that we see on earth today came from the repeated separation of species over time. This was one of the few theories that Darwin illustrated in The Origin (see Figure 15). A corollary to this theory suggests that life has continually grown in diversity throughout its history with brief interruptions caused by mass extinctions. The last, and Darwin’s most valued theory was natural selection. Curiously, very few of those in his close circle thought that life evolved by natural selection. Then, the theory fell to the scientific rubbish bin following the discovery of Mendel in 1900 when mutations seemed to be the way that species changed. Theodosius Dobzhonsky, Julian Huxley, and Ernst Mayr, in the

first half of the 20th century reconciled the problem in what was called, The New Synthesis. In this view, mutation was the source of variation on which natural selection operated. Darwin’s precious theory was restored.

FIGURE 15. Darwin’s only figure in The Origin. It was an illustration of the branching tree-like nature of evolution. OUR INHERITANCE "Natural Selection" is the key-stone of my Book & I have very great confidence it is sound.

-Charles Darwin (1858) in a letter to Hooker

Darwin succeeded in convincing science of the fact of evolution and struggled through the rest of his life to convince biologists of natural selection, the most creative contribution of Charles Darwin. He continued to explore aspects of natural selection in his books that followed Origin until his death in 1882. He was buried in Westminster Abbey right next to Isaac Newton. Natural selection as an explanation of evolution began to fall out of favor through the end of the 19th and the beginning of the 20th Centuries. Then, science began to see it at work in a most terrible way. Insects that had once been controlled by pesticides could no longer be held in check. More recently, the indiscriminate overuse of antibiotics led to the selection of highly resistant strains of staph, and tuberculosis. Other parasitic diseases such as malaria again became epidemic. Some evidence suggests that the trypanosomes that cause Chagas' disease jumped from Guinea pigs to humans after the first wave of human settlement entered South America. The triatomid bug intermediate host found humans to be a convenient and nutritious meal. The trypanosomes found the physiology of humans to be compatible to that of the original Guinea pigs. Natural selection did not have to work very hard to do the rest. By the time it had infected Darwin, Trypanosoma cruzi had become well adapted to its human hosts. Without doubt, parasites are the clearest and most tragic examples of Darwinian evolution at work.

FIGURE 16. The title page of the first edition of Origin of Species.

-2001, revised 2005 Sources That I Used to Write This Essay:

Beeson, Paul B. and Walsh McDermott, eds. 1975. Textbook of Medicine. 14th edition. W.B. Saunders Co. Philadelphia.

Bowler, Peter J. 1992. The Fontana History of the Environmental Sciences. IN: Porter, Roy, ed. Fontana History of Science. Fontana Press. London.

Browne, Janet. Charles Darwin, Voyaging. Princeton University Press. Princeton, NJ. Browne, Janet. Charles Darwin, The Power of Place. Princeton University Press. Princeton, NJ. Burkhardt, Krederick, ed. 1996. Charles Darwin's Letters, A Selection 1825-1859. Cambridge

University Press. New York. Chambers, Robert. 1870 (originally published 1844). Vestiges of the Natural Histroy of Creation.

With a Sequel. Harper & Brothers, Publishers. New York. Darwin, Charles. 1979 (originally published 1859). The Origin of Species. Gramercy Books.

New York. Darwin, Charles. (originally published 1845). The Voyage of the H.M.S. Beagle. 2nd edition. The

Heritage Press. New York. Darwin, Francis, ed. 1958 (originally published in 1992). The Autobiography of Charles Darwin

and Selected Letters. Dover Publications Inc. New York.

Darwin, Francis, ed. 1896. The Life and Letters of Charles Darwin. Vol 1-2. D. Appelton & Co. New York.

Desmond, Adrian and James Moore. 1991. The Life of a Tormented Darwin. Warner Books, Inc. New York.

Diamond, Jared. 1998. Guns, Germs, and Steel, The Fates of Human Societies. W.W. Norton and Co. New York.

Eiseley, Loren 1961. Darwin's Century: Evolution and the Men Who Discovered It. Doubleday, Anchor Books. New York.

Eiseley, Loren. 1979. Darwin and the Mysterious Mr. X, New Light on the Evolutionists. Harcourt Brace Jovanovich. New York.

Gould, Stephen Jay. 2002. The Structure of Evolutionary Theory. The Belknap Press of Harvard University. Cambridge, Mass.

Holt, Jack R. and Patricia Nelson. 2001. Paths of Science. Kendall/Hunt. Dubuque. Huxley, Thomas Henry. 1896. Darwiniana. D. Appleton & Co. New York. Huxley, Thomas Henry. 1896. Man's Place in Nature. D. Appleton & Co. New York. Irvine, William. 1955. Apes, Angels, and Victorians, The Story of Darwin, Huxley, and

Evolution. McGraw-Hill Book Co, Inc. New York. Keynes, R. D., ed. 2001. Charles Darwin's Beagle Diary. Cambridge University Press.

Cambridge. Lloyd, Elisabeth. 1994. The Structure and Confirmation of Evolutionary Theory. Princeton

University Press. Princeton, NJ. Loewenberg, Bert James, ed. 1959. Charles Darwin: Evolution and Natural Selection, An

Anthology of Writings of Charles Darwin. Beacon Press. Boston. Lyell, Charles. 1990 (originally published 1830-33). Principles of Geology. Vol 1-3. University

of Chicago Press. Chicago. Mason, Stephen F.1962. A History of the Sciences. Collier Books. New York. Mayr, Ernst. 1991. One Long Argument, Charles Darwin and the Genesis of Modern

Evolutionary Thought. Harvard University Press. Cambridge. Mayr, Ernst. 2001. What Evolution Is. Basic Books. New York. Milner, Richard. 1994. Charles Darwin, Evolution of a Naturalist. Facts on File, Inc. New York. Ruse, Michael. 1999. The Darwinian Revolution, Science Red in Tooth and Claw. The

University of Chicago Press. Chicago. Simmons, John. 2000. The Scientific 100, A Ranking of the Most Influential Scientists, Past and

Present. Citadel Press. New York. Wallace, Alfred R. 1890. Darwinism. MacMillan & Co. London. Zinsser, Hans. 1963. Rats, Lice and History, The Biography of a Bacillus. Little, Brown, and Co.

Boston. INTERNET SOURCES: http://nceas.ucsb.edu/~alroy/lefa/ http://www.ucmp.berkeley.edu/history/

Charles Darwin near the end of his life.

QUESTIONS TO THINK ABOUT 1. Why was the botfly infestation difficult to diagnose? 2. When was Darwin infected with Trypanosoma cruzi? Why does this seem likely? 3. How did Darwin spend his time in the 20 years after his return from the voyage of the

Beagle? 4. How did the Principles of Geology help Darwin to develop his evolutionary theories? 5. Distinguish between the evolutionary views of Lamarck, Cuvier, and Darwin. 6. How did the essay on population by Thomas Malthus help Darwin to develop Natural

Selection? 7. How may Blyth and Chambers have helped Darwin in the development of evolutionary

theory? 8. Hooker, Gray, Lyell, and Wallace all played prominent roles in the creation of The

Origin. What were they? 9. What types of evidence did Darwin draw upon and use in the presentation of his “long

argument”? 10. Darwin presented 5 concepts (Mayr called them 5 theories) of evolution in The Origin.

What were they?