Edward Bagnall Poulton's life and work spanned a period of immense historical and scientific change. The Crimean War was coming to an end as Poulton was born on January 27, 1856. Charles Robert Darwin's On the Origin of Species lay only three years in the future. Attending Oxford beginning in 1873, he studied under anatomist George Rolleston and eminent entomologist (and anti-Darwinian) John Obadiah Westwood. Three years later Poulton had earned an honors degree in zoology. During the closing years of the nineteenth century, Poulton lived through the zenith of British imperialism and the nadir for acceptance of natural selection as an evolutionary mechanism. Among biologists, little consensus existed on the role of natural selection. A few neo-Darwinians, led by August Weismann, accepted natural selection yet rejected most other mechanisms. Others, as did Darwin in later editions of the Origin, supplemented natural selection with other mechanisms. Many more, such as the American neo-Lamarckians, rejected natural selection outright in favor of inheritance of acquired characters and internal, directed mechanisms. Members of all these groups argued about whether evolutionary change was gradual or saltational. From the 1880s onward, although biologists agreed that organic evolution had taken place, they continued to argue about how and why.
After Westwood's death, Poulton in 1893 became the Hope professor of zoology. During his long tenure at Oxford, Poulton witnessed the rediscovery Mendelian genetics, the increased professionalization and specialization of biological sciences, the rise of population genetics and many significant changes in the biological sciences. The importance of experimental evidence as opposed to description continued to grow from the 1890s onward. The majority agreed upon evolution as such, yet argued over the appropriate methods for studying it. Poulton took part in the heated debates between paleontologists, field naturalists, geneticists and others. As Hitler was coming to power in Germany, Poulton retired from Oxford University. Having lived through the terrible destruction of the Great War, Poulton's final publication came in 1938, on the eve of the Second World War. As WWII raged, he died on November 20, 1943. Only a year prior, Julian Huxley had published his statement of the modern evolutionary synthesis. Between the Crimean and Second World War, the world had witnessed many violent changes. Similarly, between Darwin's Origin of Species (1859) and Huxley's Evolution: the Modern Synthesis (1942), evolutionary thought had seen tremendous changes. All of this had taken place during Poulton's lifetime.
With over 200 publications spanning nearly sixty years, Poulton's work (including five books, numerous addresses, articles, essays and letters) covers a wide range of topics from insect bionomics to biography, mimicry to morphology, geology to geographical distribution. Such an extended corpus provides an excellent opportunity for examining continuity and change in his thought. Many questions come to mind. How did Poulton react to the "eclipse of Darwinism" (i.e., rejection of natural selection) of the 1890s? How did his ideas change with the rise of Mendelian genetics? How did he react to the evolutionary synthesis? In short, how did Poultonís ideas develop from the early 1880s through the late 1930s? How did he respond to the wider shifts in biological thought? Addressing these questions is the primary goal of this paper. Despite all the changes, it will become apparent that Poulton's work contained a high degree of continuity.
Early Work: 1880-89
Poulton was, at heart, an entomologist. His work reflects a long-lasting interest in the study of insects. From nearly 140 works listed by entomologist George H. Carpenter, almost 70% deal directly with entomology. At the age of eighty-two Poulton reflected:
The most absorbing interest of my life, from boyhood, had been derived from the observation of insects in their natural surroundings and in the study of their life-histories and reactions to changing conditions, by means of breeding experiments.
Still, his earliest papers did not deal with insects; they dealt instead with geological and morphological studies.Geology was of little consequence to later work, so we begin with a brief overview of his morphological work. After Professor Rolleston's death in 1881 Poulton, under the new Linacre professor, Henry N. Moseley, obtained specimens from the Challenger expedition to New Zealand and Australia. This work focused on microscopical studies of the structures of marsupials and the duck-billed platypus such as taste-bulbs, hair, teeth and various tissues. These studies illustrated that Poulton believed in evolution. He described the papillae on the tongue of a specimen as "modified in a remarkable manner for the capture of small insects." He further explained that these taste bulbs had arisen from the modification and specialization of ordinary epithelial upgrowths. The primary mechanism for these modifications was natural selection. During the 1870s, Poulton had read Darwin and Wallace and apparently accepted natural selection. He was a selectionist, suspicious of Lamarckian mechanisms. In 1888, for instance, he commented that recent researches had minimized or perhaps disallowed altogether the importance of acquired characters in species construction.
Throughout his career, Poulton remained a proponent of natural selection and an opponent of the inheritance of acquired characters. He not only greatly admired Charles Darwin, but also defended the father of neo-Darwinism, August Weismann. One difficulty for natural selection during the 1880s and '90s was that the cause of individual variations was not understood. Natural selection was often attacked because it was not a true cause. Theories of heredity such as Darwin's pangenesis existed, but lacked empirical evidence or wide support. Poulton, responding to an article by George Romanes, remarked,
[Romanes] states (on page 841) that while Cope, Semper, Geddes, and Seebohm have argued "that any proof of natural selection as an operating principle opens up the more ultimate problem as to the causes of the variations on the occurrence of which this principle depends," Weismann and Poulton, on the other hand, "have not so much concerned themselves with this more ultimate problem."
Taking issue with this statement, Poulton responded that Weismann had addressed the problem of variation in recent work. Poulton noted that the Lamarckian view of "the direct influence of surroundings in causing variations" was probably mistaken. Over the next few years he defended Weismann's theory of variation, the continuity of the germ-plasm.
Poulton rejected Darwin's pangenesis in favor of Weismann's continuity of germ-plasm. Pangenesis stated that gemmules from every part of the body traveled to the sex cells during reproduction. These countless gemmules carried specific information about each part of the body and passed on parental characteristics to the offspring. Pangenesis, therefore, allowed for the inheritance of acquired characters. Poulton rejected pangenesis because of its "well-nigh insuperable" difficulties and its "almost infinite complexity." In addition, mutilations or blood transfusions, according to pangenesis, should pass characters to the offspring, yet there was no evidence from experiments by Weismann and Francis Galton that this ever occurred. Weismann's theory, on the other hand, divided heredity into somatogenic (i.e., acquired) and blastogenic (i.e., inherited) characters. Only blastogenic characters could be passed on in the germ-plasm (i.e., the sex cells). Poulton noted
The simplicity and beauty of Professor Weismann's theory of heredity commends it to our favourable attention, and demands a searching enquiry into the evidence for the supposed transmission of acquired of somatogenic characters.
If mutilations, results of training, exercise or education, or certain diseases could be shown to be inherited, then Weismann's theory would "inevitably collapse." Such direct evidences, however, failed to stand the test of thorough investigation. Not one convincing demonstration of the inheritance of acquired characteristics existed. Even so, Poulton did not totally omit the possibility of Lamarckian mechanisms. Phenomena such as individual variation, instinctual habits and use/disuse were not well understood. If any of these could be shown impossible without the transmission of acquired characters, then Poulton admitted that Weismann's theory would fail. Until shown otherwise, he would reject Lamarckism.
Poulton's entomological work from the 1880s connected with the evolutionary mechanism debates. Papers read at the Entomological Society and the Royal Society addressed issues such as the causes and changes in animal coloration, particularly in lepidopterous larvae and pupae.Taking Weismann's "The Origin of the Markings of Caterpillars" as a starting point, Poulton claimed in 1883 that natural selection took advantage of the internal organs' colors to produce markings on larvae. Through the action of natural selection resemblances gradually formed between larvae and surroundings. Protective coloration meant adaptive advantage, i.e., preventing the relatively defenseless larvae from being seen or touched. Because the larvae were especially liable to death, their means of protection was "almost always of a passive kind." When protective coloring failed, some gave off noxious odors or exhibited threatening attitudes. For Poulton, natural selection caused all of these adaptations.
Poulton also expressed his methodology in early works. In the field naturalist's tradition, Poulton stressed the importance of experiencing the true relations of the larvae and their environment. Description could never take the place of experience. Harmony between organism and environment was so subtle that protective resemblances remained inexplicable "without most careful observation of the perfectly normal and undisturbed larva on its food-plant." Even under ideal circumstances the "complex conditions. . . may not be understood in the least." Poulton clearly acknowledged the complexities of environmental relationships and the importance of observing living organisms in their surroundings.
During the 1880s, Poulton used natural selection as the primary mechanism of evolution and accepted the continuity of the germ-plasm as the most plausible theory of heredity. His work in the 1890s made even more outspoken claims about the efficacy of natural selection, strengthened the rejection of Lamarckian inheritance and laid increased stress on entomological evidence, especially with regard to insect coloration. In 1890, Poulton summarized and extended these studies in the book, The Colours of Animals.
The Next Decade: 1890-99
Animal Coloration and Butterfly Mimicry
The Colours of Animals: Their Meaning and Use, Especially Considered in the Case of Insects from 1890 was probably Poultonís most widely-known work. As the subtitle suggests, Poulton relied on entomology for most of his empirical evidence. Indeed, insects comprised the vast majority of his examples; most came from the order containing butterflies and moths, the Lepidoptera or "Scaled-Winged." The goal was clear_ to strengthen certain parts of Darwinís theory. Poulton remarked that all of his scientific work had been
inspired by one firm purpose- the desire to support, in however small a degree, and to illustrate by new examples, those great principles which we owe to the life and writings of Charles Darwin, and especially the pre-eminent principle of Natural Selection.
Colours of Animalsadhered to his plan by amassing examples of coloration. The surface colors of higher animals had developed such that "their amount, their arrangement in patterns, their varying tints, and their relation to the different parts of the body, have all been determined by natural selection through innumerable generations." Animal coloration reduced to the following classes:
1) Colors of Direct Physiological Value (e.g., dark colors absorb radiant heat)
2) Protective and Aggressive Resemblance (i.e., camouflaging of prey or predator)
3) Protective and Aggressive Mimicry (i.e., superficial resemblance of another animal to gain advantage)
4) Warning Colors (i.e., bright colored patterns advertising unpleasant attributes such as taste or smell)
5) Colors displayed in Courtship (i.e., colors modified by preference due to aesthetic sense in highest animals).
As Poulton's examples piled up, his message came across quite clearly. Natural selection (for classes 2-4) and sexual selection (for class 5) provided the best and most comprehensive explanation for an otherwise bewildering and inexplicable variety of phenomena. The strongest evidence, perhaps, came from the study of mimetic resemblance. Poulton, who published many papers on the subject, considered mimicry one of the most important aspects of animal coloration.
Early in the nineteenth century, resemblances between distant species had been recognized. For example, an entomology text from 1818 commented that insect imitation afforded "a beautiful instance of the wisdom of Providence." In another case "the Author of nature" had provided the likeness. Natural theology using the argument from design was the guiding principle explaining these mimetic likenesses. In 1861, after many years in the jungles of Brazil, English naturalist Henry Walter Bates proposed a new explanation. Bates claimed that certain palatable species of butterflies resembled other species which were conspicuously-patterned and unpalatable. The predators, usually birds, learned to recognize and avoid the warning pattern of the unpalatable model via experimental tasting. The mimic gained protective advantage because predators associated its similar warning pattern with an unpalatable model. How had the mimic come to resemble the model? Batesís answer was natural selection. Charles Darwin and naturalists such as A. R. Wallace enthusiastically embraced Batesian mimicry as evidence of natural selection in action. Batesís theory was quickly extended to other parts of the world in studies by Wallace (1866) and Roland Trimen (1870). Poulton made explicit the selective process in Colours of Animals:
Every step in the gradually increasing change of the mimicking in the direction of the [model], would have been an advantage in the struggle for existence, while the elements out of which the resemblance was built exist in the individual variability of the species, a variability which is hereditary.
The many known instances of mimicry, concluded Poulton, "strongly confirm[ed] the view that these resemblances [were] produced by the operation of natural selection." He also reiterated the conditions or "laws" of mimicry as given by A. R. Wallace:
1) Mimics inhabit the same area as model.
2) Mimics are always more defenseless than the model.
3) Mimics are always less numerous than the model.
4) Mimics differ from most of their allies.
5) Mimicry is external and visible only, not extending to internal characters.
Although naturalists like Darwin, Wallace, Trimen and Poulton accepted Batesian mimicry, many others questioned or rejected selectionist explanations. An immediate problem was Wallace's laws.
Mimicry Theory: Attack and Defense
It was noticed, even by Bates himself, that the conditions given by Wallace were not always true. The resemblance between species of bees and wasps, for example, did not require one to be more defenseless or less numerous than another. In 1879 German naturalist Fritz Müller proposed that these instances could, nonetheless, be explained by natural selection. Common warning patterns such as the black and yellow stripes of many bees and wasps gave the predators fewer patterns to learn, thereby reducing the amount of experimental tasting. Although not as quickly accepted as Batesian mimicry, Müllerian mimicry became another confirmation of natural selection. Many, however, continued questioning such explanations.
With the eclipse of Darwinism during the 1890s, few believed that natural selection was the main cause of adaptations. Rejection of selectionist explanations naturally extended to rejected of Batesian and Müllerian mimicry. In 1892, F. E. Beddard spelled out some of the difficulties with mimicry theory. He argued that with the multitude of insect species (e.g., 1500 species of Lepidoptera recorded in the Zoological Record for the year 1889 alone), purely accidental resemblances were much more likely. Other barriers to accepting selectionist mimicry involved the question of whether slight variations provided sufficient material for the action of natural selection, cases where mimicry was useless or even harmful and cases of mimicry between unprotected forms. Beddard asked how natural selection could address these problems?
Before accepting current mimicry theory, Beddard wanted more convincing proofs that selection was truly responsible. He suggested that perhaps the direct external effects of food, light or climate caused animal coloration, including mimicry. In a negative review of Beddardís book, Poulton confidently claimed:
The theory of natural selection has been pre-eminent for over thirty years as the most generally accepted explanation of organic evolution. It has, and has had throughout, many critics; but its position is strengthened by the fact that these critics invariably accept the principle as accounting for something, while most of them make it clear that they reject all other proposed substitutes, except [their own]
Poulton, overstating the widespread acceptance of natural selection, attacked Beddardís work. According to him, Beddard rejected natural selection for reasons "which will strike the instructed and uninstructed reader alike as singularly inadequate." Despite Poulton's review, Beddard was not alone in his wariness to accept natural selection or in his suggestion of external forces as the cause of mimicry.
Charles Coeís Nature versus Natural Selection (1895) took an even stronger anti-selectionist position. This book of nearly 600 pages was devoted entirely to arguments against natural selection. Coe's position was clear:
The author believes that the process of Organic Evolution has taken place, but he does not believe that Natural Selection has been the means by which that result has been brought about.
He criticized many, including Poulton, for accepting natural selection as an unquestioned law. For some, selection had become dogma rather than science. For Coe, as for Beddard, external forces were the cause. Species with similar climate, diet and living conditions were "transformed into the same outward resemblance by the same external conditions." External forces, not natural selection, were the key cause.
American paleontologist Edward Drinker Cope also doubted the efficacy of natural selection. Cope mentioned Wallace, Poulton and Beddard in his 1896 book, The Primary Factors of Organic Evolution. The first two authors, he noted, assigned natural selection as the cause of protective resemblances. Natural selection, however, could not cause variations in color. Cope, like Beddard, Coe and others, favored external physico-chemical forces such as moisture, cold or lighting as the cause. Cope also pointed out Poultonís experiments on lepidopterous larvae as evidence of the direct action of the influences of light on animal pigment.
In 1892, Poulton had compiled the results of several years of research. Like many others, he attempted to determine the effects of colored surroundings, lights and varied diets upon numerous species of Lepidoptera during different stages in their development. Poulton found that many larvae took on the colors of their surroundings (e.g., become darker when black twigs were mixed with their food, than when surrounded by green leaves and twigs). Furthermore, his experiments illustrated that color change only took place during specific developmental stages, operated within a limited color range and were not passed on to the offspring. Cope saw Poultonís experiments as clear evidence of the action of external forces. As a neo-Lamarckian he remained unconvinced that such changes were not heritable. Poulton, on the other hand, agreed that the changes were in the direction of concealment, yet noted:
I have failed to see in them what some others have believed they have seen, viz., the indications of some new power in the moulding of species, is because I have only been able to produce those changes which can be produced by a natural environment.
He concluded that this "marvellous power of individual adaptation to environment had been acquired through the ordinary processes of natural selection." The battle between natural selection and alternative mechanisms continued.
Theories of Evolution: Anti-Lamarck, Pro-Darwin
As others attributed protective resemblance, mimicry and color adaptation to external and internal forces, Poulton continued defending the explanatory power of natural selection. Early in 1894 he delivered an address entitled "Theories of Evolution." The Darwin-Wallace theory of natural selection was compared to the Lamarck-Spencer theory of acquired characters. Natural selection was based upon "undisputed, abundantly proved factors," i.e., individual variation, heredity and the struggle for existence. In contrast the Lamarckian-Spencerian theory depended on two factors i.e., acquired characters and the transmission of these characters, the latter of which was "by no means proved."
Poulton, after addressing the objections and misunderstandings related to natural selection, turned to the numerous difficulties of Lamarckism. He again explained the Weismannian distinction between blastogenic and somatogenic characters. Somatogenic (i.e., acquired characters) were produced by external forces, use and disuse, or the action of an organismís own forces. Still, there was no convincing experimental evidence that these were heritable. Natural selection could explain joint development, therefore, other causes were superfluous. Why assume two sets of mechanisms when one would suffice? For example, the development of an animalís joint explained by Lamarckian use and disuse violated Ockhamís razor because "no unnessary cause should ever be introduced into an explanation." Poulton frequently used this logical argument in conjunction with empirical evidence. Even with instinctual behaviors, which at first seemed strongly in favor of Lamarckian inheritance, Poulton concluded:
If the origin of wonderful and complex examples of instinct such as these cannot be explained by the Lamarckian theory but readily by the Darwinian, why should not natural selection also offer adequate explanation of all other cases?
In the closing years of the century, Poulton reiterated the triumphs of natural selection and the deficiencies of Lamarckianism through his addresses, historical essays and mimicry studies. During this period and into the twentieth century, Poulton met much opposition to his views.
At the Zoological Section of the British Associationís annual meeting in 1896, Poulton explained the conflicting estimates for the age of the earth given by physicists, geologists and biologists. As physicist John Perry's calculations on the heat conductivity of the terrestrial interior had shown, Lord Kelvinís estimate of 100 million years for a habitable earth could be considerably extended. The problem, of course, was allowing enough time for the gradual process of natural selection to take place. There were also uncertainties in the estimates based upon tidal retardation, solar age, denudation and thickness of geological strata. Paleontological and zoological knowledge, however, rendered more convincing evidence for Poulton and yielded much longer periods of time. His confidence in reconstructions of life's history on earth did not lie in their details, but in the general outlines. In his eyes, anti-selectionistsí arguments based upon uncertain assumptions of physicists or geologists were inadequate. Biological arguments were more compelling and allowed immenses periods of time for natural selection to operate.
In 1897 Poulton defended natural selection again at the annual meeting of the American Association for the Advancement of Science. Paleontologist H. Fairfied Osbornís alternative theory, also proposed by J. Mark Baldwin and C. Lloyd Morgan, was called organic selection. As Osborn explained, organic selection compromised Lamarckian and Darwinian theories:
While [organic selection] abandons the transmission of acquired characters, it places individual adaptation first, and fortuitous variation second, as Lamarckians have always contended, instead of placing survival conditioned by fortuitous variations first and foremost, as selectionists have contended.
Poulton responded that organic selection was not a true compromise because it did not really diminish the primacy of natural selection. Agreeing that the power of individual adaptation was extremely important, this power still had resulted from natural selection. Rejecting external and internal forces Poulton concluded, "The only remaining hypothesis is. . . the view that whenever organisms react adaptively under external forces they do so because of special powers conferred on them by natural selection." Osborn replied that, while selectionists brought every new evolutionary process under the "old comprehensive power of selection," he believed that plastic adaptation was a "fundamental property of protoplasm" not a result of natural selection. Osborn accepted directed forces inherent in protoplasm; Poulton did not. The organic selectionist and the natural selectionist reached an impasse.
Another way in which Poulton venerated natural selection was in historical essays about Charles Darwin and evolutionary thought. Several works took this approach including Charles Darwin and the Theory of Natural Selection (1896). In this book Poulton described the Origin of Species as "incomparably the greatest work" the biological sciences had seen. Critics of natural selection, Poulton contended, had not taken the time to understand it. Many writings by Poulton displayed an admiration verging on idolization of Darwin's work.
In 1909, the centenary year of Darwinís birth, Poulton wrote that the effect of the Origin of Species was "as though the sun had dispelled the mists that had long enshrouded some vast primeval continent." The same year, in a review of Darwinís critics, he remarked:
The chief results of those modern investigations. . . are now believed to be charged with menace for the Darwin-Wallace hypothesis. . .
No one of these, it is here maintained, can be reasonably held to make good the claims of the modern oponent of natural selection and evolution as conceived by Darwin.
Darwin, clearly the hero of evolutionary theory, remained an inspiration for Poulton's continuing work on butterfly mimicry. With the emergence of Hugo DeVries's mutation theory and the rediscovery of Mendelian genetics, debates about the evolution of mimicry slowly took a different direction.
Into the Twentieth Century: 1898-1915
Lamarckism to Mendelism: Butterfly Mimicry Continued
In March 1898, Poulton read a paper at the Linnean Society on mimetic resemblance and common warning colors. Batesian mimicry, he claimed, was stronger now and more widely held among students of other animal groups than among entomologists. During meetings of the Entomological Society of London the previous summer, Poulton discovered that many doubted selectionist mimicry. Of those entomologists he asked, only three excluding himself fully supported Batesian and Müllerian mimicry. The others doubted the inedibility/unpalatability of the models (some investigators even performed taste tests!) or were not convinced that birds had been proven effective selective agents. The theories of Bates and Müller required more experiment and observation. External and internal forces remained popular alternatives to natural selection.
Poulton, seemingly in the minority, constantly appealed to the broad explanatory powers of natural selection. Only under selection, he wrote, did the various resemblances of animals "fall together into a natural arrangement and receive a common explanation." Alternative theories were narrower in explanatory power or simply did not work. The theory of external influences, for instance, required the transmission of acquired characters, yet there was no evidence for this hypothesis. Other theories simply did not explain the vast amount of facts which natural selection made intelligible. This, said Poulton, "increases our confidence in the theories of Bates and Müller, while it disposes of all alternative hypotheses." Mimicry, in fact, was "one of the most notable triumphs ever won by the great theory of natural selection."
With his neo-Darwinian stance, Poulton continued meeting resistance. In 1899 botanist George Henslow took exception to his views. He concluded that selection was not a physical cause, therefore, its powers were imaginary. In a mocking tone, Henslow remarked that natural selection seemed:
. . . not unlike the wand of a fairy, which could evolve a coach and six out of a pumpkin and mice. But, in fact, all that it can tell us is, that some beings live and others die. It can neither account for the physical cause of useful variation in one organism, nor say why another organism dies. It only, so to say, registers the fact.
In his opinion, external conditions coupled with "inward adaptive powers of protoplasm" and "habits of life" caused mimetic resemblance. Henslow and numerous others relied on external and internal forces as the vera causa of evolutionary change. Henslow concluded that selection had "no raison d'être in the origin of any structure whatever."
Despite Poulton's claims that natural selection had been widely accepted, it is evident from Henslow's statements and that Lamarckian explanations had not disappeared. Poulton's repeated defenses indicate that many opponents still existed. His address to the Fifth International Zoological Congress in 1901 clearly illustrates this point. This speech evaluated specific instances of mimicry according to the rival theories, i.e., natural selection, external forces and internal forces. In cases where certain species mimicked features of distantly-related aggressive species, Poulton systematically ruled out alternative theories. He concluded:
There remains Natural Selection, which at once offers a convincing interpretation. Ants and wasps are known to be aggressive dominant insects avoided by the majority of insect-eating animals. . . It is in every way probable that a superficial resemblance to ants and wasps would be beneficial in the struggle for existence. There is, indeed, some experimental evidence to prove that real advantage is conferred. . .The results are exactly what might have been predicted. . . if Natural Selection be the efficient cause of Mimetic Resemblance.
Experimental and observational evidence alleviating doubts about bird predation was also slowly emerging. British entomologist Guy A. K. Marshall, for example had directly observed attacks on butterflies by birds and lizards. Marshall and others also discovered the wings of many specimens notched as if attacked by birds.Experiments by Lloyd Morgan illustrated that birds, rather than having inborn instincts, learned to avoid unpalatable models. Nonetheless, rival views and objections persisted. Some of them strengthened with the emergence of Mendelian genetics.
Mimicry, Mutation and Mendelism
Many biologists in the late nineteenth and early twentieth centuries questioned a mimicry theory which relied on natural selection and the accumulation of minute variations. Questions arose including: How could minute variations confer advantage and accumulate? Was there convincing evidence of bird predation to indicate selection pressure? Could tiny variations persist and alter an entire population? How were tiny individual variations passed from generation to generation? These and other questions remained problems not just for mimicry, but for natural selection in general. Some investigators including neo-Lamarckians, Mendelians and even selectionists saw part of the solution in saltational explanations. For example, American biologists David Starr Jordan and Vernon Kellogg believed in natural selection, yet wondered why small initial variations would confer advantage to the mimic. It seemed implausible that a tiny spot of color on a butterflyís wing would make a bird mistake it for another species. They argued that the leap must be saltational rather than gradual. Poulton, on the other hand, insisted that gradual accumulation of minute variations, through natural selection, was the key.
By the 1890s, Cambridge biologist William Bateson had latched on to the discontinuous mutation theory of Dutch botanist Hugo DeVries. De Vries, breeding evening primroses, discovered that these flowers suddenly changed or mutated from one generation to the next. With the rediscovery of Mendelís work (c 1900) by Bateson, DeVries and others, the discrete character of Mendelian factors held special interest for investigations which focused on discontinuity. Geneticists turned to saltational or mutational explanations for mimicry and evolutionary change in general. In 1908, Poulton described his disagreements with Bateson. He was extremely dismayed by Batesonís unfounded attacks on Darwin and natural selection. Mutationism and Mendelism, he explained, had been injurious to biological science for the following reasons:
1. The amount of dogmatism concerning work with which [Bateson] is evidently imperfectly acquainted.
2. The assumptions made by Mutationists on the slenderest evidence.
3. The appropriation under the name of Mendel of results which the present generation owes to Weismann.
4. The exaggerated estimate of the importance for Evolution of, first, Batesonís work on Variation, secondly Mendelís interesting discovery.
5. The contemptuous depreciation of other lines of investigation directly inspired by the work and teaching of Darwin and Wallace.
6. The natural consequence of this last: _ a widespread belief among the ill-informed that the teachings of the founders of modern biology are abandoned.
Consistent with his earlier work, Poulton remained a defender of the work of Darwin, Wallace and Weismann. By de-emphasizing the importance of natural selection, Bateson and the "ill-informed" had obviously erred. Mutations still needed the action of natural selection to be of any consequence; otherwise, mutation theory, said Poulton, merely revived special creation under another name. Referring to Bateson he wrote, "error is a not unnatural outcome of the depreciatory and contemptuous tone adopted by the leader of Mendelism in this country." When it came to the relative importance of Mendelian and Darwinian ideas, Poulton and Bateson were poles apart. For Poulton, although Mendelian research deserved the "warmest appreciation," it was of negligible importance compared to natural selection. Bateson accepted natural selection, but regarded it as self-evident and not very interesting. Of real importance were Mendelian genetics.
Yet again, Poulton explicitly rejected mutational jumps when it came to mimicry. He wrote:
It is as unlikely that a key could be made to fit a complicated lock by a number of chance blows upon a blank piece of metal, as that the elaborated pattern on the wings of a butterfly should have been produced on those of its mimic by Mutation.
This stance brought him in direct conflict with Bateson and one of Bateson's students at Cambridge, Reginald Crundall Punnett. The debate between Poulton and Punnett illustrates the wide gulf which separated natural selectionists from geneticists in the early twentieth century.
Poulton versus Punnett
Between 1910 and 1915, Reginald Crundall Punnett fully elaborated a Mendelian explanation of butterfly mimicry. In 1910, his paper on polymorphic mimetic butterflies of Ceylon (e.g., Papilio polytes) explained the many barriers to accepting natural selection as the formative cause of mimicry. According to Punnett, the selective value of minute variations and their inheritance still lacked experimental proofs. The absence of transitional forms and the frequent lack of mimicry in male butterflies were also troubling for selectionist theory. Finally, was the enigma of polymorphic mimicry. Some species of butterfly mimicked not merely one, but several models. In breeding experiments these polymorphs cleanly segregated according to Mendelís law of segregation. For Punnett, none these observations were explained by gradual selectionism. Instead the evidence suggested that mimicry had arisen from sudden mutational jumps. Once a mimic was formed by mutation, natural selection played only a conservative role. Selection was not the formative cause of mimicry. "Unknown conditions," noted Punnett, had brought about similar mutations in widely disparate species of butterflies which already contained similar sets of pattern factors. He compared this to similar mutant coat colors such as chocolate or agouti which appeared in both guinea pigs and rabbits.
Between 1912 and 1914, Poulton and Punnett debated mimicry in a series of articles which appeared in a short-lived journal for scientific controversy called Bedrock. Poulton repeated the unreasonableness of supposing all mimetic features arose simultaneously in one saltation. For a particular butterfly he said:
It is probable that by spontaneous variation a white band appeared in the ancestral form. . . this was from the very first sufficient to confer some advantage by suggesting the appearance of a dominant Model. . . From this point Natural Selection acting on further variations produced the detailed likeness which we see in the white band itself and in the other mimetic features.
Poulton concluded that "the hypothesis that explains mimetic resemblance explains evolution." This clearly indicates his continued devotion to natural selection and its intimate connections with mimicry.
Although both Poulton and Punnett accepted Mendelism and natural selection, they assigned distinct roles to each. They had quite different understandings of these concepts. Confirmed by breeding experiments, Poulton was perfectly willing to accept Mendelian ideas such as dominant and recessive factors. However, as reflected in the title of his next paper, "Mimicry, Mutation and Mendelism," Poulton downplayed the importance of Mendelism. Mimicry formed by natural selection was the top priority and the sudden, complete arrival of mimetic features was an "astounding hypothesis." Under a lens Poulton had seen the hard, sharp patterns of models contrasted with the soft, transitional patterns of the mimics. The scales, colors and patterns all differed. Such approximate likeness contradicted the view holding that model and mimic originated from identical sets of pattern factors. Poulton repeated that mimicry was "the result of a series of transistional steps, some of which are still preserved." Punnettís response, appropriately titled "Mendelism, Mutation and Mimicry," illustrated that his top priority. Numerous facts did not support natural selection as the cause of mimicry. Instead there was "strong evidence for supposing evolution to have proceeded by definite mutational stages."
Over the next year, the debate intensified. Late in 1913, Poulton remarked that Bateson and Punnett had been "dazzled and confused" by Mendelism and had "lost all perspective and all sense of proportion." Such irresponsible and "distorted vision" was unfortunate for English biological science. Although Poulton agreed that the initial variation must be appreciable, he refused to admit the saltational appearance of complete mimetic patterns. Punnettís response claimed that Poulton was stirring up prejudice against Mendelian views. His difficulties had not been addressed and such silence suggested to Punnett that Poulton had no answers.
While natural selection explained almost all mimicry for Poulton, Mendelian genetics generally satisfied Punnett. Poulton emphasized the existence of transitional forms distributed over the range of mimic and model, while Punnett denied true transitional forms and the selective value of minute variations. As Poulton believed that natural selection was the cause of a slow, gradual development of mimetic resemblance, Punnett saw mutation as the key to a quick, sudden development of mimicry. Poulton relegated Mendelian genetics to a subsidiary position. Punnett allowed natural selection only a conservative role. The evolution of mimetic resemblance was necessarily adaptive for Poulton, but not for Punnett. Their differences seemed irreconcilable.
In April 1914 , Poulton finally responded to Punnettís seven criticisms. In doing so, he moved toward accepting Mendelian studies of a different kind. Agreeing with the American geneticist William Castle, Poulton quoted from Castleís book:
Occasionally a unit-character is lost altogether or profoundly modified at a single step. This is mutation. But more frequent and more important, probably, are slight, scarcely noticeable modifications of unit-characters that afford a basis for slow alteration of the race by selection.
With this, Poulton accepted an emerging version of genetics which stressed the small, discrete changes rather than the large mutational ones.
Mimicry in Butterflies
In 1915 The debate between Poulton and Punnett culminated with Punnettís Mimicry in Butterflies (1915). Using mimicry as an example, the book challenged the supposed explanatory power of natural selection. As John Ray had appealed to the "Omnipotency of the Deity," Punnett said that natural selectionists had asserted the "Omnipotency of Natural Selection." Questioning their confidence he asked: "Will natural selection really serve to explain all? Must all the various characters of plants and animals be supposed to owe their existence to the gradual operation of this factor working upon small variations?" Punnettís analysis of mimicry concluded that these questions must be answered in the negative. Of the objections made in earlier exchanges with Poulton, two stood above the rest: 1) evidence of birds as selective agents was still slight and little was known of birds' discriminatory powers and 2) the theoretical process which required the gradual accumulation of minute variations did not fit with the facts of heredity. Given the evidence, natural selection was an unsatisfactory hypothesis in causing mimetic resemblance.
Poultonís review of Mimicry in Butterflies concentrated almost exclusively on Punnett's mislabelling of illustrations. Poulton implied that if Punnett could not identify butterflies correctly, why trust him in theoretical matters? Although Poulton sidestepped the difficulties of mimicry by natural selection, American reviewer John Gerould emphatically agreed with Punnett that natural selection and mimicry had many problems. In fact, Gerould claimed that the widespread skepticism of natural selection inclined many to "bury mimicry in the same grave with teleogony, prenatal influences, the inheritance of acquired somatic characters, and sexual selection." He identified Poulton as the leader of the Oxford school of zoologists with "an orthodox faith in the potency of natural selection." Mimicry, for Gerould, would sooner or later be consigned to its last resting place. Despite this bleak outlook, selectionist mimicry theory did not die.
In the 1920s, the work of William Castle, T. H. Morgan and R. A. Fisher narrowed the gap between geneticists and natural selectionists. Eventually, it made genetics more plausible to the latter and the accumulation of small variations more palatable to former. Although views relying on continuity and discontinuity were merging, the merger was neither quick nor simple. Neo-Darwinian, Mendelian and neo-Lamarckian views all persisted, competing and sometimes intermingling. Increasingly, biologists adopted and adapted ideas from nascent fields, such as population genetics, often associated with the modern evolutionary synthesis of the 1930s and 1940s. With these developments in mind, we turn to Poultonís work in the decades before the synthesis and examine how he responded to the ongoing changes in biological thought.
The Twentieth Century: 1916-1938
Poultonís entomological research did not stop once he reached sixty years old. In fact, he published at least twenty papers related to mimicry during the 1920s. Relying upon bio-geographical data gathered from his students and entomologists around the world, Poulton continued opposing mutational and Lamarckian mechanisms and supporting natural selection throughout his latter productive years. In 1916 he commented, "The inadequacy of the hypothesis that mimetic resemblance arose at a single bound can be shown by innumerable instances of such likenessess between insects of different orders." Upon winning the Linnean Societyís Gold Medal for his contributions to biological science in 1922, the presenter called Poulton "a consistent upholder of the Darwinian position with regard to natural selection as the dominant factor in evolution." Furthermore, because he rejected speculations such as Lamarckism and pangenesis, Poulton was perhaps even "more Darwinian than Darwin himself." A decade later, Nature stated that he was "known everywhere as the most prominent living upholder of the doctrine of natural selection."
Poulton, while refining and defending his mimicry theory, clearly recognized the significance of developments taking place in biology. Not surprisingly, he saw these recent changes as triumphs for natural selection. In 1931, Poulton addressed the zoological section of the British Association in a speech entitled, "A Hundred Years of Evolution." Since 1881, when he first attended the British Association, the battles raged over Lamarckism and natural selection, and especially heredity. According to Poulton, the Ďre-discoveryí of Mendelís work brought much controversy in the 1910s and 1920s because of the belief that "only large variations_ or as they were called, Ďmutationsí. . . are subject to Mendelian inheritance and. . . small variations are not inherited at all." Finally, this controversy ended with the work of Castle and Morgan who had proven that,
the smallest characters are hereditary, so that Ďthe objections raised by the mutationists to gradual change through selection are breaking down as a result of the thoroughness of the mutationistsí own studies.í To give a single illustration_ between a red-eyed and a white-eyed fruit-fly (Drosophila) seven gradations of colour intervene, each of them Ďheritable in the normal Mendelian manner.í
Ronald A. Fisherís recent book, The Genetical Theory of Natural Selection (1930) further strengthened the compatibility of natural selection and genetics. For Poulton, recent Mendelian research had vindicated the germinal qualities of Weismann and the natural selection of Darwin. Other victories for natural selection included the abandonment of the Thomson-Tait time limitation (i.e.,10-100 million years) with the discovery of radioactivity and emerging palaeontological evidence supporting continuous evolution.
Not surprisingly, Poulton devoted more than half of his address to mimicry. The explanation for mimetic resemblance, as it had been for more than half a century, was accumulation of small variations as caused by natural selection. Cases where the mimetic forms were more abundant than the model had made Poulton wonder whether selection in terms of survival of the fittest was always the cause of mimetic resemblance. In these cases, the
. . .absence of the model finally leads to the disappearance of the mimetic pattern, Ďalthough the species that bore it remains as abundant as before. The survival or extinction of the species is not affected: all that has happened is the survival or extinction of a pattern borne by a certain proportion of the individuals of the species. When these disappear, other individuals with another pattern take their place.
This solution, which Julian Huxley named intraspecific selection, reaffirmed the grand explanatory power of natural selection. Poulton addressed several more common criticisms of mimicry theory focussing on recent research. Research by G. D. Hale Carpenter, Frank Morton Jones, Guy Marshall, C. F. M. Swynnerton and W. A. Lamborn provided strengthened evidence of that models were unpalatable and that birds did commonly attacked butterflies. Alternative theories and criticisms of selection simply did not stand up to the mounting evidence. Natural selection still remained the best and broadest explanation for mimicry.
Poulton restated his views in several later addresses and papers. In 1937, he read the presidential address to the British Association at the age of 81. Poulton reviewed the history of evolutionary thought as recorded in the meetings of the British Association. The recent work of J. B. S. Haldane, R. A. Fisher and Julian Huxley was vitally important for showing the relationships between Mendelism and natural selection. The observations and experiments of Marshall, Lamborn, Carpenter, Jones, St. Aubyn Rogers, V. G. L. Someren, H. B. Cott, R. Carrick and others had "immensely strengthened and confirmed" the researches on mimicry and warning colors of pioneers like Bates, Wallace, Meldola, Trimen and Müller.
Amongst all the changes in biological thought, Poultonís views remained remarkably consistent for more than half a century. From the early 1880s through the late 1930s he used morphological studies, larvae experiments on color, adaptive insect coloration and, most importantly, mimetic resemblances as evidence pointing towards the truth of natural selection. Throughout the changing fortunes of Darwin's ideas, the disputes and battles against theories of external forces, internal forces, neo-Lamarckism and Mendelism, and the comeback and eventual acceptance of natural selection in the modern evolutionary synthesis, Poulton constantly pushed mimicry as an exemplar of natural selection in action. Darwin, Wallace and Weismann remained the heroes by which Poulton compared all other biologists. His biological thought illustrates remarkable continuity in a period of immense scientific changes. Poulton may have altered his ideas slightly or accepted new evidence or new methods outside his realm, but at heart he remained a naturalist and an entomologist devoted to the cause of mimicry and natural selection.