The Heretic in Darwin’s Court: The Life of Alfred Russel Wallace

WALLACE’S CELEBRATED OPPONENT William Benjamin Carpenter died in March 1885 after suffering severe burns from an explosion in his laboratory, but his place as major adversary was soon taken by George John Romanes. The battle between Wallace and Romanes arose on a different front. Romanes aspired to inherit Darwin’s crown, a position that Wallace had all but abandoned but clung to with surprising tenacity when he felt it was threatened. Since Darwin’s death in 1882, the theory of natural selection had once again come under assault, this time by evolutionists themselves, many of whom remained unconvinced that it was the primary force driving organic evolution. Although Wallace had shifted his focus from natural history to political theory after the publication of Island Life, he was drawn back into the conflict by Romanes and other scientists, who presented challenges he could not resist addressing.

Romanes, a lesser figure in the history of evolutionary theory, was born in Kingston, Ontario, in 1848, the year that Wallace and Bates set out for Brazil. After inheriting a considerable fortune, the family moved to London, where the four children lived in the fashion typical of the British upper classes. Romanes attended Cambridge, intending to become an Anglican minister, but after discovering the sciences he decided to devote his life to scientific research. In 1873, following the death of his father, he returned to London to live with his mother and transferred to University College, London, where he continued his experimental studies in the physiology laboratory of William Sharpey and John Burdon Sanderson. A letter submitted to the editor of Nature in June of that year, containing a somewhat abstruse discussion of the origin of coloration in an unusual specimen of a flounder at the London Aquarium, attracted the attention of Darwin, who no doubt perceived in Romanes a potential protégé whose work in experimental biology was worth encouraging.¹ Soon afterward, Romanes developed a close, almost sycophantic relationship with his mentor, which lasted until Darwin’s death. Not long after his first contact with Darwin, he discovered the rudiments of a central nervous system in jellyfish (medusae), demonstrating that, even in this primitive form, nervous tissue is fundamentally the same throughout the animal kingdom, an important confirmation of the theory of common descent. He also wrote extensively on animal psychology and the mental evolution of the human species. Darwin helped him get elected to the Royal Society in 1879, though he had twice been denied admission. Egotistical and aggressive, Romanes rarely hesitated to publish his personal views in the form of verbose and pompous letters to the editor of Nature, which tried the patience of friends and foes alike. Although his wife later wrote that he was “absolutely incapable of anything but fairness,” he could be pugnacious and indecorous.² It was this aspect of his personality that provoked clashes with Wallace.

Wallace reports in his autobiography that he first met Romanes in 1880, but he seems to have forgotten that they had exchanged a volley of letters on animal instinct in 1873. Romanes had accused Wallace of neglecting the significance of a “sense of direction” (homing instinct) in some animals. Wallace peremptorily replied that he would make no further remarks on the subject because he believed that “nothing satisfactory can be arrived at till experiments of the nature indicated in my last letter have been systematically carried out.”³ If Wallace thought nothing more of that episode, the twenty-five-year-old unknown and aspiring scientist would have remembered it clearly. They first met in person seven years later, in a “curious way,” as Wallace described it. A letter by M (the biologist St. George Mivart) in Nature in February 1880—which theorized that thought, or “brain vibrations,” could be conveyed across space to other brains, thus explaining the phenomena of clairvoyance and mesmerism—was answered by FRS, an interested skeptic. FRS asked for assistance from anyone who might help him to investigate thoroughly these so-called facts of psychic phenomena. He added that he had taken a good deal of trouble to investigate spiritualism and, while encountering a vast amount of “humbug,” had also met with one or two things that he could not explain. He would “prosecute” his researches without bias or prejudice and would declare M’s beliefs as facts if these facts were reproducible. He depicted himself as a “man of science” ready to take on “the powers of darkness”—as though his battle would end all battles. Wallace replied, reminding FRS that this ground had been covered many times before and warning that no amount of evidence would ever satisfy inveterate skeptics.⁴

A few days later, FRS wrote privately to Wallace, revealing himself as Romanes. In the letter, which was deferential and polite, he admitted to having been “staggered” by one or two facts that Wallace might find commonplace. Since the two men were not formally acquainted, he had been reluctant to make an unsolicited approach to discuss the subject, but now an opportunity had arisen. He was not as cynical as Wallace had become about the importance of his and Sir William Crookes’s inquiries. “[Pray] do not suppose that I am blind to the importance of the testimony already accumulated,” he wrote. “I should rather infer it is you who are blind to that importance; I think you underrate the impression which your own publications and that of your few scientific co-operators have produced. I know that this impression is in many minds profound.” Wallace consented to a meeting, which prompted Romanes to ask for an appointment. His chief end was to discover what the “prophets” Crookes, Wallace, and others had already discovered. Romanes and Wallace met twice, first at Croydon in late March or early April 1880, and later in London. Romanes told Wallace that a close member of his family was a medium, and that the usual rapping messages had been spelled out—some true, some not, but all believed to have been produced by some other “intelligence” and not by any of the participants. Wallace gave Romanes his best advice but heard nothing more from him, assuming that he had abandoned his investigations after they failed to produce the evidence he sought.⁵

But Romanes was not interested in further investigations. Instead, he reported to Darwin on his visit with Wallace, like a spy who had made a successful incursion into enemy territory:

Six years later, in July 1886, Romanes published an article in the Journal of the Linnean Society entitled “Physiological Selection: An Additional Suggestion on the Origin of Species,” with which Wallace vehemently disagreed. The few responses to Romanes’s argument were too weak, Wallace told his friend Raphael Meldola, so he would write one of his own “to expose the grezt presumption and ignorance … in declaring that Natfural] Selection] is nota theory of the origin of species,—as it is calculated to do much harm. See for instance the way the Duke of Argyll jumped at it like a trout at a fly!” An editorial in the Times on August 16 called Romanes “the biological investigator upon whom in England the mantle of Mr. Darwin has most conspicuously descended,” which appalled Wallace. Moreover, the anonymous commentator had remarked that if physiological selection was accepted as valid by evolutionists, Romanes’s paper would constitute “the most important addition made to the theory of evolution since the publication of ‘The Origin of Species.’” As the greatest living exponent of the theory of natural selection in its pure form, Wallace felt that he was the only man who could come to the theory’s defense. Romanes was posing as Darwin’s successor. “This should be stopped before the press and the public finally adopt him as such,” he told Meldola.⁷

Wallace’s unusually heated remarks were no doubt a reaction to what he perceived as a veiled personal attack. He responded in the Fortnightly Review, in “Romanes versus Darwin: An Episode in the History of the Evolution Theory,” published shortly after his departure for the United States. His review was intended to destroy Romanes’s objections at their roots before they could invade the garden he had so lovingly cultivated.

Romanes’s theory of physiological selection sought to solve a problem that Wallace had pointed out to Darwin in 1868: how varieties developed into species without a physical barrier separating the incipient species from its parental form. At the time, Wallace had suggested that physiological isolating mechanisms that prevented hybridization between a variety and its parental species evolved through natural selection; that is, the hybrids would have proved less successful in the struggle for survival than the pure offspring of either the variety or the parent species, and thus a variety sharing the same range as its parental form might evolve into a separate species. Darwin could find no evidence that natural selection had anything to do with what he called this disinclination to cross, but he had no other explanation for the phenomenon except to state that it was a by-product of evolutionary change. The failure to explain the disinclination to interbreed was a potential weakness in their theory, Wallace had warned at the time, one that their enemies would exploit. Now, some twenty years later, that enemy had arrived in the guise of Romanes.

According to Wallace’s understanding of Romanes’s theory—and there would be considerable confusion among naturalists over the years about what Romanes was trying to say⁸—”physiological variations” occurred first and were the actual starting point for the formation of new species. Natural selection therefore would not be a theory of the origin of species because it would not account for what Romanes maintained was the primary characteristic feature of species: the almost invariable infertility when two allied species were crossed. Moreover, individual species were not distinguished from one another solely by distinctive adaptations to the environment but by trivial, superficial, and altogether useless characteristics. To account for these characteristics, Romanes said, Darwin and his followers had suggested additional causes, such as sexual selection, Lamarckian use and disuse, and, most important, the prevention of intercrossing with parent forms. Prevention of intercrossing would occur when varieties were geographically isolated from the parental species. During that isolation, independent variability would be sufficient for the evolution of new species, for there would be an inherent tendency to develop variations, which would become fixed and be passed on to succeeding generations of animals. But, Romanes argued, most species must have originated in the midst of the parental form (sympatric speciation). Only in extreme cases, as in the Galápagos Islands, did speciation occur as a result of geographic isolation. Unless mutual infertility appeared at the outset, an incipient species would become extinct through intercrossing (“swamping effect”).

After summarizing Romanes’s theory, Wallace proceeded to deconstruct the three “great” objections to natural selection: the evolution of characteristics apparently unrelated to survival, the evolution of varieties into species even when they occupied the same range as the parent species, and the “disinclination to interbreed” among species in the wild. Wallace pointed out that an enormous number of specific characteristics that seemed useless to the observer often were not useless at all: “[T]his argument from our ignorance is a very bad one when we consider how recently whole groups of specific differences, formerly looked upon as useless, have been brought under the law of utility.” Protective coloration was one example, and he provided ample evidence to prove that trivial characters distinguishing one species from another both were useful and had been “fixed” by natural selection: “I believe, therefore, that the alleged ‘inutility of specific characters’ claimed by Mr. Romanes as one of the foundations of his new theory, has no other foundation than our extreme ignorance … of the habits and life-histories of the several allied species, the use of whose minute but numerous differential characters we are therefore unable to comprehend.”

As for the second objection, Wallace quoted Darwin, who believed in microgeographic isolation. According to Darwin, each newly formed variety is at first local, separated from the rest of the species. Similarly modified individuals exist in small local populations and interbreed. If this new variety is successful in the battle of life, it will slowly spread from a central habitat, competing with and conquering the unchanged parent species on the margins of the expanding habitat. Romanes challenged this “very large” assumption: that the same variation occurs simultaneously in a number of individuals inhabiting the same area. Wallace insisted “that which Mr. Romanes regards as ‘a very large assumption’ is … a very general fact, and, at the present time, one of the best-established facts in natural history.” There was considerable variability within a population around a mean, as measurements of body parts and examination of color and markings in a number of animal species had shown. Species could roughly be divided into two groups, with significant divergence from the mean condition:

Finally, Wallace dispatched the third objection, Romanes’s assumption that it was almost a universal rule for natural species to be incapable of interbreeding. Wallace cited the experiments of the botanist Dean Herbert, who had produced hybrids from a number of distinct genera, and these hybrids could be propagated indefinitely. “The popular ideas as to the sterility of hybrids are derived from crosses between certain domestic animals by no means closely allied,” Wallace pointed out, “such as the horse and ass, the canary and goldfinch, or the domestic fowl and the pheasant.” It was difficult to cross close species of animals under circumstances approximating natural conditions—it was even difficult to cross two members of the same species in zoos—though he knew of animals classed in distinct families, like the pheasant and the grouse, producing hybrids in a state of nature. “[This] mere fact... is in itself an argument against there being any constant infertility between the most closely allied species, since if that were the case we should expect the infertility to increase steadily with remoteness of descent till when we came to family distinctions absolute sterility should be invariable.”

While admitting that variations in fertility were highly probable, Wallace noted, “Mr. Romanes speaks of this physiological variation as if it were a simple instead of a highly complex form of variation, and as if it might occur sporadically within the limits of a species like some change of colour or modification of form.” Physiological selection necessitated large leaps, some sort of major mutation, which violated the maxim that Natura non facit saltum (Nature does not make leaps), the uniformitarian principle underlying the Darwin–Wallace conception of the evolutionary process. If one individual, or a dozen individuals, within a large species was somehow incapable of breeding with the bulk of the members of its species, the chances of that individual mating with a “physiological complement” of the opposite sex without some other distinguishing characteristic to identify the potential mate was highly improbable. Even if such a mating took place, the variation would be a rare occurrence and would not be preserved by natural selection:

In a letter to Nature on September 2, Francis Darwin corroborated Wallace’s remarks. He had evidence besides allusions in The Origin of Species that his father had been “familiar with the principle of physiological selection and, moreover, did not regard it with any great favour.” In the same journal, Romanes reproached Francis Galton and Raphael Meldola for having made similar objections. Stung by Wallace’s review, comments by Gal-ton, Meldola, and Francis Darwin, plus other critiques of his theory that were “flowing through channels other than the pages of Nature,” Romanes defended his paper, claiming that it was merely a preliminary statement of principles that he hoped would stimulate naturalists to cooperate with him in investigating and verifying his theory (though the “preliminary” statement had taken up seventy-five pages of the Linnean Society’s journal). But no one seemed interested in verifying this theory, only in refuting it. After allowing these critiques to “exhaust themselves,” Romanes published a lengthy rebuttal in the January 1887 issue of Nineteenth Century. Summarizing the main points of this rebuttal in a letter to Nature on January 13, he singled out Wallace, referring to his criticisms as “feeble.” Moreover, Wallace and other critics had misunderstood him. He was dumbfounded, he said, that sentences that were straightforward and good examples of plain English could have confused such competent readers as Wallace. Here Romanes was simply wrong; one might easily lose one’s way amid his long-winded and convoluted rhetorical style.

Wallace, who was growing tired of Romanes’s tirades, replied to his letters in Nature from the United States. Physiological selection was a weak and unsatisfactory theory, he said, and he now left the question to the consideration of those naturalists who had the time and inclination to study the problem.¹⁰ Reacting to Wallace’s and other evolutionists’ unrelenting criticisms, Romanes backpedaled. Physiological selection, he said, stood in the same relation to natural selection as did sexual selection: it was a supplementary theory, referring to only nonadaptive characters. Altering his tactics, he accused Wallace of ostracizing him from the community of evolutionists as an “arrogant heretic” who rebelled against the “highest authority.” He claimed that there were authorities elsewhere in Europe who supported his theory. He agreed with Wallace that the only way to prove or refute his theory was through further experimentation. It would take him at least another three years to amass such evidence. “[It] will be a very long time before I shall have occasion to trouble [my critics] with the theory of physiological selection,” he concluded.¹¹

After returning from the United States in August 1887, Wallace spent the rest of the year tending his garden and wading through the numerous letters and piles of journals and books that had accumulated during his absence often months. Travel by rail across a vast continent had exhausted him, and he firmly resolved to lead a quiet country life. His friend Edward Bagnall (E. B.) Poulton, a professor of zoology at Oxford University, invited him to give a series of lectures in Oxford’s university extension series, but Wallace preferred to lecture occasionally and then only to local scientific and literary societies.¹²

But he had not tired of writing. The many letters from Americans praising his “Darwinism” lectures, as well as the increasing number of attacks on the theory of natural selection, inspired him to write a popular book on the subject. He decided to “refashion” Darwin’s Origin of Species, which many found difficult to understand, in order to shed light on those areas most commonly misunderstood by both scientists and the public. A defense of natural selection, he believed, was long overdue.¹³

Wallace spent much of 1888 putting together his book, which he intended to call “Popular Sketch of Darwinism” but whose title he later shortened to Darwinism.¹⁴ The theory of natural selection alone explained the origin of species, he wrote. The publication in 1885 of August Weismann’s theory on the “continuity of the germ-plasm”—or “panmixia,” as Weismann called it—was a boon to Wallace, for it removed one of the major obstacles against the theory of natural selection by proving that acquired characteristics could not be inherited. In his youth, Weismann had been an enthusiastic naturalist who studied medicine in Frankfurt before switching to laboratory work. A serious eye disease, however, forced him to give up the microscope and devote himself to theoretical work, which eventually laid the foundation for modern genetics. Like Wallace, he was almost a “pure” natural selectionist and rejected the Lamarckian idea of the inheritance of acquired characters.¹⁵

Before Weismann enunciated his theory, no one had developed a satisfactory hypothesis to explain the phenomenon of heredity. Most naturalists believed in “blending inheritance,” but the mechanism remained a mystery. Darwin thought that he had hit on a solution and called his theory “pangenesis,” which he published in 1868 as The Variation of Animals and Plants Under Domestication. At the end of the first volume of this book, he struck a distinct Lamarckian note: “It is often sufficient for the inheritance of some peculiar character, that one parent alone should possess it.... But the power of transmission is extremely variable. In a number of individuals descended from the same parents, and treated in the same manner, some display this power in a perfect manner, and in some it is quite deficient.... The effects of injuries or mutilations are occasionally inherited; and … the long-continued use and disuse of parts produces an inherited effect.” Darwin postulated the existence of “gemmules,” or microscopic germs, “cast off’ into the circulation by the almost infinite number of cells that compose an organism and then concentrated in the generative cells, primarily the ova and sperm. If properly nourished, he argued, any gemmule could reproduce a portion of the organism or even the whole organism itself. These gemmules were inconceivably small—perhaps the size of atoms—since all could be contained in an ovum or a spermatazoon. Sometimes the gemmules remained dormant or undeveloped in an individual, only to be activated somehow in future generations. Gemmules also could be modified by external conditions. If multiplying sufficiently, they were capable of replacing the older gemmules and developing new structures. For example, a mutilation in an adult theoretically could be passed to its offspring. Pangenesis also explained the persistence of rudimentary and useless organs.¹⁶ Wallace initially embraced pangenesis as the best available theory to explain heredity. But the confusion the theory wrought among scientists caused Darwin to pronounce it “stillborn” and to predict, like a dejected Cassandra, that it eventually would reappear under some other name.¹⁷ He was wrong. One of the first experimentalists to disprove the theory of pangenesis was his cousin Francis Galton, who pioneered the study of heredity and performed blood transfusions on rabbits, replacing the entire blood volume of one animal with the blood of another and then breeding the transfused rabbits. The offspring were not in the least bit altered and resembled their biological parents, not the donor rabbits. Thus the blood did not carry the hereditary material from the body to the reproductive organs, as Darwin had speculated.¹⁸

When Weismann published his views on heredity nearly twenty years later, Wallace abandoned Darwin’s “ponderous” theory for one that he deemed far superior. He distilled Weismann’s fundamental question as follows: “How is it that in the case of all higher animals and plants a single cell is able to separate itself from amongst the millions of the most various kinds of which an organism is composed, and by division and complicated differentiation to reconstruct a new individual with marvelous likeness, unchanged in many cases even throughout whole geological periods?”¹⁹ That the union of spermatazoon and ovum reproduced not only the general character of the species but many of the individual characteristics of the parents and their remote ancestors could be explained only if the germ cells arose directly from the parent germ cells themselves. Weismann had stated that at birth a portion of the “germ-plasm,” a special molecular substance contained in the germ cells derived from the parents, was reserved unchanged to produce the germ cells of the next generation. But this was not all. Through sex, the germ-plasms of the two parents were united, intermingling the characters of many generations and creating unique individuals whose variability was precisely what natural selection worked on. This concept—the continuity of the germ-plasm—had also been postulated by Galton some years earlier.

Weismann arrived at his conclusions through induction, based on his extensive knowledge of cytology, embryology, and natural history. If his hypothesis were true, then the transmission of acquired characters could never occur because the material to be transmitted would have been segregated into germ cells at such an early stage in fetal development that climate, habits, or any other external factors could not affect it.²⁰

Weismann’s rejection of “soft” inheritance was received with great hostility by neo-Lamarckians—the majority of evolutionists at the time—and was not universally accepted for fifty years. Although Weismann did not arrive at a correct idea of the precise means of inheritance—that would have to wait until the rediscovery, at the turn of the twentieth century, of Gregor Mendel’s work; the subsequent identification of the genetic material; and James Watson and Francis Crick’s discovery in 1953 of the replicating mechanism—his general views on the segregation and transmission of genetic material are still valid. Wallace was among the first to recognize Weismann’s genius and actively promote his ideas. After reading the essays (translated from the German by his friend Meldola) in Nature, Wallace, like the duke of Argyll, jumped at germ-plasm like a trout at a fly, for it proclaimed natural selection as the only mechanism of evolution. Weismann’s work was not easy reading, but Wallace immediately understood his triumphant rebuttal of Lamarckian inheritance.²¹

Romanes and his theory of physiological selection remained a formidable obstacle. When Poulton suggested that Romanes review Darwinism, Wallace was outraged. “I think it would be almost indecent for [him] to review [it]!” he exclaimed.²² Wallace had made Romanes a major target in the course of answering several of the most prominent objections to natural selection, and this renewed antagonism had resurrected the epistolary battle that ended a few months earlier. His book also singled out the Reverend John Thomas Gulick, an American missionary, naturalist, and friend of Romanes, who in “Divergent Evolution Through Cumulative Segregation” had maintained that geographic isolation alone produced speciation, without inter- and intraspecific competition. According to Gulick, there was an inherent tendency to variation in certain divergent groups within a species (now referred to as genetic drift). When one part of a species was isolated, this tendency to variation created a persistent divergence from the rest of the population despite identical conditions, and the variant line diverged further and further from the original type; if the two groups were prevented from interbreeding, a new species would then form. Gulick used as his example terrestrial mollusks in the Hawaiian Islands, where the various species, derived from a common ancestor, inhabited “identical” though geographically separate habitats. Natural selection could not have been operating, he said, because the food, climate, and enemies were the same.²³ Strongly disagreeing, Wallace stated, “It is an error to assume that what seem to us like identical conditions are really identical to such small and delicate organisms as these land mollusks, of whose needs and difficulties at each successive stage of their existence, from the freshly-laid egg up to the adult animal, we are so profoundly ignorant.”²⁴

According to Wallace, the two greatest arguments against natural selection as the sole driving force behind speciation were hybridization and sexual selection. He therefore devoted several chapters to these difficulties. Although he had treated sexual selection in other publications and offered nothing new on the subject, his discussion of hybridization was truly novel.

The reality of hybridization called into question the very definition of species. Before 1858, when Darwin and Wallace announced their joint theory of natural selection, the defining difference between a variety and a species was the degree of fertility. In general, the varieties of any one species, no matter how different they might be in external appearance, could freely interbreed and produce offspring that could continue to freely interbreed. But distinct species, no matter how closely they might resemble each other, usually were unable to interbreed, and if they were, their offspring always were sterile. Wallace pointed out that these distinctions were once considered a “fixed law of nature” and an absolute test and criterion of a true species versus a variety. As long as species were regarded as special creations, and the origin of species and that of varieties were viewed as different, the law could have no exceptions. If two species produced a fertile hybrid and the offspring of that hybrid also were fertile, the parent species were relabeled as varieties. Likewise, if two varieties produced sterile hybrids, the varieties were considered to be separate species. “Thus the old theory led to inevitable reasoning in a circle,” Wallace said, “and what might be only a rather common fact was elevated into a law which had no exceptions.” In the Origin, Darwin had proved the fallacy of such a law by demonstrating numerous exceptions.²⁵

Unlike most of their predecessors and their fellow naturalists, Darwin and Wallace ceased to view a species as a distinct entity created by God, considering it instead as an assemblage of individuals modified to adapt to new conditions of life—an assemblage that also happened to interbreed and produce similar offspring. In a state of nature, animals preferred their own. Even slight differences of form or color were sufficient to deter them from interbreeding, in effect resulting in isolation of groups even when habitats overlapped. Wallace called this phenomenon “isolation of varieties by selective association” and felt that “the great stumbling-block of many naturalists will be completely obviated.”²⁶

Wallace posed the same hypothetical question he had put to Darwin long before: Could natural selection actually promote the failure of varieties of a species to interbreed successfully—that is, promote the development of reproductive isolating mechanisms? It was a question that turned Romanes’s theory of physiological selection on its head. Relegating natural selection to a minor role in evolution, Romanes had suggested that physiological selection was not only a fact but also the true starting point of speciation. Given the fuller knowledge of the facts of variation uncovered since 1858, Wallace felt that he could show that natural selection sometimes was able to accumulate variations in an incipient species’ ability to interbreed with another incipient species or with its parental species. His demonstration was based on a number of assumptions inferred from known facts presented by other naturalists, especially Darwin.

He first asked his readers to imagine two varieties of a species occupying an extensive area, each in the process of adapting to somewhat different conditions of existence. If the two varieties freely crossed and their offspring also were fertile, then further speciation of the two forms would be retarded or entirely prevented. He next asked his readers to suppose that the hybrids of these same two varieties were somehow less and less able to produce fertile offspring when crossed, a phenomenon correlating with the increasingly differing modes of life and the slight external or internal peculiarities that gradually arose between them. Wallace already had presented evidence from Darwin that such peculiarities were indeed real causes of the failure of two varieties of a domesticated species to hybridize. On one of the Faroe Islands, the half-wild native black sheep did not readily interbreed with imported white sheep; flocks of white and Chinese geese generally did not frequently hybridize; the differently colored herds of cattle in the Falkland Islands also tended to remain apart. Moreover, external appearances often signaled fitness or the lack thereof: white color and blue eyes in male cats correlated with deafness; pigeons of certain colors produced naked offspring; in Virginia, only black varieties of a certain pig could eat a plant called the paint-root (Lachnanthes tinctoria) without having their bones turn pink and their hooves fall off; and so forth. Although these were all examples in domesticated or partly domesticated animals, Wallace saw no reason that the tendency to mate with like-appearing members and the correlation of color with sometimes injurious constitutional peculiarities should not also occur in nature. Given the partial sterility of hybrids, it would follow that even if the two varieties freely crossed, the population of hybrids would increase less rapidly than those of the two pure forms. The offspring of the pure forms would doubtless be better adapted to their respective conditions of life. And when the struggle for existence became exceptionally severe, the smaller population of hybrids would die out before the offspring of the parent forms did, leaving the varieties pure. The greater the inability of the hybrid forms to produce fertile offspring, the likelier the pure forms of the varieties would outbreed the hybrid forms and eventually outcompete the hybrids for resources, causing their extinction:

Wallace concluded that “specialisation to separate conditions of life, differentiation of external characters, disinclination to cross-unions, and the infertility of the hybrid [products] of these unions, would all proceed pari passu, and would ultimately lead to the production of two distinct forms having all the characteristics, physiological as well as structural, of true species.”²⁷ Wallace’s theory—which holds that as two populations within a species diverge to the extent that the hybrids between them are less well adapted than either parent form, natural selection will tend to eliminate the hybrids—is recognized as valid by some evolutionary biologists and is known as the Wallace Effect.²⁸

In the fifteenth and final chapter of Darwinism, which represented nearly thirty years of reflection on the subject, Wallace examined the application of natural selection to human beings. Natural selection, he said, was sufficient to explain most of the facts of organic life, such as the origin of species, genera, families, orders, classes, and even kingdoms. But it was insufficient to explain certain major events in the history of evolution of life on earth. He therefore introduced a bit of catastrophism into his otherwise uniformitarian vision of the earth’s history. Echoing Sir Charles Lyell in The Geological Evidences of the Antiquity of Man, he pointed to three stages in the development of the organic world “when some new cause or power must necessarily have come into action.” The first stage was the change from inorganic to organic matter, when the earliest cell appeared. “[It] has been well said,” Wallace noted, “that the first vegetable cell was a new thing in the world, possessing altogether new powers—that of indefinite reproduction, and, still more marvelous, the power of variation and of reproducing those variations till endless complications of structure and varieties of form have been the result.” The second stage was the introduction of sensation or consciousness, “constituting the fundamental distinction between the animal and vegetable kingdoms.” No explanation thus far had been intellectually satisfying, he said, and the phenomenon remained a mystery. The third and final stage was the existence in man “of a number of his most characteristic and noblest faculties, those which raise him furthest above the brutes and open up possibilities of almost indefinite advancement.” These faculties, he believed, could not have been developed according to the same laws that determined the general and progressive evolution of the organic world. He was forced to conclude that these three distinct stages pointed to an unseen universe, “a world of spirit, to which the world of matter is altogether subordinate.” The spiritual nature of humanity, therefore, was not inconsistent with his and Darwin’s theory of evolution by natural selection; it depended on the same fundamental laws and causes that provided the materials for the evolutionary process to unfold. But this view of human nature should relieve us of the “crushing mental burthen” of the materialistic and fatalistic belief in our helplessness in the face of the implacable, blind forces of nature. Humankind’s struggles were not pointless. In the near future, the earth, which for untold millions of years had been slowly developing the forms of life that would culminate in human beings, would be as it had never been. Darwin must have rolled over in his crypt in Westminster Abbey at Darwinism’s final dreamy paragraphs. Had he been summoned to a séance, he would have rapped out, “No! No!”

By the time Wallace had finished the manuscript of Darwinism, he was convinced that he had demolished every one of his adversaries, including the Darwin of sexual selection. Macmillan published the book in 1889, agreeing to a few conditions that Wallace had set. He wanted his book issued with “cut edges,” not the uncut pages that resulted in ragged edges that accumulated dust and dirt over time. His concerns were economic: there seemed to be a prejudice among readers against uncut books. Darwin’s own books, he said, were neatly cut by machine at his own insistence. In 1867 Darwin had written a letter to the Athenaeum advocating the abolition of the practice of leaving pages uncut, thereby requiring separation with a knife or finger. (This would “earn the gratitude of children who have to cut through dry and unillustrated books for the benefit of their elders,” Darwin pointed out.)²⁹ Why could Wallace not demand the same conditions? “I know I shall... obtain the gratitude of all readers of my book even if you despise my bad taste,” Wallace told Alexander Macmillan.³⁰

The book was moderately successful and would go through three editions by the turn of the century. Herbert Spencer complimented Wallace on the book but chided him for a misleading modesty. “I regret that you have used the title ‘Darwinism,’” Spencer said, “for notwithstanding your qualification of its meaning you will, by using it, tend greatly to confirm the erroneous conception almost universally current.”³¹ Wallace thanked his friend Grant Allen for reviewing the book favorably in the Academy. In a letter dated July 22, 1889, he wrote that he disagreed with one point to which Allen had alluded:

Much to Wallace’s chagrin, the editor of the Contemporary Review hired Romanes to write a critique, which appeared in the August 1889 issue. Romanes’s principal objection to Darwinism was Wallace’s exclusive adherence to natural selection. “He will not have any other ‘factor,’ and therefore says natural selection must eat up sexual selection like the lean kine have the fat kine,” he told Francis Darwin before the review appeared. Pure Darwinians like Wallace and Weismann were harming the theory by discarding the “Darwinian recognition of use and disuse.” “Wallace’s jealousy … is foolish and inimical to natural selection theory itself, by forcing it into explanations which are plainly false.”³³ Romanes contended that despite its title, the book was not an exposition of Darwinism but of “pure Wallaceism.” Except for the origin of certain faculties of man, he said, the major difference between Wallace and Darwin was the scope they gave to the doctrine of natural selection. Wallace saw no limits to the mechanism, where Darwin saw several. Romanes also contended that Wallace had validated the hypothesis of physiological selection rather than disproving it.

Edwin Ray Lankester, once Wallace’s impudent opponent in the trial of the medium Henry Slade but now more or less an ally, came to Wallace’s defense in the October 10 issue of Nature. Calling the book “admirable,” he stated that it contained “an exposition of highly important and interesting views … on subsidiary matters, which have either not been published previously or have appeared in a scattered and more or less inaccessible form.” Indeed, the abundance of new facts and arguments, convincing or not, were of “extreme value and full of interest.” But he also felt that Wallace’s tendency to speculate and his failure to muster sufficient evidence to support some of his conclusions weakened the book. “With Darwin, one becomes accustomed to see no speculation put forward, no step of an argument advanced, unless there is an overwhelming weight of testimony in its favour,” he said. “‘Darwinism’ can never take the place of the ‘Origin of Species,’ but may well serve as an introduction to the study of that and the other works of Darwin—the value of which, not only as storehouses of fact and suggestion, but as classical models of scientific discussion, cannot be over-estimated and will probably never be surpassed.” Wallace’s criticism of Romanes appeared to Lankester to entirely destroy all that was novel in “that laborious attack” on Darwin’s theory of the origin of species. But Lankester lost patience with the concluding homage to the spiritualist doctrine. He could only remark—with greater diplomacy than he had exhibited in the past—that it remained an interesting problem for the “future student of human faculty” to reconcile Wallace’s “wonderful ingenuity” and reasoning skills in the field of zoology with his views on the “so-called ‘manifestations’ of spiritualists.”

Angered by Wallace’s and Lankester’s criticisms, Romanes recruited Gulick to support his cause. Gulick had hoped to reply to Wallace in Nature, but Romanes advised against it, having been chided by editor Joseph Norman Lockyer for using the magazine as his soapbox. Moreover, a rebuttal published in a periodical would have a transient effect, whereas a book would advance the truth of evolution. “If only 100 copies were sold and 100 more distributed [by] yourself,” Romanes told Gulick, “they would find their way into every mind much worth influencing in our own generation.”³⁴ In the meantime, in an article written for the Fortnightly Review, Romanes charged Wallace with plagiarism: “He presents an alternative theory to explain the same class of facts. Yet this theory is purely and simply without any modification whatsoever, a restatement of the first principles of physiological selection, as these were originally stated by myself.” He added in reference to Wallace’s concluding chapter that here “we encounter the Wallace of Spiritualism and astrology, the Wallace of vaccination and the land question, the Wallace of incapacity and absurdity.”³⁵

Wallace made no public reply to these inflammatory remarks for two months, convinced that people whose opinion he valued would condemn them. But he had an ace up his sleeve, and now he played it. Several years earlier, when he was in Kingston, Ontario, a woman who had attended one of his lectures told him that Romanes was a spiritualist and had tried to convert Darwin! Stunned, Wallace found it hard to believe that Romanes would have confided such interests to Darwin, knowing his attitude toward spiritualism. But the woman had proof. She was a good friend of Romanes’s brother, an avowed spiritualist then living in Canada. He and Romanes had written to Darwin jointly, and she still had copies of those letters in her possession. The next morning, she brought them to Wallace and gave him carte blanche to make of them what he wished. He copied them out and kept them for future reference.³⁶

“Whether or no it was good taste for you to appeal to the political and medical prejudices of your readers in a matter purely scientific … I leave others to judge,” Wallace wrote to Romanes. “But as to your appeal to popular scientific prejudice by referring to my belief in Spiritualism and astrology (which latter I have never professed my belief in), I have something to say.” He revealed that he had copies of two letters detailing Romanes’s experiences of spiritual phenomena and his conviction of the truth of these facts and the existence of spiritual intelligences:

The revelation did not faze Romanes. He replied that it had never occurred to him that he had hit Wallace below the belt. If Wallace had called his critique of Darwinism an example of “incapacity and absurdity,” he would not have objected. He was only making “fair comment” on Wallace’s “different lines” of thought. But he had erred and would not refer to such notorious matters again. Regarding the early letters to Darwin, he had forgotten the details after fourteen years. Those letters, however, were meant to be strictly confidential and to be regarded as “provisional” only. Since that time, he had changed his mind about spiritualism. He wondered how Wallace had got hold of the letters. If by some occult process, he hoped that Wallace would publish them as evidence of a spirit world. If from a member of the Darwin family, he did not know whether Wallace had a right to read them.³⁸

Wallace then challenged Romanes to deny the phenomena he had personally observed. What other explanation did Romanes have? Once again, he challenged him to tell it to the world. He himself had nothing to hide—his writings were public property, and people were free to reflect on his facts and arguments and criticize them if they wished. “After the way you have referred in print to my belief in such phenomena,” Wallace said, “most persons would think I was quite justified in making known the fact of the existence of these letters and their general tenor.” The honorable thing to do was for Romanes himself to publish the letters, with full details of the discovery of the imposture that had induced him to change those convictions so “earnestly and solemnly” expressed to Darwin. Wallace hoped that he would not be forced to reveal the letters’ existence himself.³⁹ Now Romanes was genuinely alarmed. How Wallace had obtained copies of these letters mystified him. Wallace briefed the Darwin family about the nature of the feud with Romanes and assured them that no one in the family had given him access to private material. A year later, Romanes held up a white flag of sorts, requesting a photograph of Wallace for his forthcoming book on the Darwinian theory. Wallace politely declined.⁴⁰

Wallace’s somewhat underhanded tactic had succeeded: Romanes never published another criticism of him. But Wallace neither forgave nor forgot their rancorous interchange. In 1893 Romanes developed an illness that would prove fatal. William Thistleton-Dyer, the head of the Royal Botanic Gardens at Kew and a good friend of Wallace’s, conveyed the news to him. Through Thistleton-Dyer, Romanes had made the strange request to speak with Wallace privately. Wallace was surprised. They hardly knew each other, he told Thistleton-Dyer; they had met face to face only once or twice, and only a half dozen or so letters had passed between them. The “very gross misstatements and personal attacks” on him for allegedly plagiarizing Romanes’s theory of physiological selection were still offensive to him: “This accusation he supported by such a flood of words & quotations and explanations, as to obscure all the chief issues and render it almost impossible for the ordinary reader to disentangle the facts. I told him then that unless he withdrew this accusation as publicly as he had made it I should decline all future correspondence with him, & should avoid referring to him in any of my writings.” Moreover, he concluded,

But no written retraction was forthcoming, and Romanes died in 1894 of kidney failure, his theory of physiological selection dying with him. Despite its idiosyncratic ending, Wallace’s book had dealt a fatal blow to Romanes’s attempt to subvert the theory of natural selection as the primary explanation for the origin of species. From time to time over the next two decades, Romanes’s hypothesis was revived in different guises. But other investigators after Wallace have proved that “sympatric speciation,” at least in the non-adaptive, saltationist sense that Romanes proposed, does not occur in nature. Isolating mechanisms of some sort (spatial, temporal, or behavioral), reinforced by natural selection, are essential to and responsible for the formation of separate species from divergent varieties, a conviction that Wallace maintained steadfastly.⁴²