The theory of natural selection of Alfred Russel Wallace FRS

Michael Bulmer

Summary

Wallace's 1858 paper ‘On the tendency of varieties to depart indefinitely from the original type’ is often thought to present a theory of natural selection identical with that of Darwin. Examination of Wallace's argument shows that it is different from Darwin's because Wallace thought that an inferior variety could coexist with a superior variety until environmental deterioration forced the extinction of the inferior one. Other interpretations of Wallace's argument are re-examined in the light of this finding.

Introduction

Alfred Russel Wallace (figure 1) was the first recipient of the Darwin medal of The Royal Society in 1890 ‘for his independent origination of the theory of the origin of species by natural selection’. He had developed this theory in two papers written while he was collecting professionally in the Malay Archipelago.1 The first paper, ‘On the law which has regulated the introduction of new species’, was written in Sarawak, Borneo, in 1855. From the geographical and geological distribution of animals and plants, he deduced the law that ‘every species has come into existence coincident both in space and time with a pre-existing closely allied species’. This law had been foreshadowed by Charles Darwin FRS in his Journal of Researches as ‘the law of the succession of types.’2

Figure 1

Portrait of Wallace from his autobiography My Life, Vol. 1 (Chapman and Hall, London, 1905). Copyright © Royal Society.

Wallace concealed in this paper his belief that the Sarawak law was due to transmutation, although it is clear from his contemporary letters and notebooks that he thought this to be the case.3 But he had no idea of the mechanism of transmutation. In his autobiography in 1905 he wrote: ‘My paper written at Sarawak rendered it certain to my mind that the change had taken place by natural succession and descent—one species becoming changed either slowly or rapidly into another. But the exact process of change and the causes which led to it were absolutely unknown and appeared almost inconceivable.’4

The solution occurred to him while he was recovering from a bout of malaria on the island of Gilolo in February 1858. He wrote up his thoughts, which were close to the theory developed but not yet published by Darwin, and sent his manuscript to Darwin from the neighbouring island of Ternate. This led to the reading of Wallace's Ternate paper together with extracts from Darwin's essay of 1844 and a letter from Darwin to Asa Gray at a special meeting of the Linnean Society in 1858, followed by the publication of Darwin's Origin of species in 1859.5

Both Darwin and Wallace thought that their independently derived theories of natural selection were identical. When he received Wallace's Ternate paper in 1858, Darwin wrote to Sir Charles Lyell FRS: ‘I never saw a more striking coincidence. If Wallace had my M.S. sketch written out in 1842 he could not have made a better short abstract!’6 In an introductory note to a reprint of his Ternate paper in 1891 Wallace wrote: ‘This [paper] sets forth the main features of a theory identical with that discovered by Mr. Darwin many years before but not then published.’7 But some historians have suggested that there was a marked difference between the two theories.8 I shall here argue that this view is correct and I shall put forward a new interpretation of the nature of this difference.

The argument of Wallace's Ternate paper

Wallace called his paper ‘On the tendency of varieties to depart indefinitely from the original type’. Opponents of transmutation, such as Lyell, accepted that limited changes could occur in species to form different varieties, but they argued that ‘there are fixed limits beyond which the descendants from common parents can never deviate from a certain type’ so that ‘species have a real existence in nature, and that each was endowed, at the time of its creation, with the attributes and organization by which it is now distinguished’.9 Like Darwin, Wallace admired Lyell's work but rejected his views on the fixity of species. In his Ternate paper he set out to show that varieties could depart indefinitely from the original type and to demonstrate the mechanism whereby these changes could happen. I shall provide a brief paragraph-by-paragraph summary of the paper, whose structure is not well known. Italics in all quotations are Wallace's.

In §§1–3 Wallace addressed an argument for the permanence of species, that domestic varieties are unstable and tend to revert to the parent form when left to themselves. By analogy it was inferred that natural varieties had strict limits beyond which they could not vary further, though they could return to the original type. After he had outlined his theory of natural selection, Wallace argued in §§10–13 that the tendency of domestic varieties to revert to the original type when they are returned to the wild can be explained on the same principle, because they have been bred under domestication for characteristics that are useless in the wild. ‘The great speed but slight endurance of the race horse, the unwieldy strength of the ploughman's team, would both be useless in a state of nature.’

In §§4 and 5 Wallace put forward the Malthusian argument (though he did not call it by that name) that the population of each species must be checked by density-controlling factors. ‘Even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary, or perhaps, through the influence of man, decreasing.’ He thought that the food supply and predation were the most powerful checks to population growth, and he concluded:

So long as a country remains physically unchanged, the numbers of its animal population cannot materially increase. If one species does so, some others requiring the same kind of food must diminish in proportion. The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest—the very young, the aged, and the diseased,—while those that prolong their existence can only be the most perfect in health and vigour—those who are best able to obtain food regularly, and avoid their numerous enemies. It is, as we commenced by remarking, “a struggle for existence,” in which the weakest and least perfectly organised must always succumb.

It looks as if Wallace was about to embrace a Darwinian view at this point, but he did not do so, possibly because he was thinking of the weakest who died first as suffering from an accidental rather than a heritable weakness; the examples he gives are being very young or old, which are not heritable conditions, and being diseased, which might be plausibly attributed to bad luck rather than to a heritable predisposition to disease.

Instead, Wallace extended the Malthusian argument in §§6–7 to explain the relative abundances of allied species of a group:

Those [species] which are best adapted to obtain a regular supply of food, and to defend themselves against the attacks of their enemies and the vicissitudes of the seasons, must necessarily obtain and preserve a superiority in population; while those species which from some defect of power or organisation, are the least capable of counteracting the vicissitudes of food-supply, etc., must diminish in numbers, and, in extreme cases, become altogether extinct. Between these extremes the species will present various degrees of capacity for ensuring the means of preserving life; and it is thus we account for the abundance or rarity of species.

Wallace concluded ‘that the comparative abundance or scarcity of the individuals of the several species is entirely due to their organisation and resulting habits’, less well-adapted species being less abundant than better-adapted species.

In §§8–9 Wallace argued that these conclusions could be applied to the consideration of varieties. An antelope with shorter or weaker legs would suffer more from predation, and

the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers. …All varieties will therefore fall into two classes—those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority.

He then supposed that the physical conditions deteriorated:

Now, let some alteration of physical conditions occur in the district—a long period of drought, a destruction of vegetation by locusts, the irruption of some new carnivorous animal seeking ‘pastures new’—any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination; it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organised variety would suffer first, and, were the pressure severe, must soon become extinct. The same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. The superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety.

Wallace concluded that

the variety would now have replaced the species. …Such a variety could not return to the original form; for that form is an inferior one, and could never compete with it for existence. Granted, therefore, a ‘tendency’ to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions again alone survive.

This passage reveals some ambivalence about what would happen when the physical conditions improved. Wallace continued that the variety could in turn give rise to an even better-adapted variety, which would in turn become predominant: ‘Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur.’

In §14 Wallace concluded that these principles account more easily than the hypothesis of Lamarck for the progressive changes in species.

Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes10 with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them.

The same principle explains the cryptic colours of many insects: ‘though in the course of ages varieties of many tints may have occurred, yet those races having colours best adapted to concealment from their enemies would inevitably survive the longest.’

Interpretation of Wallace's argument

Wallace's argument summarized above can be encapsulated in five propositions. (i) The population of each species is controlled at a constant level by Malthusian checks, such as the food supply and predation. (ii) The comparative abundance or scarcity of related species is determined by their relative level of adaptation, less well-adapted species being less abundant than better-adapted species. (iii) This argument can be extended to different varieties within a species, less well-adapted varieties being less abundant than better-adapted varieties. (iv) If the environment deteriorates, the less numerous and less well-adapted variety will become extinct, so that the better-adapted variety would have replaced the less well-adapted variety: ‘It is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organized variety would suffer first, and, were the pressure severe, must soon become extinct’. This also explains why domestic varieties revert to the original type when they are returned to the wild, where the latter is better adapted. (v) Continuation of this process through changing environmental conditions over a long time explains ‘the tendency of varieties to depart indefinitely from the original type.’

The first proposition embodies the Malthusian argument applied to animal populations, which was the starting point for the theories of both Darwin and Wallace. But the remaining propositions differ from the Darwinian argument that an advantageous variation would gradually increase and its parental form would decrease in frequency until it became extinct. Instead Wallace drew an analogy from the population ecology of closely related species, and came to the conclusion that an advantageous heritable variation and its parental form could coexist, although with the former more numerous than the latter, until an environmental deterioration forced the extinction of the parental form.

The Darwinian theory and Wallace's original theory can be formalized in terms of what is called today the carrying capacity of the environment, usually denoted by K; in Wallace's words, this is the level at which ‘the population must have reached its limits, and have become stationary.’ Suppose that the carrying capacity of the parental form on its own is K, and that the carrying capacities of the parental form and the advantageous variation when they coexist are K1 and K2, respectively. Under Darwin's theory K1=0, whereas K2 is equal to or perhaps slightly greater than K, so that the parental form eventually becomes extinct even in a constant environment. Under Wallace's theory both carrying capacities are greater than zero, with K1<K2, so that both forms can coexist; if the environment deteriorates, both carrying capacities decrease, and if the deterioration is severe K1 becomes 0, so that the parental form becomes extinct. When the environment recovers, the carrying capacities return to their original values so that both types can again coexist. Wallace considered this possibility in §9 in the passage quoted above ‘The variety would now have replaced the species….’, although he was ambivalent about what happens when the environment recovers.

Wallace did not discuss the relationship between K1, K2 (the carrying capacities of the species and the superior variety when both are present) and K (the carrying capacity of the species on its own). In §8 he wrote, ‘All varieties will therefore fall into two classes—those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority.’ This might suggest that the parent species had the same carrying capacity K in the presence and in the absence of the superior variety, whereas the superior variety had a higher carrying capacity; this would imply that there was no competition between them. Against this interpretation is the statement in §5: ‘So long as a country remains physically unchanged, the numbers of its animal population cannot materially increase. If one species does so, some others requiring the same kind of food must diminish in proportion.’ Thus Wallace probably thought that, when both the parent species and the superior variety are present, K1 is somewhat less and K2 somewhat greater than K/2.

Wallace arrived at this theory by assuming that closely related species depend on the same limiting resource. He inferred that related species depending on the same resource can coexist, although with the better-adapted more numerous than the less well-adapted species (§6). He extended this conclusion to different varieties within a species (§8). He argued that, if environmental conditions deteriorated, placing pressure on both varieties, the less numerous and less well-adapted variety would become extinct first (§§8–9). Finally, he observed that continuation of this process through time would explain ‘the tendency of varieties to depart indefinitely from the original type.’

In his 1855 Sarawak paper Wallace drew a clear distinction between phyletic evolution within a single lineage and cladistic evolution after a species splits into two:

So long as each species has had but one new species formed on its model, the line of affinities will be simple, and may be represented by placing the several species in direct succession in a straight line [phyletic evolution]. But if two or more species have been independently formed on the plan of a common antitype, then the series of affinities will be compound, and can only be represented by a forked or many-branched line [cladistic evolution].11He attributed the splitting of lineages to geographical isolation or isolation by distance:

A country having species, genera, and whole families peculiar to it, will be the necessary result of its having been isolated for a long period, sufficient for many series of species to have been created on the type of pre-existing ones. …If in any case the antitype had an extensive range, two or more groups of species might have been formed, each varying from it in a different manner.12

He went on to consider the role of geographical isolation through the elevation of mountains or the formation of islands in causing speciation.

In the Ternate paper Wallace was concerned solely with the process of phyletic evolution within a single lineage, although it follows from his argument that two geographically isolated lineages subject to different environmental circumstances will tend to diverge. There is one passage in the paper that might be interpreted in the latter sense. After arguing in §9 that a superior variety will ultimately extirpate the original species, Wallace wrote:

But this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do frequently occur.

Was he referring to divergence from a common ancestor within a lineage or between two geographically isolated lineages? It seems natural to assume the former because the latter idea does not occur elsewhere in the Ternate paper. In any case he was not referring to Darwin's principle of divergence, which involved ecological competition between sympatric species, an idea not mentioned in either the Sarawak paper or the Ternate paper.13

Other interpretations of Wallace's argument

Two other interpretations of Wallace's argument will now be contrasted with the interpretation put forward above.

Competitive selection Embedded Imageenvironmental selection

The population ecologist A. J. Nicholson has presented the following interpretation of the difference between Darwin's and Wallace's theories of natural selection:Darwin's arguments were concerned dominantly with competitive selection, which causes the less fit forms to be displaced as a secondary effect of the preservation of fitter forms, whereas Wallace, when illustrating the operation of natural selection, referred almost exclusively to what may be called ‘environmental selection’, the active principle of which is the direct elimination of the unfit.14In supporting this interpretation, Malcolm Kottler has pointed out that competitive and environmental selection are examples of what Bruce Wallace has called soft and hard selection.15 Under competitive (soft) selection an individual is struggling against other members of the same species for limited resources; an individual of a less well-adapted variety could survive perfectly well in the absence of a better-adapted variety but not in its presence. Under environmental (hard) selection an individual is struggling against the physical environment; an individual will survive only if it can endure, say, the rigours of winter, regardless of the characteristics of its conspecifics.

Nicholson based his conclusion that Wallace was primarily concerned with environmental selection on the passage at the end of §8 of his paper, beginning ‘Now, let some alteration of physical conditions occur in the district...’ (see the quotation above). After quoting this passage, Nicholson commented: ‘Apparently he [Wallace] did not clearly realise that competition plays an important part in selection, in addition to being the mechanism causing the numerical limitation of populations in their environments.’

Nicholson correctly identified the element of environmental selection in Wallace's theory of selection between varieties, but he did not identify the underlying reason why Wallace's theory took its particular form. Wallace did take competition between varieties into account. But by an incorrect analogy with competing species, which can coexist if they have different niches, he inferred that competing varieties can coexist, with the inferior variety less numerous than the superior one, until a deterioration in the environment forces the inferior variety into extinction.

Individual selection versus selection between varieties or between groups

Peter Bowler has argued that Wallace's initial concept of natural selection in 1858 differed from Darwin's because Darwin considered competition between individuals whereas Wallace was concerned with competition between varieties:There are good reasons for suggesting that Wallace's initial concept of selection differed considerably from Darwin's, or at least was expressed in very different terms. From the beginning, Darwin took as his starting point the existence of individual differences within a species, arguing that the struggle for existence would favor those variants with an advantageous feature. He realized that if the species were subjected to different conditions in the various parts of its geographical range, selection would produce well-marked permanent varieties adapted to these conditions. Further selection would turn the varieties into distinct species, and Darwin eventually realized that the varieties themselves might come into conflict, producing a second level of selection in which the fittest varieties would bring about the extinction of the less well favored ones. Wallace's paper, on the other hand, approached selection only through this last phase of the process. He was concerned with the survival of the permanent varieties once they have been formed, not with their actual formation through the action of selection on individual differences.16

Bowler assumes that Wallace's varieties differ from each other far more than Darwin's individual differences. Kottler traces this line of thought back to H. F. Osborn ForMemRS in 1894:Darwin dwells upon variations in single characters, as taken hold of by Selection; Wallace mentions variations, but dwells upon full-formed varieties, as favourably or unfavourably adapted. …With Wallace, Varieties are already presupposed by causes which he does not discuss, a change in the environment occurs, and those varieties which happen to be adapted to it survive.17E. B. Poulton FRS disagreed with this view and asked Wallace to comment on it. Poulton wrote:Further consideration tends to obliterate the supposed distinction. Although Wallace used the term ‘variety’ as contrasted with ‘species’, the whole context proves that he, equally with Darwin, recognised the importance of individual variations and of variations in single characters. This becomes clear when we remember his argument about the neck of the giraffe, the changes of colour and hairiness, the shorter legs of the antelope, and the less powerful wings of the passenger pigeon. Wallace has kindly written to me (May 12th, 1896) stating the case as I have given it, and he further explains—‘I used the term “varieties” because “varieties” were alone recognised at that time, individual variability being ignored or thought of no importance. My “varieties” therefore included “individual variations”.’18

On the basis of Poulton's statement of Wallace's views I argue with Kottler and against Bowler that Wallace's variations did not differ from each other much more than Darwin's individual differences. This conclusion is consistent with what Wallace wrote in the Ternate paper. For example, he wrote in §8: ‘Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. Even a change of colour might, by rendering them more or less distinguishable, affect their safety.’

The second question raised by Bowler is whether Wallace's varieties were variant individuals in a population or whether they were variant populations, so that we must consider what Wallace meant by the words ‘variation’, ‘variety’ and ‘race’. He used ‘variety’ and ‘race’ interchangeably. For example, in §2 he wrote: ‘Equally general, however, is the belief in what are called “permanent or true varieties,”—races of animals which continually propagate their like, but which differ so slightly (although constantly) from some other race, that the one is considered to be a variety of the other.’ He was less consistent in his usage of ‘variation’, sometimes using it interchangeably with ‘variety’. But he usually used ‘variation’ to mean a heritable variant character, or an individual possessing such a character, whereas he used ‘variety’ for the set of individuals with that character. This distinction is exemplified in §9: ‘Variations in unimportant parts might also occur, having no perceptible effect on the life-preserving powers; and the varieties so furnished might run a course parallel with the parent species, either giving rise to further variations or returning to the former type.’ Kottler has drawn attention to the ambiguity of Wallace's usage.19 But I conclude that by ‘variation’ Wallace usually meant a heritable character (such as a long neck in the giraffe) or an individual possessing such a character, whereas ‘variety’ or ‘race’ usually means the set of individuals with this character; I use the word ‘set’ rather than ’population’ to avoid implications about population structure.

Wallace was silent in the Ternate paper about the population structure of varieties except that they were ‘races of animals which continually propagate their like’, which is reminiscent of Darwin's statement in the Origin that ‘like begets like.’ It is consistent with a population structure in which the varieties belong to a single interbreeding population without blending inheritance. It is also consistent with a population structure in which the two varieties form two non-interbreeding populations; for some reason, the members of the two varieties do not mate with each other although they occupy the same geographical area. Kottler has shown that Wallace was considering competition between varieties in the same geographic area rather than geographically isolated varieties,20 but it is possible that he was thinking of them as being reproductively isolated. Although one might be tempted to say that selection is acting in the second case on varieties rather than on individuals, the population sizes of the two varieties behave in exactly the same way under the two models. Thus there is no operational distinction in the absence of geographical isolation between selection on individuals and selection on varieties, provided that complications due to blending heredity are ignored. It may be remembered that the subtitle of the Origin was The preservation of favoured races in the struggle for life.

I therefore follow Kottler in rejecting Bowler's conclusion that there are two levels of selection, Darwinian selection on individuals and a second level of selection on varieties, and that Wallace was only concerned with the second level. Bowler cites three passages in the Origin in which Darwin considers competition between varieties.21 The first and third are concerned with what happens when varieties meet that have developed in different directions under selection in geographical isolation. The second develops the idea of divergence of character. None of them is relevant to Wallace's much simpler argument in the Ternate paper.

Nevertheless, Osborn and Bowler were correct in observing that Wallace placed exclusive emphasis on the population sizes of varieties, understood as sets of individuals that ‘propagate their like’ for some character in which they differ from each other. I contend that Wallace had the same idea as Darwin of competition between individuals, but that he differed from Darwin because he thought that an inferior could coexist with a superior variety, though at a lower population size, until it was forced into extinction by an environmental deterioration. He was led to this conclusion by analogy with the relative abundances of closely related species.

Jean Gayon has argued that Wallace and Darwin had ‘similar theories but different hypotheses’ of natural selection, in that ‘Darwin…conceived of competition as taking place between individuals. Wallace, on the other hand, thought that the “struggle for existence” involved competition between populations.’22 This interpretation is similar to Bowler's, which has been discussed above. I would argue that Darwin and Wallace had similar hypotheses of natural selection but that Wallace misinterpreted the consequence of this hypothesis through misunderstanding the population ecology of competing species.

Gayon also suggested that ‘from the point of view of contemporary debates about the “units” and “levels” of selection, the different ideas of Darwin and Wallace are reminiscent of current discussions as to the possibility of a “group selection” different and separate from the “individual selection” that Darwin defended so vigorously.’23 Michael Ruse defined group selection as ‘selection in some way causing characteristics which help others, including non-relatives, in an individual's group. …There is not necessarily any hope of return for the individual.’24 He showed that Darwin relied almost exclusively on individual as opposed to group selection in this sense, though he used the latter to explain the evolution of the moral sense in man, tribes containing many members with a high moral sense being victorious over other tribes.25 But this is not relevant to the characters considered by Wallace in the Ternate paper, such as a giraffe with a long neck or a cryptically coloured insect, which are clearly advantageous to the individuals possessing them rather than to other individuals in the group.

Conclusion

When he afterwards discussed how he came to discover natural selection, Wallace wrote as if he had adopted the Darwinian viewpoint in his Ternate paper. In his autobiography in 1905, after describing how he had thought of the Malthusian checks to population increase while he was suffering from an attack of intermittent fever, he continued:It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more rapidly than does mankind, the destruction every year from these causes must be enormous in order to keep down the numbers of each species. …Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, Why do some live and some die? And the answer was clearly, that on the whole the best fitted live. From the effects of disease the most healthy escaped; from enemies, the strongest, the swiftest, or the most cunning; from famine, the best hunters or those with the best digestion; and so on. Then it suddenly flashed upon me that this self-acting process would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior remain—that is, the fittest would survive. Then at once I seemed to see the whole effect of this, that when changes of land and sea, or of climate, or of food-supply, or of enemies occurred…and considering the amount of individual variation that my experience as a collector had shown me to exist, then it followed that all the changes necessary for the adaptation of the species to the changing conditions would be brought about. …I waited anxiously for the termination of my fit so that I might at once make notes for a paper on the subject. The same evening I did this pretty fully, and on the two succeeding evenings wrote it out carefully in order to send it to Darwin by the next post, which would leave in a day or two.26

He wrote in similar Darwinian terms in three other accounts of his discovery of natural selection.27

This reconstruction of the theory of natural selection, written long after Wallace became familiar with Darwin's development of the theory in the Origin of Species, adopts the Darwinian argument that an advantageous variation would gradually increase and its parental form would decrease in frequency until it became extinct. But it does not agree with the chain of arguments developed in the Ternate paper, which concludes that an advantageous variation and its parental form could coexist, although with the former more numerous than the latter, until an environmental deterioration forced the extinction of the parental form.

With the benefit of hindsight we can see that the latter is less satisfactory than Darwin's theory; one would expect an inferior variety to be eliminated through competition with a superior variety in a stationary population (Darwin's theory) rather than having to wait for a population crash (Wallace's original theory). It is not surprising that Wallace's original theory was imperfect given that the Ternate paper was conceived and written in three days, whereas Darwin's theory of natural selection in the Origin was perfected over 20 years. But it is puzzling that Wallace never recognized the difference between his original theory and Darwin's theory.

Acknowledgements

I thank the referee for valuable suggestions.

    Notes

    1. In the first edition of his Journal of Researches published in 1839, Darwin wrote, in connection with his discovery of the fossil bones of Machrauchenia in Patagonia, of ‘the confirmation of the law that existing animals have a close relation in form with extinct species. . The law of the succession of types, although subject to some remarkable exceptions, must possess the highest interest to every philosophical naturalist’ in the second edition, published in 1845, he wrote, ‘This wonderful relationship in the same continent between the dead and the living will, I do not doubt, hereafter throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts.’

    2. In the Sarawak paper, Wallace wrote of new species ‘coming into existence’ or ‘being formed’ or ‘being created’ without any indication of how this happened. Darwin noted on his copy of the paper: ‘Laws of Geograph. Distrib. nothing very new—.It is all creation, but why does his law hold good. .I should state that put generation for creation & I quite agree.

    3. On the tendency of varieties to depart indefinitely from the original type.

    4. Throughout the Sarawak paper Wallace used the archaic word ‘antitype’ instead of ancestor, although its dictionary meaning is the opposite; likewise he used the word ‘prototype’ instead of descendant, whereas it should mean ancestor. This was probably part of his strategy to avoid discussing transmutation, although it is unfortunate that he reversed the meaning of these two words.

    5. For Darwin's principle of divergence see

    6. Darwin also used a group selection argument as a possible explanation for a male–biased sex ratio in some animals: ‘With animals living in herds or troops, in which the males come to the front and defend the herd, as with the bisons of North America and certain baboons, it is conceivable that a male–producing tendency might be gained by natural selection; for the individuals of the better defended herds would leave more numerous descendants’

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