Download The Theory of Natural Selection of Alfred Russel Wallace FRS

Notes Rec. R. Soc. (2005) 59, 125–136
doi:10.1098/rsnr.2004.0081
Published online 15 May 2005
THE THEORY OF NATURAL SELECTION OF
ALFRED RUSSEL WALLACE FRS
by
MICHAEL BULMER
The Old Vicarage, Chittlehampton, Umberleigh EX37 9RQ, UK
([email protected])
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.
Keywords: Alfred Russel Wallace; Charles Darwin; natural selection;
Ternate paper
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
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
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M. Bulmer
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-byparagraph 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
Wallace’s theory of natural selection
127
Figure 1. Portrait of Wallace from his autobiography My Life, Vol. 1 (Chapman and Hall, London, 1905). Copyright
q Royal Society.
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
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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.
Wallace’s theory of natural selection
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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 K1Z0, 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
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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].11
He 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
Wallace’s theory of natural selection
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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 versus environmental 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.14
In 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
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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.17
E. 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 lifepreserving powers; and the varieties so furnished might run a course parallel with the parent
Wallace’s theory of natural selection
133
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
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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
Wallace’s theory of natural selection
135
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
2
3
4
5
6
7
8
9
10
B. G. Beddall, ‘Wallace, Darwin, and the theory of natural selection: a study in the development
of ideas and attitudes’, J. Hist. Biol. 1, 261–323 (1968); J. L. Brooks, Just before the origin
(Columbia University Press, New York, 1984); H. L. McKinney, Wallace and natural selection
(Yale University Press, New Haven, CT, 1972).
A. R. Wallace, ‘On the law which has regulated the introduction of new species’, Ann. Mag. Nat.
Hist. 16, 184–196 (1855). 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.’
See Darwin on evolution (ed. T. F. Glick and D. Kohn) (Hackett, Indianapolis, 1996), pp. 17–18;
C. Darwin, The voyage of the Beagle (Harmsworth Library, London, 2nd edn 1845, reprinted
1905), p. 170.
McKinney, op. cit. (note 1). 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.’ See The correspondence of Charles Darwin (ed. F. Burkhardt and S. Smith), vol. 5
(Cambridge University Press, Cambridge, 1989), p. 522. Darwin later wrote of the Sarawak law:
‘I now know from correspondence, that this coincidence he [Wallace] attributes to generation
with modification.’ See C. Darwin, On the origin of species (Murray, London, 1859), p. 355.
Wallace, A. R. (1905). My Life, vol. 1, p. 360, Chapman & Hall, London p. 360.
Wallace, A. R. (1859). On the tendency of varieties to depart indefinitely from the original type.
Proc. Linn. Soc. Lond. 3, 53–62. For accounts of the well-known story, see references in note 1.
Burkhardt, F. & Smith, S., eds (1991). The correspondence of Charles Darwin, vol. 7, p. 107,
Cambridge University Press, Cambridge.
Wallace, A. R. (1891). Natural selection and tropical nature. Macmillan, London p. 20.
M. J. Kottler, ‘Charles Darwin and Alfred Russel Wallace: two decades of debate over natural
selection’, in The Darwinian heritage (ed. D. Kohn) (Princeton University Press, 1985), pp. 367–
432; A. J. Nicholson, ‘The role of population dynamics in natural selection’, in Evolution after
Darwin (ed. S. Tax), vol. 1 (Chicago University Press, 1960), pp 477–522; P. J. Bowler, ‘Alfred
Russel Wallace’s concepts of variation’, J. Hist. Med. 31, 17–29 (1976); J. Gayon, Darwinism’s
struggle for survival (Cambridge University Press, 1998), pp. 19–59.
Lyell, C. (1832). Principles of geology, vol. 2. Murray, London pp. 23 and 65.
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
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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.
Wallace, op. cit. (note 2), p. 186.
Wallace, op. cit. (note 2), p. 188.
For Darwin’s principle of divergence see D. Ospovat, The development of Darwin’s theory
(Cambridge University Press, Cambridge, 1981); D. Kohn, ‘Darwin’s principle of divergence as
internal dialogue’, in The Darwinian heritage (ed. D. Kohn) (Princeton University Press,
Princeton, 1985), pp. 245–257; S. J. Gould, The structure of evolutionary theory (Harvard
University Press, Cambridge, MA, 2002), pp. 224–250. A. C. Brackman, A delicate arrangement
(Times Books, New York, 1980), and Brooks, op. cit. (note 1), have suggested that Darwin stole
the principle of divergence from Wallace, but this conspiracy theory is no longer taken seriously.
See Gould, op. cit. (above); D. Kohn, ‘On the origin of the principle of diversity’, Science 213,
1105–1108 (1981); Burkhardt and Smith, op. cit. (note 6), pp. xvii–xviii.
Nicholson, op. cit. (note 8), p. 491.
Kottler, op. cit. (note 8), pp. 371–374; B. Wallace, Topics in population genetics (Norton,
New York, 1968).
Bowler, op. cit. (note 8), pp. 18–19.
H. F. Osborn, From the Greeks to Darwin: an outline of the development of the evolution idea
(Columbia University Press, New York, 1894 ) p. 245, quoted in Kottler, op. cit. (note 8) p. 378.
E. B. Poulton, Charles Darwin and the theory of natural selection (Cassell, London, 1896), p. 80,
quoted in Kottler, op. cit. (note 8), pp. 378–379.
Kottler, op. cit. (note 8).
Kottler, op. cit. (note 8), p. 376; Brooks, op. cit. (note 1), pp. 50–51.
Bowler, op. cit. (note 8), p. 23.
Gayon, op. cit. (note 8), p. 25.
Gayon, op. cit. (note 8), p. 31.
Ruse, M. (1980). Charles Darwin and group selection. Ann. Sci. 37, 615–630.
Ruse, op. cit. (note 24), pp. 626–627. 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.’ C. Darwin, The descent of man and selection in relation to sex (Murray, London,
2nd edn 1874, reprinted 1913), p. 399. In the first edition Darwin suggested that equality of
the sex ratio evolved because of a reproductive advantage to the rarer sex, in line with modern
theory, but he abandoned this idea in the second edition. See A. W. F. Edwards, Natural selection
and the sex ratio: Fisher’s sources, Am. Nat. 151, 564–565 (1998).
Wallace, op. cit. (note 4), pp. 361–363.
McKinney, op. cit. (1), pp. 160–163.