536 



NA TURE 



[April lo, 1890 



I cannot allow the present communication to appear in these 

 columns without again recording my conviction that the writer 

 is the most profound of living thinkers upon Darwinian topics, 

 and that the generalizations which have been reached by his 

 twenty years of thought are of more importance to the theory of 

 evolution than any that have been published during the post- 

 Darwinian period. George J. Romanes. 



London, March 10. 



I FOLLOW Prof. Lankester in the use of bionomics to designate 

 the science treating of the relations of species to species. If 

 the theory of evolution is true, bionomics should treat of the 

 origin, not only of species, but of genera, and the higher groups 

 in which the organic world now exists. 



In his very suggestive review of " Darwinism," by Mr. A. R. 

 Wallace, in Nat ure of October 10, 1889 (p. 566), Prof. Lankester 

 refers to " his (Mr. Wallace's) theory of the importance of the 

 principle of ' like to like' in the segregation of varieties, and the 

 consequent development of new species." Prof. Lankester has 

 here alluded to a principle which I consider more fundamental 

 than natural selection, in that it not only explains whatever 

 influence natural, selection has in the formation of new species, 

 but also indicates combinations of causes that may produce 

 new species without the aid of diversity of natural selection. 

 The form of like to like which Mr. Wallace discusses is " the 

 constant preference of animals for their like, even in the case of 

 slightly different varieties of the same species," which is con- 

 sidered not as an independent cause of divergence, hut as pro- 

 ducing isolation which facilitates the action of natural selection. 

 If he had recognized this principle, which be calls selective 

 association, as capable of producing in one phase of its action 

 sexual and social segregation, and in another phase sexual and 

 social selection, he would perhaps have seen that its power to 

 produce divergence does not depend on its being aided by 

 natural Felection. 



Mr. Wallace's view is very clearly expressed in the following 

 passages, though I find other passages which lead me to think 

 that the chief reason he does not recognize segregation as the 

 fundamental principle in divergence is that he has not observed 

 its relations to the principle of like to like. He says : — " A great 

 body of facts on the one hand, and some weighty arguments on the 

 other, alike prove that specific characters have been, and could 

 only have been, developed and fixed by natural selection because 

 of their utility " (" Darwinism," p. 142). " Most writers on the 

 subject consider the isolation of a portion of a species a very 

 important factor in the formation of new species, while others 

 maintain it to be absolutely essential. This latter view has 

 arisen from an exaggerated opinion as to the power of inter- 

 crossing to keep down any variety or incipient species, and 

 merge it in the parent stock " (" Darwinism," p. 144). 



I think we shall reach a more consistent and complete ap- 

 prehension of the subject by starting with the fundamental laws 

 of heredity, and refusing to admit any assumption that is opposed 

 to these principles, till sufficient reasons have been given. Laws 

 which have been established by thousands of years of experiment 

 in domesticating plants and animals, should be, it seems to me, 

 consistently applied to the general theory of evolution. P'or 

 example, if in the case of domesticated animals, "it is only by 

 isolation and pure breeding that any specially desired qualities 

 can be increased by selection " (see "Darwinism," p. 99), why is 

 not the same condition equally essential in the formation of natural 

 varieties and species? If in our experiments we find that careful 

 selection of divergent variations of one stock does not result in 

 increasingly divergent varieties unless free civssing be ween the 

 varieties is prevented, why should it be considered an exaggeration 

 to hold that in wild species " the power of intercrossing to keep 

 down any variety or incipient species, and merge it in the parent 

 stock," is the same. Experience shows that segregation, -which is 

 the bringing of like to like in groups that arc prevented from 

 crossing, is the fundamental principle in the divergence of the 

 various forms of a given stock, rather than selection, which is like 

 to like throrigh the pr,vcnticn of certain forms from propagating : 

 and I think we introduce confusion, perplexity, and a network 

 of inconsistencies into our exposition of the subject, whenever we 

 assume that the latter is the fundamental factor, and especially 

 when we assume that it can produce divergence without the co- 

 operation of any cause of segregation dividing the forms that propa- 

 gate into two or more groups of similars, or when we assume that 

 segregation and divergence cannot be produced without the aid 

 of diverse forms of selection in the difierent groups. The theory 



of divergence through segregation states the principle through 

 which natural selection becomes a factor promoting sometimes 

 the stability and sometimes the transformation of types, but never 

 producing divergent transformation except as it co-operates with 

 some form of isolation in producing segregation ; and it main- 

 tains that, whenever variations whose ancestors have freely inter- 

 generated are from any combination of causes subjected to 

 persistent and cumulative forms of segregation, divergence more 

 or less pronounced must be the result. The laws of heredity on 

 which this principle rests may be given in the three following 

 statements : — 



(i) Unlike to unlike, or the removal of segregating influences, 

 is a principle that results either in extinction through failure to 

 propagate, or in the breaking down of divergences through free 

 crossing. 



(2) Like to like, when the individuals of each intergeneVating 

 group represent the average character of the group, is a principle 

 through which the stability of existing types is promoted. 



(3) Like to like, when the individuals of each group represent 

 other than the average character of the group, is a principle 

 through which the transformation of types is effected. 



In my paper on " Divergent Evolution" (Linn. Soc. Journ., 

 Zoology, vol. XX. pp. 189-274), I pointed out that sexual and 

 social instincts often conspire together to bring like to like in 

 groups that do not cross, and that in such cases there will be 

 divergence even when there is no diversity of natural selection in 

 the different groups, as, for example, when the different groups 

 occupy the same area, and are guided by the same habits ir> 

 their use of the environment. There is reason to believe that 

 under such circumstances divergence often arises somewhat in 

 the following way. Local segregation of a partial nature results 

 in some diversity of colour or in some peculiar development of 

 accessory plumes, and through the principle of social segregation,, 

 which leads animals to prefer to associate with those whose 

 appearance has become familiar to them, the variation is pre- 

 vented from being submerged by intercrossing. There next 

 ari es a double process of sexual and social selection, whereby 

 both the peculiar external character and the internal instinct 

 that leads those thus characterized to associate together are 

 intensified. The instinct is intensified, because any member of 

 the community that is deficient in the desire to keep with com- 

 panions of that kind will stray away and fail of breeding with the 

 rest. This process I call social selection. The peculiarity of 

 colour or plumage is preserved and accumulated, because any 

 individual deficient in the characteristic is less likely to succeed 

 in pairing and leaving progeny. This latter process is sexual 

 selection. It can hardly be questioned that both these principles 

 are operative in producing permanent varieties and initial 

 species ; and in the circumstances I have supposed, I do not see 

 how the process can be attributed to natural selection. Varieties 

 thus segregated may often develop divergent habits in their use 

 of the environment, resulting in divergent forms of natural 

 selection, and producing additional changes ; but so long as 

 their habits of using the environment remain unchanged, their 

 divergencies cannot be due to natural selection. 



Mr. Wallace's very interesting section on "Colour as a 

 Means of Recognition," taken in connection with the section 

 on " Selective Association," already referred to, and another 

 on " Sexual Characters due to Natural Selection," offers an 

 explanation of "the curious fact that p ominent differences of 

 colour often distinguish species otherwise very closely allied to 

 each other" (p. 226). Ilis exposition differs from mine in that 

 he denies the influence of sexual selection, and attributes the 

 whole process to natural selection, on the ground that "means 

 of easy recognition must be of vital importance " (p. 217). The 

 reasoning, however, seems to me to be defective, because the 

 general necessity for means of easy recognition is taken ai^ 

 equivalent to the necessity for a specialization of recognition 

 marks that shall enable the different varieties to avoid crossing. 

 In the cases I am considering, there is, however, no advantage 

 in the separate breeding of the different varieties, and even in 

 cases where there is such an advantage (as there would be if the 

 variety had habits enabling it to escape from competition with 

 the parent stock, but only partially preventing it from crossing 

 with the same), it does not appear how this advantage can pre- 

 vent the individual that ii defective in the special colouring from 

 following and associating with those that are more clearly marked. 

 The significant part of the process in the development of recogni- 

 tion marks must be in the failure of such individuals to secure 

 mates, which is sexual selection ; or in the unwillingness of th« 



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