APPENDIX III. 



LETTERS PUBLISHED IN NATURE. 

 I. "LIKE TO LIKE" -A FUNDAMENTAL PRINCIPLE IN BIONOMICS.* 



I follow Professor Lankester in the use of the term 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 Professor I/ankester's very suggestive review of "Darwinism," 

 by Mr. A. R. Wallace (Nature, October 10, 1889, p. 566), reference is 

 made 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." Professor 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 considered not as an independ- 

 ent cause of divergence, but as producing isolation which facilitates 

 the action of natural selection. If he had recognized this principle, 

 which he 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 

 selection. 



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

 ages, though I find other passages which lead me to think that the 

 chief reason he does not recognize segregation as the fundamental prin- 

 ciple in divergence is that he has not observed its relations to the prin- 

 ciple 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 



* Published in Nature, April 10, 1890. 



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