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 Lankester’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, 
an that rt 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 ard 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 tolike. Hesays: ‘‘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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