258 APPENDIX in — LETTERS PUBLISHED IN NATURE. 



papers on "Divergent Evolution through Cumulative Segregation" 

 and "Intensive Segregation" I have endeavored to show that there 

 must be several principles somewhat similar to sexual selection, which 

 I have grouped with it under the names reflexive segregation and 

 reflexive selection. In the former of these papers, pages 212-214, I 

 have pointed out that of freely crossing forms of any species it is 

 only those that are most successful that are perpetuated; while of 

 forms that have by isolation escaped from competition with the 

 original stock and are not crossing with it every variation is perpet- 

 uated that is not fatally deficient in its adaptations to the environ- 

 ment; and this will be the case whether the forms are held apart by 

 reflexive or environal segregation. 



2. A Difference in Use that is not a Useful Difference. 



Let us consider the case of two allied species occupying the same 

 area, and differing from each other in what Dr. Wallace has so appro- 

 priately called their recognition marks, and in the segregating sexual 

 and social instincts correlated with these marks. If investigation 

 justifies the belief that an early stage of divergence, due, perhaps, to 

 local segregation, resulted not only in sexual and social segregation, 

 but also in what I have called divergent social selection (or what Dr. 

 Wallace prefers to call selective association), then we are warranted 

 in the belief that this segregative and selective principle was sufficient 

 to perpetuate and intensify the new character, although the section of 

 the species possessing the new character had not migrated into any 

 new environment, and had not been exposed to any change in the old 

 environment, and although it had not gained any new adaptation to 

 the common environment of the two sections and, therefore, while 

 both sections of the species were equally subject to identical forms of 

 natural selection. 



Now, seeing that the individuals of the segregated sections are able 

 to find and keep company with associates, and in the season to pair 

 with suitable mates, as effectually, but no more effectually, than be- 

 fore they were segregated, what shall we say of the usefulness of the 

 distinctive characters that produce the segregation? It is plain that 

 these divergent characters are in constant use ; but does that prove 

 that the divergence is a useful divergence? Is it not possible that there 

 should be a difference in use which is not a useful difference? And if 

 nothing has been gained by the difference either in maintaining the con- 

 ditions of individual life, or in propagating the species, how can we call 

 it a useful difference? And how can we attribute the divergence to 

 natural selection, seeing that natural selection is the superior mainte- 



