254 APPENDIX III LETTERS PUBLISHED IN NATURE. 



form incipient species, and each belongs to the environment of the other. 

 While they are members of the same intergenerating group, their 

 mutual influence results in reflexive selection, which maintains the 

 correspondence with each other by which power to cross is preserved ; 

 while they are members of groups that do not cross, their mutual influ- 

 ence results in cumulative segregation ; for it inevitably tends toward 

 the preservation of variations that, through greater divergence, best 

 escape from competition. I have elsewhere defined reflexive selection 

 as being the exclusive propagation of those better fitted to the rela- 

 tions in which the members of the same species stand to each other, 

 resulting from the failure to propagate of those less fitted. Among 

 those that are equally fitted to the environment of the species, and, 

 therefore, equally preserved by natural selection, there is often great 

 difference in the degrees of fitness for sustaining such relations to the 

 rest of the species as will secure an opportunity to propagate. To 

 this class of influences belong the different forms of sexual selection 

 through which the sexual instincts and the correlated sexual charac- 

 ters of the different sexes are kept in full coordination. In like man- 

 ner we must believe that the pollen of any species is kept up to its full 

 degree of potency by the constant selection which results from the 

 failure to propagate of the individuals whose pollen is less potent or 

 whose germs are more difficult to fertilize than the average. We call 

 this potential selection. Again, there is a constant selection of ani- 

 mals that are suitably endowed with the recognition marks and calls 

 by which the different members of the species know each other, and 

 that have the corresponding instincts leading them to associate with 

 their own kind. I have elsewhere called this principle of social 

 coordination "social selection," and have classed it as a form of 

 reflexive selection. 



2. The Cessation o] Reflexive Selection between Isolated Sections Causes 

 Divergence as Soon as Heredity Weakens. 



Independent breeding is in its very nature the suspension, not only 

 of one form, but of all forms of reflexive selection between the separated 

 portions of the species. The importance of the cessation of natural 

 selection in producing the different stages of the degeneration of organs 

 that are disappearing has been fully discussed by Professor Romanes 

 (see Nature, vol. XLI, p. 437, and previous communications there re- 

 ferred to), who points out that, as the power of the special form of 

 heredity by which any organ is produced has been built up by the 

 many generations of natural selection that have acted on the organ, so 

 the gradual weakening of that power follows the cessation of the natu- 

 ral selection. Professor Weismann seems to appeal to the same prin- 



