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 formsof sexual selection 
through which the sexual instincts and the correlated sexual charac- 
ters of the different sexes are kept in full codrdination. 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 of 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. XL, 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- 
