136 
Rev. J. T. Gulick on the 
from generation to generation by a law that is quite distinct 
from natural selection. It was also shown that endowments 
only partially segregative (as, for example, somewhat diver- 
gent habits of feeding), when not concurrent with any forms 
of cross incompatibility, are liable to be obliterated by 
crossing ; but, when associated with segregate fertility and 
cross infertility, will increase from generation to generation, 
even if the mongrels are as well adapted to the environment 
as the pure forms. I at the same time called attention to the 
fact that, when associated with some form of partial positive 
segregation (as divergent habits of feeding or segregative 
sexual and social instincts), greater vigour of pure forms, as 
contrasted with the mongrels, would have the same effect as 
their greater fertility. In other words, Segregate Vigour 
would preserve a partially segregated variety as effectually as 
Segregate Fecundity. 
Incompatibilities will disappear unless preserved by 
Positive Segregation . 
Mr. Wallace has given a very instructive computation on 
pages 181-184 ; but it does not seem to me to prove, as he 
supposes, that infertility between the individuals of a species 
cannot increase u unless correlated with some useful variation,” 
but that it cannot arise, except as a transitory variation, 
unless associated with some positively segregative principle, 
causing those to pair together which are fertile with each 
other. My contention is that, without some positive form of 
segregation, fecundity and cross sterility can never arise, and 
that, after it has arisen under segregation, no amount of corre- 
lation with useful variation will preserve it if the positive 
segregation is removed. If, for example, all the species of 
humming-birds were brought together in one country, and 
w T ere deprived of all segregative habits and instincts, it cer- 
tainly would not require many generations to reduce them to 
one species. If equally adapted to the environment, the 
species that would succeed in perpetuating itself would be the 
one represented by the largest number of individuals ; or, if 
several species were entirely cross fertile and were in the 
aggregate represented by a larger number of individuals than 
any other similar group of species or than any single species, 
then the resulting species would be the hybrid descendants of 
this most numerous group. All the other species would be- 
come extinct through failing to mate with u physiological 
complements.” 
