262 MONTGOMERY. [Vor Xi. 
that where the adjoining areas of geographical races overlap, 
the struggle for existence would be keenest, so that the indi- 
viduals occupying this intermediate area would in time become 
extinct. But he has overlooked the fact that until such exter- 
mination has been brought about, z.e. as long as the races con- 
tinue to intergrade, the intermediate area would continue to be 
the vortex of development, and the sharp struggle there would 
itself instigate the wandering of individuals into the adjoining 
areas, where again they must adapt themselves to new environ- 
ments. Now, when in any species the individuals occupying 
the areas transitional between those proper to the several races 
have become exterminated, the races must cease to intergrade, 
fewer individuals will continue to wander into other areas, and, 
the struggle for existence becoming less sharp, the main factor 
in the process of development would disappear. But, as we 
have shown above, when the races cease to intergrade, and 
become more distinctly pronounced, they can sensu stricto be 
no longer termed races, but rather distinct species. Accord- 
ingly, the presence of geographical races being correctly con- 
sidered a criterion of continuing development, we should expect 
our data to show, as indeed they do, that, other factors being 
equal, species with geographical subspecies evince a greater 
amount of variation than do species which present no geograph- 
ical races. 
It is noteworthy that extensive periodical migrations act as 
a restraint upon the production of geographical races. And 
I account for this fact! by assuming that the migratory 
species, being influenced in winter by an environment to some 
extent different from that which it experiences in summer, must 
be equally adapted to both environments (equally, at least, if it 
remains under the influence of both environments for the same 
length of time), and hence, the winter environment exerting a 
restraint upon the production of adaptations suited to the 
summer environment alone, such a migratory species would 
not be capable of producing geographical races to the same 
1In a paper which has not yet appeared, but which will be published in the 
American Naturalist for June, 1896, dealing with migration as a check upon 
geographical variation in birds. 
