292 MONTGOMERY. PVOL. XII 
species evinces a greater amount of variation than the “stable” 
species ; and that species which undertake extensive migra- 
tions exhibit more individual variation, other factors being 
equal, than do species which do not migrate. Now in the first 
part of this second section, we have found that the presence 
of geographical races (subspecies) and migration are two 
criteria of continuing development. Therefore, the fact that 
the amount of variation is greater in migratory species, and in 
species which exhibit geographical races, than in non-migratory 
species, or than in species which present no geographical sub- 
species, is a sufficient proof for the assertion that continuing 
development is always associated with variability. In other 
words, individual variation is greater in amount in those 
species which we must consider under the influence of a con- 
tinuing process of development than in those species which 
we must consider as being influenced by no process of develop- 
ment at all, or by a much less energetic development. And as 
we have found variation is as a rule more marked in migratory 
than in non-migratory species, and in extensively migratory 
than in less migratory species ; and further, that as a rule, 
greater individual variation is present in the several races of a 
species which exhibits a large number of races than in the 
races of a species exhibiting a smaller number, — therefore we 
must conclude that the amount of individual variation stands 
in a direct ratio to the activity and energy of the operating 
process of development. In short, the logical sequence from 
the facts given is plainly that the amount of individual 
variation stands in a direct ratio to the degree of complex- 
ity of the environmental forces which influence the organism. 
A species with an extensive breeding area, or one which 
meets with environmental changes in the course of its 
migrations, is more variable than a species with a restricted 
and little diversified breeding area, or than a non-migratory 
species, which does not come into contact with new environ- 
ments. 
The fact that the dimensions of birds are more variable in 
the males than in the females is interesting, as offering a 
parallel to the case in man, where, too, the males are more 
