1891. ] NEW YORK ACADEMY OF SCIENCES. 79 
to-day. A third period, differing much from either of the two 
preceding, has now opened ; its culmination and final results 
cannot as yet be foreseen. 
During the Bairdian period of North American mammalogy, 
the discovery of new forms was the paramount incentive to in- 
vestigation. The method of study was hence intensely analytic ; 
differences, rather than resemblances, filled the mind of the in- 
vestigator. At this period the subject of variation under cli- 
matic influences, the evolution of species by environment, had 
attracted little attention. But the facts of evolution were being 
minutely recorded ; a harvest was being reaped which was to 
form the basis of important generalizations, as then unsuspected 
even by the most industrious of the reapers. But they did their 
work well, and it is not to their discredit that they failed to see 
at first some of the fundamental principles of the evolution of 
life under the influences of environment. We have always first 
to gather our facts before we can generalize. Professor Baird 
himself, however, was the first to perceive and formulate some 
of the fundamental laws of geographical variation, as regards 
both North American birds and mammals, in the study of which, 
be it said to the credit of American naturalists, such princi- 
ples were first recognized and established. Buird early perceived 
that, as a rule, individuals of the same species decreased in size 
from the north southward ; that the animals from the arid plains 
were paler in color than their nearest relatives of the wooded 
region to the eastward; and that over the heavy rainfall belt of 
the Northwest they assumed a depth of coloring met with no- 
where else on the continent. He also recognized that, in some 
instances, the size of peripheral parts, especially in birds, in- 
creased in size from the north southward, while the general size of 
the individual decreased. Later it was also noticed that, asa rule, 
individuals of the same species became brighter colored at the 
southward as compared with their northern relatives—that black 
bars and streaks were broadened at the expense of the interven- 
ing lighter spaces; that birds with metallic tints became more 
iridescent, and that the duller colors, of both birds and mammals, 
became deepened and intensified. ’ 
In subsequent years, as material increased and large series of 
specimens of the same species and from the same locality were 
brought together, it was found that the range of purely individ- 
ual variation, in respect to every feature, was far greater than 
had previously been suspected. ‘This, of course, seemed to in- 
validate certain characters on which species had previously often 
_!' See Allen,‘‘ Mammals and Winter Birds of East Florida,” Bull. Mus. 
Comp. Zo6l., IT., No. 3, 1871. 
