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- 

 Testigation. 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 tlien 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. Baird 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 ef 

 the individual decreased. Later it was also noticed that, as a 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, vvas far greater than 

 had previously been suspected. This, of course, seemed to in- 

 Talidate certain characters on which species had previously often 



' See Allen," Mammals and Winter Birdsof East Florida," Bull. Mus. 

 Comp. Zoo)., II., No. 3, 1871. 



