28 ZOOLOGY. 



tudinal variation.* In respect to both, differentiation occurs in different 

 decrees in different groups, in accordance with their general tendency to 

 variation, or, as it were, in proportion to their normal degree of plasticity. 

 In regard to variation with latitude, the modifications are apparently more 

 general than in what I have termed longitudinal variation : the differentia- 

 tion affects not merely color but size and the details of structural parts, 

 whereas color appears to be the main element affected by longitudinal vari- 

 ation. The fact of variation in size has been conceded as a general law by 

 the majority of at least American ornithologists and mammalogists, since it 

 was so fully established by Prof. S. F. Baircl, in 1857 and 1858, in his admi- 

 rable reports on the Mammals and Birds of North America, published in the 

 series of the Government reports on the exploration and survey of the vari- 

 ous Pacific Railroad routes. 



Professor Baird then and subsequently called attention to the fact of the 

 greater length of the tail in several specimens of birds at certain localities, 

 and cites instances of the larger size of the bill at southern points, and the 

 paler color of the plumage of the birds of the plains and the arid peninsula of 

 Lower California. All his subsequent works have furnished numerous cita- 

 tions of similar variation with locality; but, instead of insisting upon any com- 

 mon tie connecting these phenomena as the result of general laws, they are 

 viewed as evidences of specific differentiation. The differences are indeed so 

 great between many of the forms now known to intergrade that it is not sur- 

 prising that they were regarded as different species when known from only a 

 few examples, apparently unconnected by any intermediate form. Subse- 

 quently, however, it has been found that they are not trenchantly separated, 

 intermediate forms so linking them together that they can be only vaguely 

 diagnosed. These connecting links, inhabiting at least in the breeding- sea- 

 son localities intermediate in geographical position and in climatic condi- 

 tions to those frequented by the more extreme forms, suggest an intimate 

 genetic relationship and a differentiation mainly or wholly through climatic 

 influence, or the diverse conditions of environment. 



" Latitudinal variation presents the following phenomena, which are of 



* See Bulletiu of the Museum Comparative Zoology, vol. ii, April, 1871. 



