278 Prof. S. F. Baird on the Distribution and 



other table ; but this has been in most cases the result of recent 

 determinations and different identifications from those of other 

 authors. 



Having thus briefly indicated the boundaries of the principal 

 provinces of the North American ornithological fauna, I propose 

 to call attention to some generalizations that have suggested 

 themselves in reference to certain influences exerted upon species 

 by their distribution according to latitude, longitude, and eleva- 

 tion, and by their association with each other. The most im- 

 portant of these is the law that North American birds of 

 wide distribution in latitude, whether migrant or residents, 

 will be found to be larger the higher the latitude of their place 

 of birth. 



It is well known to zoologists, that of all animals, birds are 

 most constant in their dimensions — so much so, indeed, that 

 size is generally considered a most important specific character. 

 The comparison of many specimens of the same species from 

 widely remote localities has shown me, however, that there is a 

 certain variation in size, dependent on the extension northward 

 and southward of the limits of distribution during the breeding- 

 season, the more northern being the larger, the more southern 

 the smaller. Nor does this depend upon a greater development 

 of body by more constant use of the muscular system in flight, 

 as suggested by Gloger, who observed the same fact in Europe 

 (but confined especially to the increase in length of wings and 

 tail), or upon a greater variety or amount of food, since the dif- 

 ference is as strongly marked in species constantly resident, as 

 in those which migrate over great distances ; and the develop- 

 ment extends to the bill, feet, and all parts of the body. And, 

 in fact, birds most remarkable for their great range show the 

 least variation in size, while the variation is most evident in 

 certain species of Woodpeckers, as Picus villosus and Hylotomus 

 pileatus, which have a very wide distribution in latitude, without 

 any special migration at all. In these Woodpeckers the dif- 

 ference between specimens from Florida and from Canada is so 

 great as to have given rise to the impression of there being 

 several species of each, differing in size. 



In nearly every instance where I have compared summer 



