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



of supposed species to this rank, and have aided in materially 

 diminishing the accepted number of species of both mammals 

 and birds. 



Another fact which may be mentioned in reference to birds of 

 the diflferent provinces is, that specimens from the Pacific coast 

 are apt to be darker in colour than those from the interior^ the 

 latter frequently exhibiting a bleached or weather-beaten ap- 

 pearance, possibly the result of greater exposure to the elements 

 and less protection by dense forests. 



In a careful study of large series of birds of any two repre- 

 sentative species collected near the line of junction of their re- 

 spective provinces, a combination of characters of both species 

 will often be met with, explicable only on the supposition of the 

 hybridization of the two. Whether such hybrids are themselves 

 fertile, or whether the cross is kept up by the constantly 

 recurring union of individuals of pure breed of either species, I 

 am not prepared to say ; but the general facts appear to be as 

 stated. A notable instance of this is seen in the two northern 

 species of Colaptes, one, C. mexicanus, characterizing the Western 

 and Middle provinces, the other, C auratus, the Eastern. The 

 lines of distribution of the two intersect on the upper Missouri 

 near the mouth of the Yellowstone River ; and all along that 

 portion of its course we find Colaptes of every possible grade of 

 transition, or combination of the several characters of the two 

 species, scarcely any two exactly alike, and the same individual 

 not even agreeing in the markings of opposite sides'^. A 

 similar combination of characters of Cyanura stelleri, and C. 

 maci'olopha is met with on the headwaters of the Columbia, and 

 on the Yukon, of Junco hyemalis and J. oregonus, and of Hel- 

 minthophaga celata, and H. peregrina. Other instances can be 

 adduced ; but these will be sufficient to illustrate the facts. 



The possibility of hybridity as referred to, is another element 

 to be taken into consideration in discussing the claim of a 

 supposed new species to that rank. 



Having thus discussed the laws of distribution and migration 

 of the birds of North America on the continent itself, and the 

 influence of region upon the development of the individual, I 



* See Baird, ' Birds of North America,' 1858, p. 122. 



