5C6 



THE AMERICAN NATURALIST 



[Vol. L 



breeds in the north and reaches our latitude only in 

 winter, finds its nearest relative not in an equatorial 

 species, but in Lams dominicensis of the southern hemi- 

 sphere, below 10 c south latitude. This bird is more or 

 less of an intermediate between Larus marinus (the 

 great black-back), and Lams fuscus (the lesser black- 

 back) which occurs with the former in Europe and is 

 represented by allied forms eastward to the Pacific coast 

 of North America, the mantle of dominicensis is very 

 dark as in the larger bird; its size about that of the lesser 

 ones. These are four in number— Larus fuscus occurs 

 in Europe east to the Dwina, Larus affinis from the 

 Dwina eastward across Asia, Larus schistisagus in 

 northern Japan and Bering Sea, Larus occidentalis on 

 the west coast of the United States. The four birds of 

 this series are most readily separated by size and pro- 

 portions of feet. Affinis resembles schistisagus in the 

 former, f uscus in the latter ; schistisagus resembles affinis 

 in the former, occidentalis in the latter. That is affinis 

 and schistisagus are more or less intermediates struc- 

 turally between the species they separate geographically. 

 For convenience we will call them " adjacent intermedi- 

 ates" as opposed to dominicensis which we will call a 

 "foreign intermediate" between marinus and birds of 

 fuscus group. 



A species of vertebrate animals distributed over a wide 

 geographical area often varies in the different regions it 

 inhabits sufficiently to be separable into different inter- 

 grading races. Ordinarily no two of these races from 

 the very nature of their origin will be found inhabiting 

 the same region, sometimes they mingle in migration. 

 Often these races are in direct response to different en- 

 vironmental conditions, but sometimes this response 

 can not be traced. Such races form a series comparable 

 to the gulls of the Larus fuscus group, and we may call 

 them "adjacent races" and the gulls of the group re- 

 ferred to "adjacent representatives," also we may call 

 Larus dominicensis a "foreign representative" of the 



