THE GENUS CHORDEILES SWAINSON — OBERHOLSER. 31 



Transition, Canadian, and Hudsonian zones of the Nearctic Region. 

 Four of the subspecies, Chordeiles mrginianus aserriensis,^ Chordeiles 

 virginianus chapmani^ Chordeiles virginianus vicinus, and Chordeiles 

 virginianus minor, are confined to a single life zone; one, Chordeiles 

 virginianus sennetti, to two zones, and each of the four remaining 

 races is spread over several zones. The ranges of all conform very 

 satisfactorily to the boundaries of their respective zones. 



Phylogeny. — As a species Chordeiles virginianus is, as already re- 

 marked,- of South American origin. To the dispersal of strong- 

 winged birds like this, physical features, such as high mountain 

 ranges and wide expanses of water or desert, present no obstacle; 

 and an unsuitable summer temperature and the lack of proper food 

 at this season are therefore apparently its only real barriers. Thus 

 its emigration from South America, from whatever primal cause, 

 was easy, as was also the finding of a new home, since on account of 

 its annual migratory return to the south, the only requisite for its 

 new abode was a suitable place for reproduction. Its present dis- 

 tribution, routes of migration, and the evidently close relationship 

 of the Cuban Chordeiles virginianus minor with Chordeiles acfdtipen- 

 nis acutipennis from South America all indicate that Chordeiles vir- 

 ginianus reached the North American Continent through the West 

 Indies. Birds that remained to breed on the Greater Antilles re- 

 tained more nearly the original characters of the species, while those 

 that pushed on northward and westward became more and more 

 differentiated. The Floridian Chordeiles virginianus chapmani is, 

 of course, the form least removed from the island birds, both geo- 

 graphically and phylogenetically. From this area the species prob- 

 ably spread northward and northwestward, where it developed into 

 the large, dark race, Chordeiles virginianus virginianus. This, in 

 turn, spread westward and southwestward over nearly the whole 

 western United States, where environmental and perhaps other 

 factors have produced the four large paler races of the more or less 

 arid interior : Chordeiles virginianus hespens, Chordeiles virginianus 

 sennetti, Chordeiles virginianus howelli, and Chordeiles virginianus 

 henryi; of which the last is undoubtedly phylogenetically as well as 

 geographically farthest removed from Chordeiles virginianus vir- 

 ginianus. Simultaneously, in all probability, the advance of Chor- 

 deiles virginianus chapmani along the Gulf coast to southern Texas 

 ultimately resulted in the development of the small, pale race in semi- 

 arid southern Texas. The uniformity of Chordeiles virginianus vir- 

 ginianus throughout its wide breeding range from ocean to ocean is 

 apparently due to the comparative uniformity of climatic and en- 



__,___^ • 



1 See p. 71. 



2 See p. 12. 



