464 The American Naturalist. [June, 



that of the species, one being west of, while the other is east of 

 the Rocky Mts. And hence, since not only in its breeding area 

 but also in its migration range, the variety is subjected to 

 conditions of environment different from those influencing the 

 type species, we would naturally expect that the species (as a 

 whole) would become differentiated into two geographical 



The reason for the law, that extensive migration acts as a 

 check upon the production of geographical varieties, is not far 

 to seek. The barn swallow, for instance, remains in its breed- 

 ing area from four to five months each year, spending the re- 

 mainder of its time, except that consumed by its actual migra- 

 tion to and fro, in its tropical winter quarters. Roughly 

 speaking, we may say that it spends about half a year in its 

 breeding area, and the remainder in its winter home. In 

 other words, the swallow is subjected to one environment for 

 half the time of its existence, and to a more or less different 

 environment during the remainder of its life. The result of 

 this on the organism is obvious : the action of the two en- 

 vironments during approximately the same length of time, 

 would prevent it from becoming more particularly adapted to 

 the one than to the other, and would lead to the production of 

 more generalized characters, fitted to respond more or less 

 equally to both environments. In this way individuals of the 

 species could not become especially adapted to a certain por- 

 tion of the breeding area, if such adaptations should be un- 

 favorable for its existence in the winter quarters, and rice 

 versa ; in other words, the influence of the winter environment 

 acts as a check upon the acquisition of adaptations suited 

 alone to the summer environment. This is, to my mind, the 

 only adequate explanation for the law that extensive migra- 

 tion exerts a check upon the production of geographical vari- 

 eties. Species with wide-ranging breeding areas, on the other 

 hand, but with none or only restricted migrations, may give 

 rise to geographical varieties, suited respectively to the diverse 

 conditions found in different portions of its habitat, since such 

 species are influenced by the conditions of but one environ- 

 ment, owing to the absence or restriction of migration. 



11 Sept., 1895. 

 Wfcst Chester, Penna. 



