1896.] Extensive Migration in Birds. 461 



It is at once apparent that almost all the species of North 

 American birds which are divisible into geographical varieties 

 are classed in this third list, that is, that those species evincing 

 the greatest tendency to geographical variation, are also those 

 which undertake migrations of the least extent. Thus, for in- 

 stance, Mehspka fasciata is usually resident in most localities 

 throughout the whole year, and has become differentiated into 

 a number of geographical races, while Melospiza georgiana is 

 migratory, and though it breeds in an area nearly equal in 

 extent to that of fasciata, has not produced local varieties ; the 

 non-migratory Megascops asio shows great geographical varia- 

 tion, while the migratory Asio acciplrinus, though almost cos- 

 mopolitan in its breeding area, shows no tendency toward 

 such variation. And, in fact, an examination and compari- 

 son of List III with Lists I and II, will lead to the conclusion, 

 that given any two species of equally extensive breeding 

 areas, the one with the smaller range of periodic migration 

 will, as a rule, evince a greater tendency to produce geo- 

 graphical varieties than will the species with the greater range 

 of migration. This conclusion may be concisely formulated 

 as follows ; it is the rule that the amount of geographical variation 

 in species with more or less extensive breeding areas, stand* in in- 

 verse ratio to the extent of its periodic migrations. Naturally, this 

 law is only applicable to species with extended breeding areas, 

 since diverse conditions in different sections of this area are 

 necessary, according to the theory of Natural Selection, for the 

 production of geographical subspecies or varieties ; and in a 

 limited breeding area, throughout which the conditions of the 

 environment are similar, there could be no cause to produce 

 geographical varieties, irrespective of the migratory or non- 

 migratory habits of the species. 



I have' not meant to imply, in the preceding pages, that spe- 

 cies with migration ranges of 30° lat, or more, are all sharply 

 definable, i. e., that such species are never divisible into geo- 

 graphical varieties ; but, on the contrary, that this tendency 

 to produce geographical races is less in the species with exten- 

 sive migrations, than in those with shorter ranges of migration. 

 For it is usual, even in species with extensive migrations, whose 



