q8 Allen, Origin and Distribution of N. A. Birds. [ April 



in detail from the standpoint of birds. As an introduction to 

 Part I a few pages are devoted to questions having a more or less 

 direct bearing upon the general subject. 



I. — The Geographical Origin and Distribution of North 

 American Birds. 



It has long been recognized by nearly all writers on geograph- 

 ical biology that the two leading factors governing the distribution 

 and dispersal of life over the globe are the land connections which 

 now exist, or have formerly existed, between the principal land 

 masses, and climate ; and that by far the most potent of the 

 climatic influences is temperature. In considering the fauna! 

 relations of North America to the Old World, it is important, 

 therefore, to keep in mind the present slight separation of 

 northern North America from Eurasia, and that, as currently 

 believed by many geologists, the shallow basin now forming 

 Bering Sea was at one time dry land, and thus formed a broad 

 land connection between northwestern North America and north- 

 eastern Asia, during at least a portion of the Tertiary. 



As is well known, a large proportion of the genera, and many 

 of the species, of both animals and plants occurring in North 

 America have a circumboreal distribution, even in many cases 

 where their present habitats do not extend quite to the Arctic 

 regions. Furthermore, that many genera, particularly of birds, 

 which are at present limited to the warm temperate and tropical 

 latitudes, are common to both the Old World and the two Ameri- 

 can continents. It is, in certain cases, hard to see how their pres- 

 ent dispersion could have been brought about under the geographic 

 and climatic conditions now existing. Geology here comes to our 

 assistance, furnishing evidence that in earlier times the climate of 

 the globe was not only more uniform, but also much warmer over 

 the regions now buried half the year under snow and ice. It is 

 well known that in Miocene times a warm temperate flora pre- 

 vailed over the present Arctic regions, and that subtropical plants 

 flourished in Central Europe and in corresponding latitudes in 

 North America. Also that many types of mammals, now repre- 

 sented only in the tropics, formerly ranged over the greater part 

 of the northern hemisphere, as shown by their fossil remains, long 



