DISTRIBUTION OF MAMMALS 161 



vances of such nature are likely to be shared by all the animals 

 of a region. 



The ways in which mammal species and mammal faunas move 

 from their favorable original environment into some newly avail- 

 able but just as favorable one, seem almost analogous to the 

 creeping expansion of pools of water here or there on a dry, 

 nearly level floor. This analogy is useful, provided that one re- 

 members that the water (a fauna) is a solution of many liquids 

 (species) which are slowly changing into other liquids, some- 

 times meeting and dissolving or obliterating still others, and 

 that the floor itself and the local climate above it are also grad- 

 ually changing. 



Geographical movements of species depend fundamentally 

 upon random movements of individuals through successive gen- 

 erations. Several deer, for example, born at locality "A," move 

 fifty miles to the northeast and produce offspring at "B." If 

 this takes place within the established range of the species it has 

 little significance, but if it represents penetration into new 

 territory for that species, perhaps repeated a thousand times 

 along the perimeter of the range, it may well have significance. 

 Such movements, generation by generation, represent the way 

 by which land animals spread into favorable environments. This 

 procedure continues until stopped by contact with unfavorable 

 conditions. But random movement into and accidental trans- 

 portation into the unfavorable environment persists. Rarely 

 accident permits passage through a barrier into favorable con- 

 ditions. 



THE PASSAGE OF NATURAL BARRIERS 



Barriers to the spread of animals on the continents are some- 

 times incomplete because not sufficiently extensive, or are com- 

 pletely efficacious against one species while scarcely hindering 

 another. The functioning of a barrier may vary with the season 

 of the year, or perhaps with sun-spot cycles. In the course of 



