160 MAMMALS OF THE PACIFIC WORLD 



fossils" today, reached favorably situated islands in the dis- 

 tant past. A few bats and hardy, raft-riding mammals often 

 are found on such islands. The remote Pacific islands had no 

 mammals at all until the first men landed. 



RANGE AND DISPERSAL OF MAMMALS 



At all times during the history of the evolution of a species of 

 mammals its environmental conditions have necessarily been 

 generally favorable. For although the environment best suited 

 to a species probably can never exist, neither can markedly 

 unfavorable conditions be long sustained. Similar or even more 

 favorable locations elsewhere may remain unoccupied by that 

 species only because of intervening environmental barriers. 



Favorable conjunction of animal needs with suitable sur- 

 roundings may be temporary only. The animal is continu- 

 ously evolving; the physiography, climate, and vegetation of 

 its habitat are constantly changing. Let us assume a possible 

 case, under which the surface of the land becomes lower, the 

 climate hotter and drier, the character of the vegetation more 

 and more desert-like. Simultaneously with this environmental 

 change, involuntary evolutionary experiments are being con- 

 ducted by the animal. Some of the products of variation are 

 more suited to the drying habitat, others to a region even 

 more humid than the original one. The former persist and re- 

 produce their kind ; the latter die. Thus an animal species has 

 changed, and, through the selective action of a changing habitat, 

 the changed species survives as a race distinct from the orig- 

 inal one. 



In a second situation, the environmental conditions at the 

 margin of the range of a particular species of mammal gradu- 

 ally become favorably modified; what before was desert be- 

 comes grassland or forest, favorable to the species. Then, the 

 species as a whole can advance, generation by generation, into 

 the now favorable area until stopped by some new barrier. Ad- 



