THEORY OF EVOLUTION 499 



the Pliocene epoch tapirs were distributed over nearly all of North 

 America, Northern Asia, and Europe. Following that time they 

 were gradually decreased due to discontinuous distribution until 

 the one-time world-wide distribution is now isolated in two widely 

 separated regions. Long isolation of genera in different environ- 

 ments will bring about definite specific differences. As an example, 

 a litter of foreign rabbits was introduced to the islands of Porto 

 Santo during the fifteenth century and by the middle of the nine- 

 teenth century the descendants had become so distinct from the 

 original ancestors that they were described as a new species. There 

 are numerous instances of this effect, demonstrated by isolation on 

 ocean islands. The islands are either continental, with fauna similar 

 to those of the nearby continent from which the animals have come, 

 as the British Isles, or they are oceanic, with a very bizarre assem- 

 blage of animals which have either drifted in or have been carried 

 there, as the Haw^aiian Islands. Many of the animals on these 

 oceanic islands are peculiar and are found nowhere else on earth. 

 Australia has a group of animals which are very different from those 

 of Asia because the two have been so long separated. Europe, Asia, 

 Africa, and North America have been connected with each other by 

 land bridges in recent- enough times that the mammals show simi- 

 larity. The distribution, of the species of a genus often radiates 

 from the more generalized species which occupy the center of the 

 range of the genus, and the more specialized species are found in 

 the scattered outskirts of the range. 



From the preceding statements concerning distribution it seems 

 that any given species originates in a definite locality, that it multi- 

 plies there and migrates in all possible directions. It modifies as 

 it goes in response to the various new conditions prevailing and 

 becomes divided into local varieties which in the course of time 

 become species. Thus the working- method of animal distribution, 

 as it has been presented, is the principle of dtscent from preceding 

 generations with modification. 



Morphological Evidence. — Classification of the animals shows in 

 fact something of the morphological evidence, since current classi- 

 fication is based chiefiy on anatomical features and comparative 

 anatomy. The groups of the classification are established largely 

 on anatomical similarities. The differences existing among the rep- 

 resentatives of all the classes of vertebrates are relatively slight 



