PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 877 



locality which is designated as the common center of origin. A sec- 

 ond conception is that as the ancestral form became established and 

 multiplied, mig-ration in search of food and more suitable conditions 

 occurred. Barriers, many of which were geographic, determined 

 the direction and extent of this migration. Large bodies of water 

 blocked the passage of terrestrial animals, as of course land was a 

 barrier to aquatic animals. High mountains or deserts were barriers 

 to all animals unable to withstand low temperatures and altitudes 

 on the one hand, or high temperature and drj^ness on the other. 

 These forces, and others, are believed to account for the natural 

 distribution of animals. The eases of the camel group, originating 

 in North America, migrating to South America and Asia by the 

 land connections of the Eocene to Pliocene epoch, and the tapirs, 

 which are represented by distinct species in two widely separated 

 regions, South Asia territory, and the Central America-South Amer- 

 ican territory are examples. Here again paleontologieal data show 

 that in the Pliocene epoch tapirs were distributed over nearly all of 

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

 they were graduallj^ 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 island of Porto 

 Santo during the fifteenth century and b}^ 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 eifect, 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 Hawaiian Islands. Many of the animals on these 

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

 Australia has a group of animals, examples of which are very dif- 

 ferent from those of Asia because the two have been so long separated. 

 Europe, Asia, Africa, and North America have been connected with 

 each other hy land bridges in recent enough times that the mam- 

 mals show similarity. The distribution of the species of a genus 

 often radiates from the more generalized species which occupy the 



