CENOZOIC ERA. AGE OF MAMMALS. 405 



especially the weaker indigenes. Thus have resulted the 

 mammalian fauna of Africa. 



3. The British Isles. The fauna and flora of the 

 British Isles are almost identical with those of Europe, 

 but not quite. They differ in two respects, a. There 

 are varietal, though perhaps not specific, differences of 

 form. b. The number of species is much less than on 

 the continent. This is especially true of Ireland. Thus, 

 of 90 species of European mammals only 40 are found in 

 England and 22 in Ireland. Of 22 European species of 

 reptiles and amphibians only 13 are found in England 

 and 4 in Ireland. The migrations of glacial times com- 

 pletely explain this. Before glacial times Great Britain 

 was a part of Europe and had the same fauna and flora. 

 During the glacial times it was covered with the ice-sheet, 

 and all life destroyed or driven southward. After the 

 glacial times it was still connected with the continent, 

 and began to be recolonized by migration from Europe. 

 But before the colonization was completed, especially for 

 more distant Ireland, it was again separated by subsidence 

 from the continent, and at the same time Ireland was 

 separated from England. The time since has not been 

 sufficient to make species, although it has been enough 

 to make incipient species, i. e., geographical varieties. 



4. Coast Islands of California. The flora of Cali- 

 fornia consists of two groups of species, the one charac- 

 teristically Californian, the other more widely diffused. 

 The first is undoubtedly indigenous, the second is prob- 

 ably composed of invaders from the north. Now, off the 

 coast of Southern California there is a string of bold, 

 rocky islands about 2,000 feet high and separated from 

 the mainland by a deep channel 50 miles wide. The flora 

 of these islands is very peculiar. Of 300 known species 

 about 50 are wholly peculiar to the islands and not found 

 elsewhere in the world. The remaining 250 are all 

 characteristic California species. Now the explanation. 



