488 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



i. Alpine Species. — We have already spoken of the 

 remarkable fact that alpine species of plants and of in- 

 sects are very similar (though not usually identical) in 

 Europe and America, although so widely separated and 

 completely isolated. The key to the explanation is 

 found in the additional fact that they are both similar 

 to arctic species, and the explanation of both is found in 

 the migrations of the glacial epoch. 



We have already seen that arctic species are the 

 same on the two continents, because they are circum- 

 polar and in substantial connection all around. Now, 

 as the glacial cold came on, the ice sheet advanced south- 

 ward, driving the arctic species before it on both conti- 

 nents, until they invaded and occupied all America to 

 the Gulf and all Europe to the Mediterranean. As the 

 ice sheet retreated, arctic species followed it, step by step, 

 back to their present arctic home. But many individ- 

 uals sought arctic conditions by retreating up the slopes 

 of mountains, and were left stranded there on both con- 

 tinents. Since then they have been changed somewhat, 

 in different directions by isolation, but the time has not 

 been long enough for great divergence. There is some 

 difference, but not so great as the wide separation would 

 lead us to expect. 



2. Australia. — Of all known faunas and floras, this is 

 the most distinct. It is so because longest isolated. We 

 have seen that all its mammals are nonplacentals — i. e., 

 marsupials and monotremes — except a few introduced 

 accidentally or by man. Furthermore, that monotremes 

 are found nowhere else, and marsupials nowhere else, 

 except a few opossums in America. But this has not 

 always been so. In Jurassic (middle geological) times 

 the earth was everywhere abundantly inhabited by mar- 

 supials and monotremes, but not by eutheres or typical 

 mammals. These last were introduced subsequently in 



