ORGANIC AGENCIES. 125 



We said the difference is almost complete. There are, 

 therefore, some exceptions, but these only confirm the 

 principles on which the rule is founded. They are of three 

 kinds : 1. Hardy or widely migrating species. Some 

 hardy species range through No. 3 into No. 4, and these 

 may pass over from continent to continent. Some birds, 

 like the Canada goose and mallard duck, migrate in sum- 

 mer to No. 4, and thence in winter southward in both 

 continents. 2. Introduced species, which have become 

 wild. 3. Alpine species, mostly of insects and plants. It 

 is a curious fact that species of plants and insects, 

 isolated on the tops of high mountains near the snow- 

 line, are similar to each other on the two continents, 

 and also similar to Arctic species. This latter fact gives 

 the key to the explanation. The geological epoch imme- 

 diately preceding the present (glacial epoch) was charac- 

 terized by extension of Arctic conditions southward even 

 to the shores of the Mediterranean and the Gulf of Mex- 

 ico. At that time, therefore, Arctic species occupied all 

 Europe and the United States. As the cold abated, Arc- 

 tic species mostly went northward to their present home 

 in the Arctic zone. But some followed the receding Arc- 

 tic conditions upward to the tops of mountains, and were 

 left stranded there, both in Europe and this country. 



In. No. 1 the species on the two continents are still 

 more markedly different, the difference extending even 

 to families and in some instances to orders. Thus, for 

 example, among plants, the cactus order is confined to 

 America. Among animals, the great pachyderms, e. g., 

 elephants, rhinoceroses, hippopotamuses, also the camels, 

 horses, and tailless monkeys, are confined to the Old 

 World, while the sloths, the armadillos, the prehensile- 

 tailed monkeys, the whole family of humming-birds (of 

 which there are over four hundred species), and the 

 family of toucans, are confined to the New. 



South of the equator the continents do not again ap- 



