478 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



corresponding numerals. We may leave out No. 5, as 

 this is unknown and largely uninhabited. In No. 4 the 

 fauna is practically the same all around, because of the 

 close approximation of the lands of the two continents 

 here and the easy communication over the solid ice. 



But in Nos. 3 and 2, which include the United States 

 and Europe, we find the species are substantially all dif- 

 ferent. Even many of the genera and some families 

 are peculiar to each continent. Some species, indeed, 

 are representative or resembling species, but not identi- 

 cal. To this general statement there are some excep- 

 tions, but these are of the kind which prove the rule, 

 or rather the principle on which the rule is founded. 



Exceptions. — 1. Hardy or widely migrating species, such 

 as geese and ducks. These are the same on the two 

 continents, because they range also into No. 4, and thence 

 go down on either continent. 



2. Introduced species, such as all the useful plants and 

 domesticated animals and all the noxious weeds and 

 animal pests — flies, rats, etc. — which follow the footsteps 

 of civilization. We not only find these on both conti- 

 nents, but they often do as well, or even better in their 

 new homes than in their native places. They were not 

 in those new homes before only because they could not 

 get there. 



3. Alpine Species. — It is a curious fact that animals 

 and plants inhabiting the tops of high mountains in Eu- 

 rope and in the United States are extremely similar, and 

 even sometimes identical, even though so widely and 

 impassably separated. The explanation of this will come 

 up later. 



In No. 1, or tropic zone, the difference is still greater; 

 not only all the species in South America and Africa are 

 wholly different, but many whole families are entirely 

 peculiar to one or the other continent. For example, 



