604 CLASS XVII. 



part even to the air (Galeopithecus, the Ghiroptera}. This differ- 

 ence of resort is naturally in relation with the general bodily form 

 and the constitution of its different parts, especially of the organs 

 of motion and of sense. Thus, for example, the limbs are changed 

 into fins in the whales, or are provided with a membrane for swim- 

 ming in most of the other water-animals; the eyes are small in 

 those animals that burrow under ground; the external ear is small, 

 or is absent in these last, as well as in those mammals that live 

 constantly in water. 



In the geographic distribution of the mammals in the first place 

 the general law again receives confirmation, that the number 

 as well of genera as of species increases from the poles to the 

 equator. Still, however, the true cetaceans and the seals must be 

 excepted from it, for to them the same rule applies as to the 

 swimming-birds, of which the species are more numerous in the 

 polar regions. There are species in the north polar regions com- 

 mon to the old and the new world, as Canis Lagopus, Ursus mari- 

 timus and Cervus Tarandus; without the polar circle also some 

 species are found in the northern countries of both hemispheres, as 

 Mustela Maries and Mustela erminea and Castor Fiber. (Some 

 writers indeed maintain that the beaver of America is specifically 

 different from that of the old world.) In the temperate parts of 

 North America almost all the species are such as do not occur in the 

 eastern hemisphere; in South America no single species is found 

 which also lives in the old world, nay, even the genera differ for 

 the most part from those of the old world. South American genera, 

 of which no species in the old world are hitherto known, are the 

 following: Dicotyles, Auchenia, Dasypus, Myrmecophaga, Brady- 

 pus, Gavia (or the genera, of more modern writers Gcelogenys, 

 Dasyprocta, Hydrochosrus), Loncheres, Nasua, the genera of the 

 bat-tribe: GlossopJiaga, Phyllostoma, Molossus, Noctilio, and many 

 genera of quadrumanes, namely Callithrix, Ateles, Mycetes, Pithecia, 

 Hapale. Procyon is peculiar to the new world in the northern and 

 southern hemispheres. Fiber is an animal form of North America. 

 Other genera are peculiar to the eastern hemisphere, as Sus, Eguus, 

 Camelus, Rhinoceros, Manis, Myoxus, Spalax, Cricetus, Viverra, 

 Herpestes, Erinaceus, the genera of bats: Megaderma, Nycteris, 

 Rhinolophus, Pteropus, the family of the Lemurids, the genera of 

 the apes : Gercopithecus, Semnopithecus, Inuus, Cynocephalus, Hylo- 



