MAMMALIA. 593 



CHAPTER XXXI. 



GEOGRAPHICAL DISTRIBUTION OF 

 MAMMALIA. 



MAMMALS, with the exception of the aerial {Chirop- 

 terd) and aquatic {Sirenia and Cetaced) types, lend 

 themselves specially to the solution of geographical problems, 

 because, as a rule, a strait of water of a few miles (twenty or 

 so) forms an effective physical barrier to their migratory 

 progress. Hence the first important fact of mammalian 

 distribution is their entire absence from (i) all oceanic 

 islands, i.e.., from islands raised above the level of the sea 

 by volcanic agency or by the growth of coral ; and (2) 

 all islands which were separated from the mainland at a 

 date antecedent to the evolution of mammals {e.g.^ New 

 Zealand). 



Leaving these islands out of consideration, we find that 

 there is great diversity in the occurrence of Mammalia in 

 certain districts. This diversity, like that of organic struc- 

 ture, must be primarily due to diversity in the physical 

 environment. 



It must be remembered that certain mammals are adapted 

 for certain habitats. Thus arboreal forms are confined to 

 forest lands, others to the open plains, and so on. The par- 

 ticular kind of habitat affected by a mammal is called its 

 station^ and as these natural conditions recur throughout all 

 the large regions, they do not affect the general problems of 

 geographical distribution. As an example, if we say that 

 marmosets are characteristically found in South America, we 

 do not mean to imply that they occur in the open '' pampas " 

 of the Argentine, but that having a forest station they usually 

 occur in the forests of South America. 



Coming to the prime physical factors which govern the 

 spreading or distribution of mammals, we find that they act 

 through one of the two primary functions of locomotion and 

 food. 



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