MAMMALIA, 503 
CHAPTER XXXI. 
GEOGRAPHICAL DISTRIBUTION OF 
MAMMALIA. 
AMMALS, with the exception of the erial (Cirvop- 
Lh tera) and aquatic (S¢venza and Cetacea) 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 (1) all oceanic 
islands, z.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.¢., New 
Zealand). 
Leaving these islands out of consideration, we find that 
there is great diversity in the occurrence of Mammata 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 s¢azzon 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. 
M. 39 
