804. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 
there are some four species in South and Central America, 
while the only other species occurs in Malacca and Borneo. 
Similarly the Camelide are represented by one genus in 
the Old World and another in South America, and the 
insectivorous Centetidz are represented by five genera in 
Madagascar, and one in Cuba and Hayti. 
The factors determining distribution.—There are six factors 
which combine to determine the particular distribution of an animal. 
‘These may be conveniently considered in pairs. 
(a) Distribution is in part determined by the constitution of the 
animal and by the physical conditions of the region, Thus snakes 
diminish rapidly in numbers towards the poles, their constitution being 
in most cases ill-adapted to withstand cold; thus crayfishes are absent 
from districts where the fresh water does not contain sufficient lime salts 
for their needs. 
(4) Distribution is in part determined by the position of the animal’s 
original home (which is often an unknown fact), and by the available 
means of dispersal. Thus, so far as we know, the Old World has been 
the exclusive home of the anthropoid apes, and there they have 
remained ; thus bats, being able to fly, have a more cosmopolitan 
‘distribution than most other mammals; thus amphibians, being unable 
to withstand salt water, are absent from almost all oceanic islands. 
(c) Distribution is in part determined by the actual changes (geo- 
logical, climatic, etc.) which have affected different regions, and by 
,‘ bionomic ” factors, z.¢. the relations between the animal in question 
and other organisms, whether animals, plants, or man. Thus it is 
plain that we cannot understand the fauna of Australia without knowing 
the geological fact that part of this island was once connected with 
the Oriental continent by a bridge of land across the Java Sea. The 
AustraJasian mammalian fauna consists of survivals and descendants 
of Mesozoic Marsupials which have been exterminated everywhere else, 
except the American opossums and Cazolestes. The original Australian 
mammals were saved, not by any virtue of their own, but by the earth- 
change which insulated them. Similarly, it is the geologist who helps 
us to understand the faunal diversity on the two sides of ‘‘ Wallace’s 
line,” or the absence of amphibians, reptiles, and mammals from the 
Canaries. That much will also depend on the animal’s power of 
surviving the struggle for existence in different regions is too obvious 
to require exposition. We need only think of the way in which man 
has in a few years altered the distribution of many birds and mammals, 
Bemis lms indeed reducing it to #z/, or increasing it with disastrous 
results. 
To sum up: the chief factors determining geographical 
distribution are—(1) the constitution of the animal, (2) the 
physical conditions of the region, (3) the position of the 
original home, (4) the means of dispersal, (5) the historical 
