DIVISIONS OF ZOOLOGY. 13: 
Distribution is the study of the relations of organisms 
to external conditions—the study of the relation between 
external form, internal structure, and habits of life. It 
enquires by what means animals live, what food they eat, 
why certain species are rare while others are common ; and 
it aims ultimately at answering the questions, How did 
animals get where we now find them? and, Why are they 
not found in other places? It is chiefly observational in its 
method, but experminent can also often be used. An 
examination of the animals inhabiting different parts of 
the world, that is their fawna, has shown that different 
districts are characterised by different animals; and it has 
been found that while some species and groups are almost 
cosmopolitan, others are restricted to small areas. As the 
conditions of life are not evenly distributed over the earth, 
this might have been expected, but it is found that climate 
will account for a comparatively small number of the facts. 
The chief boundaries of species, or groups, are oceans, 
mountains, deserts, and rivers; but it is also found that some 
species are prevented from spreading into an adjacent 
district by that district being pre-occupied by some animal 
in sufficient numbers to consume all the appropriate food. 
These boundaries are called barriers. There are, however, 
several means of dispersal, by which animals can occasionally 
overcome these barriers, the principal of which are winds, ice- 
bergs, drift-wood, the attachment of the ova of fish, molluses, 
&e,, to the feet of birds. Thus the fauna of any particular 
district can be divided into two groups——(1) the aborigines, 
or original inhabitants of the district, and (2) the colonists, 
who have overcome the barriers.* 
* All animals which have come to a country by natural causes are 
called indigenous, while those that have been brought by the agency of 
man are called introduced. 
