GEOGRAPHICAL DISTRIBUTION OF ANIMALS 983 
To processes of this kind, on a larger or smaller scale, 
the variety in the animal life of the globe is very largely 
due. Isolation and adaptation give the clew to the forma- 
tion of a very large proportion of the “new species” in 
any group. 
153. Effect of barriers—It will be thus seen that geo- 
graphical distribution is primarily dependent on barriers or 
checks to the movement of animals. The obstacles met 
in the spread of animals determine the limits of the spe- 
cies. Each species broadens its range as far as it can. It 
attempts unwittingly, through natural processes of increase, 
to overcome the obstacles of ocean or river, of mountain or 
plain, of woodland or prairie or desert, of cold or heat, of 
lack of food or abundance of cnemies—whatever the bar- 
riers may be. Were it not for these barriers, each type or 
species would become cosmopolitan or universal. Man is 
pre-eminently a barrier-crossing animal. Hence he is found 
in all regions where human life is possible. The different 
races of men, however, find checks and barriers entirely 
similar in nature to those experienced by the lower animals, 
and the race peculiaritics are wholly similar to characters 
acquired by new species undcr adaptation to changed con- 
ditions. The degree of hindrance offered by any barrier 
differs with the nature of the species trying to surmount it. 
That which constitutes an impassable obstacle to one form 
may be a great aid to another. The river which blocks the 
monkey or the cat is the highway of the fish or the turtle. 
The waterfall which limits the ascent of the fish is the 
chosen home of the ouzel. The mountain barrier which 
the bobolink or the prairie-dog does not cross may be the 
center of distribution of the chief hare or the arctic blue- 
bird. 
154, Relation of species to habitat—The habitat of a 
species of animal is the region in which it is found ina 
state of Nature. It is currently believed that the habitat 
of any creature is the region for which it is best adapted. 
