316 EVOLUTION AND ANIMAL LIFE 



will be formed, like the parent species in general structure, but 

 having gained new traits adjusted to the new environment. 



To processes of this kind, on a larger or smaller scale, the 

 variety in the animal life of the globe must be largely due. 

 Isolation and adaptation through selection probably give the 

 clew to the formation of a very large proportion of the "new 

 species " in any group. 



It will be thus seen that geographical distribution is primar- 

 ily dependent on barriers or checks to the movement of animals. 

 The obstacles met in the spread of animals determine the 

 limits of the species. Each species broadens its range as far 

 as it can. It attempts, unwittingly, of course, through natural 

 processes of increase, to overcome the obstacles of ocean and 

 river, of mountain or plain, of woodland or prairie or desert, of 

 cold or heat, of lack of food, or abundance of enemies — what- 

 ever the barriers may be. Were it not for these barriers, each 

 type or species would become cosmopolitan or universal. 



Man is preeminently a barrier-crossing animal; hence, in 

 different races or species, man 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 peculiarities 

 are wholly similar to characters acquired by new species under 

 adaptation to changed conditions. 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 little chief hare or the Arctic bluebird. 



The term fauna is applied to the animals of any region 

 considered collectively. Thus the fauna of Illinois comprises 

 the entire list of animals found naturally in that State. It 

 includes the aboriginal man, the black bear, the fox, and all its 

 animal life down to the Amoeba and the microbe of malaria. 

 The relation of the fauna of one region to that of another de- 

 pends on the ease with which barriers maj^ be crossed. Thus 

 the fauna of Illinois differs little from that of Indiana or Iowa, 

 because the State contains no barriers that animals may not 



