XXVlll INTRODUCTION. 



exist in nature. The latter must be received 

 with caution, since they are, for the most part, 

 pure hypotheses. 



Numerous observations show that some ani- 

 mals are very widely distributed, while others are 

 restricted within narrow limits. Again, some 

 districts contain a fauna {i. e. animal population) 

 peculiar, or nearly so, to themselves ; whilst 

 others are peopled almost exclusively with de- 

 rived species. 



To account for these, and numerous other facts 

 of a similar kind, it has been assumed that all the 

 individuals of each species proceeded from one 

 originally created pair ; or, in the case of bisexual 

 organisms, from a single parent. Each species 

 must, therefore, have been at first confined within 

 a very limited area ; but being endowed with a 

 power of diffusing itself, its descendants, after the 

 lapse of time, became dispersed to a greater or 

 less extent. ^^ 



There are, however, various causes which would 

 tend to keep in check the migration of animals. 

 Among these, an inability to exist outside a 

 certain range of temperature is not the least im- 

 portant. For it has been proved that contiguous 

 areas, with different climates, are inhabited by 

 different species of animals. On the other hand, 

 the effect of climate in determining the distri- 

 bution of organised beings has been, in many 



