III. 



Conditions affecting distribution.— Climate. — Food-supply.— Barriers to mi- 

 gration. — Migrations of mammals and birds.— Dispersal of insects and 

 moUusks. 



Of the Conditions which affect or limit Distribution among 

 Animals. — Climate. — It Is a common belief that the principal fac- 

 tor limiting or regulating the distribution of animals is constituted 

 by climate ; in other words, certain groups of animals are associated 

 with certain grades or conditions of climate, beyond the reach of 

 whose interacting influence they could no longer maintain an exist- 

 ence. Thus, among quadrupeds, the elephant, camel, and tiger are 

 popularly associated with the hottest climates of the earth's surface ; 

 the reindeer and moose with climates of equal, but opposite, sever- 

 ity. And, similarly, among birds, the ostriches and hummers are 

 considered to be particularly indicative of hot or tropical climates, 

 and the auks, guillemots, puffins, and penguins, as products of the 

 cold northern or southern climes. That climate does regulate dis- 

 tribution, or impose a bar upon the migration of certain forms of 

 life, there can be no manner of doubt ; but that it does not exercise 

 the paramount influence that ia generally attributed to it there can 

 likewise be no question. Taking, for example, some of the in- 

 stances that have just been mentioned as indicating the supposed 

 association between animal distribution and given conditions of 

 climate, we find that the tiger, while its home, par excellence, may 

 be considered to be the hot districts of India and the Indian Archi- 

 pelago, is in no way restricted in its range to those regions, or to 

 regions having at all a similar climate. Thus, the animal is found 

 in the elevated regions of the Caucasus and the Altai chain, and in 

 the Himalaya range its footprints are not infrequently found im- 

 pressed in the fields of snow. It is a permanent inhabitant of the 



