10 PHILOSOPHY OF ZOOLOGY. 



perature, equally hurtful to the system, and fatal to its 

 continuance. The polar bear appears to be accommodated 

 to live in a region, whose mean annual temperature is be- 

 low the freezing point. In the summer temperature of 

 Edinburgh, however well supplied with food, he appears to 

 languish in misery. Cold spring-water poured upon him 

 seems to revive him for a little ; but all relief is temporary, 

 the climate is too hot for the enjoyment of life. Destined 

 to live in a climate where the system is required to secrete 

 heat chiefly, it seems incapable of generating the cold re- 

 quisite to counteract the effects of even a temperate climate. 

 The inhabitants of the torrid regions, on the other hand, 

 seem capable of generating cold chiefly, all their organs 

 being adapted for resisting high temperatures ; and hence, 

 when brought to cold districts, they are incapable of gene- 

 rating the requisite degree of heat. 



In those districts where the individuals of a species are 

 most vigorous and prolific, the temperature most suitable 

 for existence prevails. The native country of the horse is 

 probably Arabia. There he exists in a wild state in the 

 greatest numbers. In the Zetland Islands, where he is 

 nearly in a state of nature, he is approaching the polar li- 

 mits of his distribution. He has become a dwarf. He 

 does not reach maturity until his fourth year, seldom con- 

 tinues in vigour beyond his twelfth, and the female is never 

 pregnant above once in two years. At the line where the 

 energies of the horse terminate, however, the rein-deer be- 

 comes a useful substitute. Its equatorial limits do not 

 reach the shores of the Baltic. 



The variations of the seasons, which bring along with 

 them corresponding changes of heat and cold, exercise a 

 powerful influence on the distribution of animals, in refe- 

 rence to temperature, Some species appear to possess a 

 considerable range of temperature, within which life can be 



