Iviii INTRODUCTION. 



commode the animals, and therefore, we may suppose, they 

 disappear ; and were peacocks to be reared under similar 

 circumstances, it is probable, that, in the course of successive 

 generations, they would lose the beautiful appendage which 

 they bring from their native jungles. 



The effects, likewise, of altitude are to be numbered 

 amongst those which modify the characters of animals. In 

 general, the animals of mountains are smaller and more agile 

 than those of the same species inhabiting plains. In man, 

 the pulse increases in frequency as he ascends into the at- 

 mosphere, so that, while at the level of the sea. the number 

 of beats is 70 in a minute, at the height of 4000 feet the 

 number exceeds 100. The air being rarer, a greater quan- 

 tity of it must be drawn into the lungs to afford the oxygen 

 necessary to carry off the excess of carbon in the system. 

 But gradually, as man and other animals become naturalized 

 in an elevated country, the digestive and respiratory organs, 

 and with these the capacity of the chest and abdomen, become 

 suited to their new relations. Humboldt remarks on the ex- 

 traordinary development of the chest in the inhabitants of 

 the Andes, producing even deformity ; and he justly observes, 

 that this is a consequence of the rarity of the air which de- 

 mands an extension of the lungs. 



The effects have been referred to of use or exercise in mo- 

 difying certain parts of the animal form. The limbs of many 

 animals inured or compelled to speed become extended in 

 length, as of the dogs employed in the chase of the swifter ani- 

 mals. The limbs of an animal deprived of the means of mo- 

 tion become feeble and small, as the wings of domesticated 

 birds. In the natural state, the cow has a small udder, yet 

 sufficient to contain the milk which her young requires ; in 

 the domesticated state, by milking her, the organ becomes 

 enlarged, so as to contain a quantity of milk, beyond what 

 the wants of her own offspring demand. Nor are the charac- 

 ters thus acquired confined to the individuals on which they 

 have been impressed, but may be transmitted to their pos- 



