70 ON THE REVOLUTIONS OF 



-were as constant as our own ; or at least because the catastrophe 

 that destroyed them had not left them time to give evidence of the 

 changes ? 



As to the naturalists who allow that the varieties are confined 

 within certain limits fixed by nature, it is necessary, in order to 

 answer them, that we should examine what may be the extent of 

 these limits — a curious research, very interesting in itself in many 

 respects, and yet one which has hitherto excited but very little 

 attention. 



This inquiry calls for the definition of a species, which may serve 

 as the foundation for the use which is made of the term. A species 

 then includes the i?idividuals ivhich descend from one another, or from 

 common parents, and those which resemble them as strongly as they resem- 

 ble one another. Thus we only call the varieties of a species those 

 races, more or less different, which may have proceeded from them by 

 generation. Our observations on the distinctions between ancestors 

 and descendants are consequently our only rational rule ; for every 

 other would enter into hypothesis without proofs. 



But in thus considering the variety, we observe that the differences 

 which constitute it depend on determinate circumstances, and that 

 their extent increases with the weight of these circumstances. 



Thus the most superficial characteristics are the most changeable ; 

 colour depends much on light ; the thickness of the hair on the heat ; 

 the size on the great supply of food. But in a wild animal, even 

 these varieties are very much limited by its habits, for it will not 

 willingly leave the place where it finds, in a quantity suited to its 

 wants, all that is necessary for the support of its species, and then 

 only to those where it may find them as equally well supplied. Thus, 

 although the wolf and the fox are found from the torrid to the icy 

 zone, we rarely find in this vast space very little other difference than 

 a little more or less beauty in their fur. I have compared the skulls 

 • of foxes of the north and those of Egypt with those of France, and 

 have only found individual differences. 



Those savage animals which are confined to more limited spaces 

 vary still less, particularly those which are carnivorous. A thicker 

 mane makes the only difference between the hyena of Persia and those 

 of Morocco. 



Herbivorous wild animals feel rather more sensibly the influence of 

 climate, because it more affects their food, which differs in abundance 

 and quality at various times. Thus elephants will be greater in one' 

 forest than in another ; they will have tusks larger in those places 

 where the nourishment is more congenial to the formation of the 



