120 THEORY OF THE EARTH. 



rately what is, or ought to be, understood by the 

 word species, which may be thus expressed : Jl 

 species comprehends all the individuals which descend 

 from each other, or from a common parentage, and 

 those ivhich resemble them as much as they do each other. 

 Thus the different races which they have genera- 

 ted from them are considered as varieties but of 

 one species. Our observations, therefore, respect- 

 ing the differences between the ancestors and 

 the descendants, are the only rules by which we 

 can judge on this subject ; all other considerations 

 being merely hypothetical, and destitute of proof. 

 Taking the word variety in this limited sense, we 

 observe that the differences which constitute this 

 variety depend upon determinate circumstances, 

 and that their extent increases in proportion to 

 the intensity of the circumstances which occasion 



them. 



* 



Upon these principles it may be observed, that 

 the most superficial characters are the most va- 

 riable. Thus colour depends much upon light ; 

 thickness of hair upon heat; size upon abundance 

 of food, &c. In wild animals, however, even these 

 varieties are greatly limited by the natural habits 

 of the animal, which does not willingly migrate 

 from the places where it finds insufficient quanti- 

 ty what is necessary for the support of its species, 

 and does not even extend its haunts to any great 

 distances, unless it also finds all these circumstan- 

 ces conjoined. Thus, although the w r olf and the 





