NATURAL SELECTION. I39 



to observe the reversion of the modern sheep to its 

 aboriginal form is utterly impossible. That the horse 

 is derived from a striped aboriginal species is probable ; 

 but notwithstanding the many generations during Vv-hich 

 the great herds of feral horses in South America have 

 propagated themselves undisturbed, no such species 

 has been produced. Riitimeyer's minute researches on 

 domestic cattle have shown that, in Europe at least, 

 three well-Jiefined species of the Diluvial period have 

 contributed to their formation, Bos primigenius, longi- 

 frons, and frontosus. These species once lived geo- 

 graphically separate, but contemporaneously ; and they 

 and their specific peculiarities have perished, to rise 

 again in our domestic races. These races breed to- 

 gether with unqualified fertility ; in the form of skull 

 and horns they recall one or other of the extinct 

 species ; but collectively they constitute a new main 

 species. That from their various breeds, the three or 

 any one of the aboriginal species would ever emerge in 

 a state of pristine purity, would be an utterly ludicrous 

 assertion. 



In all these domestic animals^dog, sheep, goat, horse, 

 and cattle — the transformation was initiated in an era of 

 civilization in which there was no idea of artificial breed- 

 ing in the modern sense, and in which the main factor of 

 transformation, independently of involuntary and uncon- 

 scious selection, consisted simply in the altered mode 

 of life. This introduces us to variations in a state of 

 nature, and to Natural Selection. Natural as well as 

 artificial selection both rest on the undisputed fact of 

 the idiosyncrasies of the most closely allied vegetal and 

 animal individuals; and it has already become manifest 



