NATURAL SELECTION. 139 
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 which 
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-defined 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 
