456 THE CAUSES OF THE xI 
as the members of any great natural group have 
with a real or imaginary typical form. _ Now, we 
know that all varieties of pigeons of every kind 
have arisen by a process of selective breeding from — 
a common stock, the rock-pigeon ; hence, you see, 
that if all species of animals have proceeded from 
some common stock, the general character of their 
structural relations, and of our systems of classifi- 
cation, which express those relations, would be just 
what we find them to be. In other words, the 
hypothetical cause is, so far, competent to produce 
effects similar to those of the real cause. 
Take, again, another set of very remarkable 
facts;—the existence of what are called rudi- 
mentary organs, organs for which we can find 
no obvious use,’ in the particular animal econ- 
omy in which they are found, and yet which are 
there. 
Such are the splint-like bones in the leg of the 
horse, which I here show you, and which corre- 
spond with bones which belong to certain toes and 
fingers in the human hand and foot. In the horse 
you see they are quite rudimentary, and bear 
neither toes nor fingers; so that the horse has 
only one “finger” in his fore-foot and one “toe” 
in his hind-foot. But it is a very curious thing 
that the animals closely allied to the horse show 
more toes than he ; as the rhinoceros, for instance : 
he has these extra toes well formed, and anatomi- 
cal facts show very clearly that he is very closely 
