192 Darwin, and after Darwin. 
able throughout their entire length, in the deer, and 
still more in the camel, their union is more complete, 
so that they go to constitute a single bone, whose 
double or compound character is indicated externally 
only by a slight bifurcation at the base. Nevertheless, 
if we examine the state of matters in the unborn 
young of these animals, we find that the two bones 
in question are still separated throughout their length, 
Fic. 85.—Aipparion. (New World Pliocene.) 
and thus precisely resemble what used to be their 
permanent condition in some of the now fossil species 
of lioofed mammalia. 
Turning next from bones of the limb to other parts 
of the mammalian skeleton, let us briefly consider the 
evidence of evolution that is here likewise presented by 
the vertebral column, the skull, and the teeth. 
As regards the vertebral column, if we examine this 
