10 THE DOCTRINE OF DESCENT. 
extremity a small bone supporting the four toes. Earlier 
writers were content to say that the astragalus (c) re- 
places the tarsus and metatarsus. But this is not the 
case ; for the chick in the egg (B) shows that the bird’s 
leg consists of the thigh, or femur (a), and the shank or 
tibia (2), two tarsal (# x), and three or four metatarsal 
bones (c), and the toes, or phalanges; that the upper 
tarsal bone is anchylosed with the tibia, and the lower 
one with the consolidated metatarsus. Only thus do 
we obtain a true perception of the fact manifested in 
A, although the cause of the fact does not as yet 
appear. 
The next example is rather more difficult. With- 
out the history of development, comparative anatomy 
is incapable of explaining why man possesses three 
little bones in the auditory apparatus, the bird only one. 
The history of development shows that out of the ma- 
terial which in man is applied to the formation of the 
malleus and incus, two other portions of the skull are 
evolved in the bird, having little or nothing to do with 
the auditory mechanism. In short, the history of deve- 
lopment, which describes the gradual formation of the 
organism, is at every step a beacon to comparative 
anatomy. In itself, however, the history of development 
does not as yet exceed the rank of a merely descriptive 
branch of erudition. 
But if we now perceive how the evolutionary stages of 
individuals represent series from the lower to the higher, 
analogous to the various members existing side by 
side in the same group of animals,—how, for instance, 
the mammal passes through stages at which the lower 
vertebrata remain fixed,—a connection, at first sight 
