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 (d), two tarsal (m ;/), 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 



