114 HYPOTHESIS OF THE DEVELOPMENT OF 



and tending to impress its own features on that which suc- 

 ceeds. 



This principle is partly matter of familiar observation. It 

 is obvious to all, that an ordinary mammalian quadruped has 

 a strong analogy of form to the human being ; its head, its 

 forelegs, its hinder extremities, have each their representative 

 parts in our frame. But the ordinary observer is surprised to 

 learn how much further the principle is carried. For example, 

 the hind leg of the horse looks very different from one of our 

 limbs, in as far as it seems to have a knee presented backwards, 

 and possesses no toes. In reality, the part of the horse 

 corresponding to our knee is high up near the body of the 

 animal, and the hock corresponds to our heel. It has toes, 

 moreover ; but they are sunk in the hollow of a hoof, which 

 serves as a shield to that part. The horse, the dog, and many 

 other quadrupeds, may be said to walk upon their toes (hence 

 called digitigrade) : others, as the bear and badger, present 

 the whole foot to the ground, as man does (hence called 

 plantigrade). Thus, too, the wing of the bird contains bones 

 representing those of our arm, though modified for so different 

 a purpose. The paddles of the whale tribes and seals are 

 other curious modifications of a member substantially the 

 same. The bat, again, has the bones of its hand developed to 

 an unusual extent, so as to become a frame for the membrane 

 by which it flies : in the extinct pterodactyle, the same pur- 

 pose was chiefly served by a development of the forefinger 

 alone. The fundamental resemblance which lurks below 

 various appearances is often startling. Thus, the giraffe, 

 with its long neck, has, in that part, no more bones than are 

 to be found in the neck of the elephant or pig, which hardly 

 seem to have any neck at all. The cervical vertebrae are but 

 seven in every one of the mammalian animals. Sometimes, 

 an organ appears entirely wanting in one family, as feet 

 in the serpent tribes, a pelvic region in the whale, the wing 

 in the bird called the apteryx ; and yet it is not truly 

 wanting. Usually, some rudiment of it appears, as if nature 

 had been willing to give it, but had kept it back from a com- 



