The Meaning of Embryonic Structures 133 



sheltered, and it cannot move about either in flight or pur- 

 suit. In animals like the insects, the problems of adaptation 

 means at each stage a mode of living that cannot be identical 

 with that of hypothetical ancestors or real ancestors. 



With all of these qualifications, however, there remain 

 facts that do not lend themselves to interpretation except 

 on the assumption that there has been modification of types 

 in the course of descent. Embryos of all classes of animals 

 are found to contain structures which are meaningless if we 

 think of a purposeful adaptation of either the embryo or of 

 the adult. But these structures agree in detail with the as- 

 sumption of descent with modifications. The ruminants 

 like the sheep, cow, buffalo, deer and camel have no collar 

 bone in the adult stage. In the embryo of the sheep, the 

 beginning of such a bone is formed. Before the animal is 

 born this is absorbed. Adult whales differ from other mam- 

 mals in having no hair covering on the body. We may ac- 

 cept this fact as quite in keeping with the theory of pur- 

 poseful adaptation since these animals living in the water 

 have no need for hair, although the seal and the otter and 

 beaver, also living in the water, have the finest fur in the 

 world. The whale has, besides, a thick layer of blubber which 

 protects adequately against changes of temperature and loss 

 of heat. In the fetus of the whale, however, there is a rich 

 coat of hair, which disappears before birth. 



The scarcity of hens' teeth is proverbial, and no mod- 

 ern birds have teeth although the fossils of the earliest un- 

 mistakably bird-like animals so far found (the archeopteryx) 

 show unmistakably tooth-like structures. Later birds also 

 had teeth as the record in the rocks tells us. "While adults 

 of modern birds do not have teeth, the embryos of many 

 species of birds have the germs of teeth, definite beginnings 

 while still in the tg^, and lose them before emerging from 

 the Qgg. The hind legs of the Greenland whale have a defi- 

 nite hip bone and thigh bone and a shin that never get out 

 far enough to reach the water (see Fig. 27) . In the fin-back 

 whale, there is a reduced hip bone and a bit of the thigh bone. 



