OKIGIN OF THE SPECIFIC TYPE 



313 



say, to a class whose structure was built for life on the land. The 

 ancestors of Cetaceans were similar 

 to the other mammals, and possessed 

 a coat of hair and four legs, and a 

 body the mass of which was so dis- 

 tributed that it could be borne by 

 those four legs. But all the modern 

 Cetaceans live in the sea, and they 

 have therefore entirely changed their 

 bodily form ; they have become spindle- 

 shaped like fishes, well adapted for 

 cleaving the water, but incapable of 

 moving upon land. At the same time, 

 their hind-legs have completely disap- 

 peared, and can now be demonstrated 

 only as rudiments within the mass of 

 muscle (Fig. 130, Br, Tr, Fr), while 

 the fore-legs have been transformed 

 into flippers, in which, however, the 

 wdiole inherited, l)ut greatly shortened, 

 skeleton of the mammalian arm is 

 concealed (OA, UA, H). The skin has 

 lost its covering of hair so completely 

 that in some cases no traces of it are 

 demonstrable except in the embryo. 

 All these changes are adaptations to an 

 aquatic life, and could not have been 

 produced independently of the in- 

 fluence of external conditions. But 

 there is much more than this. A 

 thick layer of blubber under the 

 skin gives this w^arm-blooded animal 

 an effective protection against being 

 cooled down by the surrounding water, 

 and at the same time gives it the 

 appropriate specific gravity for life in 

 the sea ; an enormous tail-fin similar 

 to that of fishes, but placed hori- 

 zontally, forms the chief organ of 

 locomotion, and for tliis reason the 

 hind-legs became superfluous and de- 

 generated. Similarly, the muscles of 



Fig. 130. Skeleton of a Grreenland 

 Whale with the contour of the body. 

 Ok, upper jaw. L'A-, lower jaw. Sch, 

 shoulder-blade. OA, upper arm. 

 UA, bones of fore-arm. H, hand. 

 Br, vestige of the pelvis. Fr, vestige 

 of the femur. Tr, vestige of the lower 

 part of the leg. After Claus. 



