ORIGIN OF THE SPECIFIC TYPE 



say, to a class Avhose structure was Ijuilt for life on the land. 



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 



whole inherited, but 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 independentl}' of the in- 

 fluence of external conditions. But 

 there is much more than this. A 

 thick layer of blubber under the 

 skin gives this warm-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 this reason the 

 hind-legs became superfluous and de- 



3b3 



The 



generated. 



Similarly, the muscles of 



Fig. 130. Skeleton of a Greenland 

 Whale with the contour of the body. 

 Ok, upper jaw. I'k, 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 Clans. 



