THE ALLIES OF THE SQUIRREL 417 



though functionally reduced to mere paddles, with no power of 

 motion except at the shoulder- joint, have beneath their smooth 

 and continuous covering all the bones, joints, and even most of 

 the muscles, nerves, and arteries of the human arm and hand ; 

 the rudiments of hind legs, found buried deep in the interior of the 

 animal, apparently subserve no useful purpose, but point an in- 

 structive lesson to those who are able to read it. 



Cetaceans are found in all seas, and feed on fishes, crus- 

 taceans, and the smaller floating animal life of the ocean 



Fig. 219. Comparison of the size of an elephant with that of a whale 1 



generally. They vary from four to eighty feet in length, 

 some of the whales being the largest of existing animals 

 (Fig. 219). There is every reason to believe that the group 

 is descended from land mammals. The whalebone whales 

 are species without teeth, but with a development of baleen, 

 or whalebone, in the upper jaw, which acts as a strainer. By 

 means of the closely set, flexible strips of whalebone the 

 small animals on which they feed are retained, while the 

 water is forced out. Several species_are found in the Atlantic 

 and Pacific oceans. The sperm whale (Physe'ter macro- 

 ceph'alus) has a square head, within which is a cavity con- 

 taining oil which, on being refined, yields spermaceti. 



Hoofed Mammals. The great assemblage of animals called 

 Ungula'ta (Lat. unguis, "nail") includes the hippopota- 



1 From R. S. Lull's Organic Evolution. Used by permission of The MacMillan 

 Company. 



