52 THE THEORY OF NATURAL SELECTION 



the creature would very soon cease to cumber the earth. 

 But the student of adaptation goes into details, and 

 endeavours to find a use for every minute point of 

 structure, on the assumption, which we shall presently 

 see to be open to criticism, that but for their useful- 

 ness these details would not exist. We may proceed 

 to glance at one or two examples of the kind of thing 

 which is meant when it is said that an animal or plant 

 exhibits very marked adapt ative features. 



The order Cetacea belongs to the class of mammals 

 of which the more typical members are land animals 

 possessing four legs, and having their bodies covered 

 with hair. The true whalebone whales, a sub-order 

 which includes the Greenland whale, are in many ways 

 the most specialized members of the group. 



The Greenland whale* has a spindle-shaped body 

 like that of a fish, and its fore limbs are modified into 

 flippers resembling the pectoral fins of fishes. The 

 hind legs are only represented by a few rudimentary 

 bones, which are completely hidden within the body 

 wall, and the function of propulsion, which is performed 

 by the hind legs in such less completely aquatic animals 

 as seals, is here taken over by a great tail-fin which 

 resembles that of a fish, except that it is placed hori- 

 zontally. Hair is absent, but under the skin a thick 

 layer of blubber is developed, which prevents a too 

 rapid loss of heat, and at the same time adjusts the 

 specific gravity of the body to that of the surrounding 

 water. External ears are entirely wanting, and the 



* Weisinann, ' The Evolution Theory,' English edition, 



