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 adaptative 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 
* Weismann, ‘The Evolution Theory,’ English edition, 
41. 313. 
