ADAPTATIONS OF WHALES 55 



front ; the neck is not marked ; the back rises in a 

 curve that gives easiest passage through the water, 

 and finishes off behind in a tail flattened from above 

 downwards. The general aspect and build resemble 

 those of a free-swimming fish, but, whilst fish have 

 a vertically flattened tail for horizontal movement, 

 whales require a horizontally compressed tail to make 

 those periodical ascents requisite for breathing air. 



Forward movement is maintained slowly or at 

 a prodigious speed by the huge longitudinal tail- 

 muscles swaying the body from side to side and making 

 effective undulations on the principle of a fish. The 

 legs are hidden and rudimentary, and their muscles 

 are added to those of the back : whilst the arms are 

 converted into steering-paddles as near a return to 

 fins as is well possible. The fingers, though still five 

 in number, are composed of more joints than the other- 

 wise universal number, three. The nails have dis- 

 appeared. Weight and size, no longer limited by the 

 supporting power of the limb, may reach colossal 

 dimensions ; whales of eighty feet in length and 

 weighing eighty tons still abound. 



To acquire powers of flight is perhaps an even 

 greater feat than the conversion of a terrestrial animal 

 into a whale. Amongst various groups of mammals 

 the first stage of flight — the arrest of the body in a 

 gliding descent, instead of in a straight fall, has been 

 accomplished. Some of the opossums have extended 

 that fold of skin which allows movement of our arms 

 at the armpit, into a membrane that stretches from 



