ADAPTATION OF LAND REPTILES TO LIFE IN WATER 67 



required when the body is surrounded on all sides by water of nearly 

 the same specific gravity as the body itself. And it is doubtless 

 for the same reasons that the articulations of all strictly aquatic 

 reptiles have for the most part become looser and less firm, espe- 

 cially those between the different vertebrae. 



The same looseness of articulation is also found in the ribs of 

 aquatic animals. In most animals, and in all those which walk 

 erect, Hke the mammals, each rib is firmly attached to the back- 

 bone by two distinct joints, the head and tubercle, with an interval 

 between them. This double attachment prevents much in-and-out 

 movement of the ribs and gives a firm support for the attach- 

 ment of the muscles of respiration, as well as for those supporting 

 the viscera. This firmness is unnecessary in animals hving always 

 in the water, and the ribs therefore in all aquatic animals tend to 

 become single-headed and loose. The lower or capitular articula- 

 tion has been lost in part, or almost wholly, in many cetaceans. It 

 has been said that a whale cast up on land will die of suffocation, 

 not for the lack of air, for it is an air-breathing animal like ourselves, 

 but because it can no longer use its respiratory muscles attached 

 to the loosely articulated ribs; it suffocates because the ribs 

 collapse. 



As would be expected, the greatest modifications of structure 

 in the adaptation of air-breathers to water life are found in the 

 hmbs. No other parts of the body have such different functions in 

 water and on land as the limbs and fins. The limbs of a dog, or a 

 cat, or a man are feeble organs for swimming in comparison with the 

 fins of a fish, and if the land animal must compete with fishes to 

 prey upon them for food it must acquire hke swimming powers. 

 As a matter of fact, the hmbs of all typically aquatic air-breathing 

 animals have lost nearly all external resemblance to the legs of 

 walking and running animals, and have become more or less fin-hke 

 in function — fin-like in shape and function, but never fin-like in 

 actual structure. No creature can go back and begin over again, 

 any more than a man can again become a child with all its possi- 

 bilities for improvement and development. If an animal cannot 

 modify the organs it already possesses so as to adapt them to new 

 and changed uses by the aid of evolutionary forces it must fail in 



