CHAPTER XV 

 ADAPTATION 



Every animal seems to be fitted at least moderately well to its 

 environment. A certain amount of harmony, between the 

 anatomical structure and functions of organisms and the environ- 

 ment, is evident to even the casual observer. The common 

 grass frog, Rana pipiens, for example, possesses structural and 

 functional features that can be understood if interpreted with 

 reference to an amphibious habitat. Its well-developed hind- 

 limbs, provided with webbed toes, and its shorter forelimbs 

 enable it to move about in water or on land with equal facility. 

 It has lungs with which it can obtain oxygen by breathing air; 

 but in addition a considerable amount of oxygen can also be 

 absorbed through the skin. It can remain for a long time under 

 water. Its protective coloration renders it inconspicuous. The 

 young develop from eggs laid in the water. The tadpoles upon 

 hatching are capable of living entirely in water until sufficient 

 time has elapsed for organs to develop to the point where the 

 animals can live on land as well. The adults can survive a con- 

 siderable range of temperatures. A frog may be frozen in a 

 block of ice and, when the ice is thawed, resume its normal 

 activities none the worse for the change. The frog is more or 

 less specialized for the conditions under which it lives, but this 

 specialization in turn imposes limitations. Thus a frog requires 

 a moist environment. Because its skin cannot prevent a loss of 

 water in a dry atmosphere, a frog left overnight in a warm dry 

 room may be dead in the morning, its body shrunken to a fraction 

 of its normal size. 



There are various views about the origin and significance of 

 adaptation. The old traditional view of adaptation, such as was 

 held by Georges Cuvier (1769-1832), the greatest comparative 

 anatomist of his time, looked upon adaptation as a universal 

 property of living matter. The adaptive relation of organisms to 

 environment is an expression of a plan of nature ruled by Provi- 

 dence. It is an a priori established harmony in living things 



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