2 74 READINGS IN BIOLOGICAL SCIENCE 



ample, has a dozen unique properties that condition life. Carbon dioxide 

 could not be replaced by any other substance. In brief, given the environ- 

 ment ns it is, life could not be other than it is. The evolution of the environ- 

 ment and the evolution of the organisms have gone hand in hand. 



In the case of plants the action of the environment is remarkably direct; 

 for the plant cannot get away from a fixed environment. If the environ- 

 ment undergoes material change, the plant's only response is a structural 

 one. For example, if plants that are accustomed to a relatively humid cli- 

 mate are grown in the desert they develop numerous xerophytic adapta- 

 tions such as small leaves with greatly diminished transpiration surface, 

 a thick epidermis, hairs, or spines, small stature, deep-root system, and 

 other similar protections against the inimical desert conditions. Similarly, 

 plants accustomed to grow in relatively dry soil, if grown in soil that is 

 covered over with water, will produce aquatic leaves and roots and un- 

 dergo appropriate changes in epidermis and loss of supporting tissues, for 

 plants that are buoyed up by water need little support. 



Animals, on the other hand, are for the most part not so intimately 

 related to a local environment as are plants. They are characteristically 

 mobile creatures with varying capacities for wandering about and select- 

 ing the habitat that best suits them. "Animals select their habitats. By this 

 we do not mean that the animal reasons, but that selection results from 

 regulating behavior. The animal usually tries a number of situations as the 

 result of random movements, and stays in the set of conditions in which 

 its physiological processes are least interfered with," according to V. E. 

 Shelford. 



Many special adaptations may be explained through habitat choice. 

 Thus animals such as the duckbill platypus, the lung-fishes, and others 

 whose teeth are replaced by bony or chitonous plates that are used for 

 crushing the hard shells of molluscs and crustaceans, may not confidently 

 be said to have developed these crushing appUances in adaptation to a habit 

 of feeding upon hard-shelled prey; but rather it seems more likely that the 

 loss of teeth and the development of crushers occurred through a degener- 

 ative process incident to racial senescence and that the possession of the 

 crushing equipment enabled them to avail themselves of a new type of 

 food, formerly unavailable to them. 



SOME SPECIAL ADAPTATIONS 



The mammary glands of mammals are skin glands usually with well-de- 

 fined ducts leading to the surface and terminating in teats. In the lowest 

 mammals, the monotremes or egg-laying mammals, these glands are rela- 

 tively poorly developed and difi^use; also they are known to be developed 

 through a regional specialization of sweat glands. In the true mammals the 

 glands are modified sebaceous or oil glands and may be seen to develop 

 from the same embryonic rudiments as the latter. 



