196 EVOLUTION, GENETICS, AND EUGENICS 



dry soil, if grown in soil that is covered over with water, will produce 

 aquatic leaves and roots and undergo 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 characteristi- 

 cally mobile creatures with var3dng capacities for wandering about and 

 selecting the habitat that best suits them. 



"By virtue of being unlike or possessing different properties," 

 says Shelford,* " the various animal species require different conditions 

 for the best adjustment of their internal processes. For example, the 

 carp lives in shallow and muddy ponds and rivers, while the brook 

 trout Hves only in clear swift streams. These two organisms are able 

 to move about and find places to which they are suited. The differ- 

 ences between them are clearly indicated by the differences in the 

 habitats which they prefer. 



"By observation and by experimentation it has been shown that 

 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. This process is called selection by 

 trial and error. If animals are placed in situations where a number of 

 conditions are equally available, they will almost always be found liv- 

 ing in or staying most of the time in one of the places. The only 

 reason to be assigned for this unequal or local distribution of the ani- 

 mals is that they are not in physiological equilibrium in all the places. 

 However, some animals move about so much that it is with some 

 difficulty that we determine what their true habitats are." 



This idea of habitat preference and habitat selection is extremely 

 important for a correct understanding of adaptation, or the fitness of 

 organisms to environments. Much of the observed fitness may be due 

 to the fact that an organism has chosen out of a wide range of environ- 

 ments the one that best suits it. We cannot in such a case say that the 

 environment has had a direct influence in shaping the organism any 

 more than we could say that, when a man tries on various shoes and 

 finds a pair to fit, he has been responsible for the fitness of the shoes. 



Many special adaptations may be explained through habitat 

 choice. Thus animals such as the duckbill platypus, the lung-fishes, 



' V. E. Shelford, Animal Communities in Temperate America (igi3). 



