190 READINGS IN 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 varying 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 lives 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 Joca] 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. Wecannot in sucha 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 (1913). 
