No. 643] 



EFFECTS OF ENVIRONMENT 



157 



for two days ; and makes similar changes in the absence of 

 light for about a week. There is a physiological rhythm 

 that corresponds to periodic environmental changes. 



A fourth generalization must again relate chiefly to 

 adaptation — Though animals possess considerable power 

 of adjustment to new or changed factors in their environ- 

 ment, they apparently do not usually become adapted as 

 species to physiographic changes, but are eliminated by 

 the variation of factors beyond their limit of toleration. 

 One species or group of species succeeds another during 

 physiographic succession. However, animals do respond 

 in an adaptive way to rhythmical daily, monthly and 

 seasonal successions. Some animals show adaptive re- 

 sponses to rln'thmical environmental changes only once 

 during their life cycle. Salmon, for example, do not 

 migrate up rivers to spawn until they have reached a 

 certain age. Animals apparently become most special- 

 ized, or adapted to particular environments, when con- 

 ditions are most stable. Even the striking instances of 

 adaptations to rhythms show this tendency of adaptation 

 to attain stability — in this case a regularly changing sta- 

 bility. 



Environmental changes have been important in their 

 effects on the evolution of animals. In this paper it has 

 been shown that living systems of activities are adapted 

 to the environment ; that they respond to the environment 

 by transformation, selective survival, or migration; that 

 each habitat limits the patterns of the systems that exist 

 within it; and, that, though adaptation to environment 

 may permit precise adjustment to rhythmical changes 

 extending over considerable periods, and though animals 

 generally become most specialized when conditions are 

 most stable, there is no evidence that living systems are 

 caused to change from one species to another by the 

 transformations of habitats due to physiographic suc- 

 cession. The pattern of evolution is set by environment, 

 but there is little or no evidence that changing envi- 

 ronment causes adaptive variations of such a degree 

 that new species are produced. Animals adapt them- 



