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THE AMERICAN NATURALIST [Vol. LVI 



selves to environment by changing their systems of ac- 

 tivities, but such responses are apparently limited in 

 extent to the inherent possibilities of variation already 

 within the system. Animals have great powers of adapta- 

 tion to environment, but are not fundamentUy changed 

 by it. Environment permits evolution and controls its 

 course, but does not appear to cause it. If variations fit 

 environment, they are adaptive; if they do not, systems 

 cease to exist. Environment does not appear to cause 

 variation. The living mechanism still holds the mystery 

 of variation within itself. Until there is conclusive evi- 

 dence, this one great remaining problem of evolution 

 can not be solved. Yet, notwithstanding this lack of evi- 

 dence, there are still those who belive the environment 

 does cause evolution — ^though their only foundation for 

 such belief is what Bergson calls " intuition." Until 

 there is proof, science, if it would be scientific, must keep 

 in mind that these " faithful " believers may be right, 

 and be content to wait, perhaps a hundred thousand years 

 — for evidence. 



SUMMAEY 



1. Animals are systems of activities that are adapted 

 to environment. 



2. Animals become adapted to the environment by 

 transformation, selective survival, migration. 



3. Each habitat limits the patterns of systems of ac- 

 tivities that may result from reactions within it. The 

 type of adaptation is set by the environment. 



■1. Though animals possess considerable power of ad- 

 justment to changes in environment, there is no evidence 

 that they became adapted as species to slow changes due 

 to p]iysi()ora].]iic succession. They do respond to rhyth- 

 mical daily, iiKintlily, and seasonal changes in an adaptive 

 way. Animals a])i»ear to become most specialized, or 

 adapted to i>articu]ar environments, when conditions are 

 most stable. 



5. Environment permits and directs evolution, but does 

 not appear to cause it by forcing the acquirement of new 

 characters. 



