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can be caused by the environment, but the fact that individual 

 adjustments do have definite relations to the environment, has 

 served to sustain a belief in the environmental causation of evo- 

 lution. All species have, of course, fitness for their environ- 

 ments ; otherwise they would not continue to exist. They must 

 be more fit than other species which have had access to the 

 same environments, or they would be driven out. Neverthe- 

 less, inside of the general environment, or place of the species in 

 the economy of nature, there is still a very great diversity of 

 individual experience to which each organism must adjust itself. 

 The environment at all times determines the relation of fitness, 

 but the characters which afford the fitness are as truly results of 

 evolution as any other characters. It has not been shown that 

 they are caused by the environment or that they can be inherited 

 from it. 



The doctrine of environmental causation of evolution supports 

 one assumption by another equally baseless. It takes for 

 granted that adjustment differences between individuals of the 

 same species are caused by the environmental differences which 

 are met by these same adjustments. It also takes for granted 

 that the general fitness or adaptation of the species is merely a 

 product of the fitness of individual adjustment, whereas there 

 are two phenomena of fitness which are quite distinct in their 

 relations to the problem of evolutionary causes, though neither 

 of them affords any special indication regarding the nature of 

 such causes. The adjustment of individuals to differences of 

 environment is a form of organic elasticity which permits lateral 

 vibrations or displacements of characters, while the fitness of a 

 species or genus as a whole is, obviously, an accomplished result 

 of evolution instead of being a formative principle or cause. 



ADAPTIVE VERSATILITY OF ORGANISMS. 



To say, as has been the custom of writers on evolution, that 

 organisms are plastic or susceptible of environmental influences, 

 is only half of the truth. Organisms are not merely plastic, 

 but versatile. Under different conditions they are able to grow 

 in different ways, and often in ways which qualify them better 

 for existence in these particular conditions, though not neces- 



