372 THE VARIATION OF ANIMALS IN NATURE 



means. This analogy cannot have much weight in face of 

 the very convincing suggestion that a great deal of specific and 

 racial differentiation is due to isolation and chance sur- 

 vival. Finally, we believe that the special weaknesses of the 

 selection theory render it unsuited to explain the origin 

 of complex organs, and the more profound co-ordinative 

 principles. 



As the case for Natural Selection is of such a kind as to require 

 what is virtually a suspense of judgment, we are driven to 

 inquire as to the claims of the other theories. 



When we turn to the suggestion that the prime factor in 

 evolution is the inheritance of induced modification or of the 

 effects of use and effort, it is possible to speak with more 

 assurance. 



As far as the experimental evidence is concerned, we 

 believe that there is some likelihood that mutations may be 

 induced by the direct effect of environmental factors on the 

 germ cells. For the inherited effects on structure 1 of use and 

 effort we find no evidence. We must admit that the time- 

 factor has to be taken into account. The hereditary behaviour 

 of ' Dauermodifikationen ' suggests that the germinal material 

 is susceptible to temporary modifications, and we regard it as 

 an open question whether stimuli applied for periods far 

 exceeding those employed in experiment might not produce 

 stable modifications. It is possible and even likely that 

 such influences might account for much local differentiation, 

 though we have little evidence for the transformation of whole 

 populations by their means. But we do not believe they are 

 capable of producing adaptations with their long-sustained 

 history of modification in a given direction. ' Lamarckian ' 

 processes involving long-continued use and effort would be 

 suited to produce such results ; but we have no evidence for 

 their occurrence. 



The theory that various phenomena of determinate 

 variation, excessive growth, and complexity are to be attributed 

 to an inner momentum also labours under the disability that 

 it does not account for the transformation of populations 

 except on the assumption that such changes occur en masse. 



1 We think it possible that modifications of habit, perhaps not due to 

 mutation at all, may nevertheless become permanent. The matter is still under 

 investigation, but its importance in evolution may well be found to be con- 

 siderable. 



