I VARIATION AND NATURAL SELECTION 25 



It is generally recognised that it is one of the main elements 

 of Darwin's, as well as of every other theory of evolution, that 

 there is in every individual organic being an innate tendency 

 to vary from the standard of its predecessors, but that this 

 tendency is usually kept under the sternest control by the 

 opposite tendency to resemble them a force to which the 

 terms "heredity" and "atavism" are applied. The causes 

 of this initial tendency to vary, as well as those of its limits 

 and prevailing direction, and the circumstances which favour 

 its occasional bursting through the constraining principle of 

 heredity, offer an endless field for speculation. Though 

 several theories of variation have been suggested, I think that 

 no one would venture to say we have passed beyond the 

 threshold of knowledge of the subject at present. 



Taking for granted, however, as we all do, that this ten- 

 dency to individual variation exists, then comes the question, 

 What are the agents by which, when it has asserted itself, it 

 is controlled or directed in such a manner as to produce the 

 permanent or apparently permanent modifications of organic 

 structures which we see around us ? Is " survival of the 

 fittest " or preservation by natural selection of those variations 

 best adapted for their surrounding conditions (the essentially 

 Darwinian or still more essentially Wallacian doctrine) the 

 sole or even the chief of these agents ? Can isolation, or the 

 revived Lamarckian view of the direct action of the environ- 

 ment, or the effects of use or disuse accumulating through 

 generations, either singly or combined, account for all ? or is 

 it necessary to invoke the aid of any of the numerous subsidiary 

 methods of selection which have been suggested as factors in 

 bringing about the great result ? 



Any one who has closely followed these discussions, especially 

 those bearing most directly upon what is generally regarded as 

 the most important factor of evolution natural selection, or 

 "survival of the fittest" cannot fail to have noticed the 

 appeal constantly made to the advantage, the utility, or 

 otherwise of special organs or modifications of organs or 

 structures to their possessors. Those who have convinced 

 themselves of the universal application of the doctrine of 

 natural selection hold that every particular structure or 



