OTHER THEORIES OF SPECIES-FORMING. 199 



variations a contradiction which formed a serious stum- 

 bling-block to the theory of selection. Though still assum- 

 ing that primary variations are 'accidental,' I yet hope to 

 have demonstrated that an interior mechanism exists which 

 compels them to go on increasing in a definite direction, the 

 moment selection intervenes. Definitely directed variation 

 exists, but not predestined variation running on independ- 

 ently of the life conditions of the organism as Nageli, to 

 mention the position that the most extreme advocate of 

 this doctrine has assumed : on the contrary, the variation is 

 such as is elicited and controlled by those conditions them- 

 selves, though indirectly." 



Obviously Weismann in his theory of germinal selection 

 has preserved the actuality of a struggle and a selection, but 

 with a "rehabilitation" of natural selection in the real Dar- 

 winian meaning and only fair application of the phrase the 

 new theory has nothing to do. It is, much more, a distinct 

 admission of the inadequacy of natural selection to do what 

 has long been claimed for it. It is the first serious attempt 

 at a causo-mechanical explanation of a theory of ortho- 

 genesis, that is, variation along determined lines. 



As to our acceptance or non-acceptance of such a theory 

 we need say little. It consists of two purely speculative basic 

 assumptions : First, Weismann's particular theory of the 

 ultimate structure of the germ-plasm, namely, the theory of 

 biophors and determinants ; and secondly, the assumption , 

 that there is a struggle for food among the determinants. 

 There is no proof of pure observation or experiment for 

 the theory, and there is some proof directly against it. And I 

 yet the great need of a working hypothesis for the causo- 

 mechanical explanation of determinate variation makes us 

 give such a pure speculation more attention than it might 

 otherwise get. Unfortunately the attention thus given to 

 this particular theory seems toAiave resulted in the bringing 

 forward of some rather serio)us objections to the possibility 



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