Tendency to Vary. 249 



The law of natural selection, as a fundamental 

 process, has long since passed the stage of discussion. 

 But there has always remained one unsettled ques- 

 tion lying at its very base, which Darwin himself 

 admitted to be an open one. That question con- 

 cerns the cause itself of variation. It is granted 

 that, admitting the tendency to vary, all the results 

 claimed for natural selection must follow ; but many 

 declare that, in this very tendency to vary, there is a 

 mystery as great as the mystery of life itself. 



It is only in this work on the " Power of Move- 

 ment in Plants " that Darwin has really assailed this 

 last fortress of supernaturalism. Not that he has 

 avowed any such purpose, for of this he would have 

 been incapable, but so skilfully and so powerfully 

 has he marshalled the facts that the conclusion fol- 

 lows without being stated. No one can doubt that 

 he perceived this, and I, for one, am convinced that 

 he saw it from afar, and that it was the great end of 

 his labours ; but with his characteristic wisdom he has 

 declined to invoke the odium theologicum, correctly 

 judging that the truth must ultimately assert itself. 



The tendency to vary, then, is a mechanical result 

 of the proved fact of universal movement coupled 

 with the admitted law of natural selection. By 

 means of the former all plants and growing parts of 

 plants are perpetually exploring their immediate 

 surroundings in search, as it were, for conditions 

 favourable to development. By means of the latter 

 they are able to avail themselves of such favourable 

 conditions when found. Nothing further than this 

 is required to complete the. natural explanation of all 



