PRESENT ASPECTS OF EVOLUTION 307 



erred in being too exclusiyely mechanical in their 

 theories. It is the main business of the scientific man 

 to discover and study mechanisms. But he must re- 

 member that mechanism does not produce force, it 

 only transmits it. If he maintains that he has noth- 

 ing to do with anything outside of mechanism, that 

 the invisible and imponderable force lies outside of 

 his domain, he has handed over to metaphysics the 

 fairest and richest portion of his realm. In our fear 

 of being metaphysical we have swung to another ex- 

 treme, and have lost sight of valuable truth which lay 

 at the bottom of the old vitalistic theories. Cells, 

 tissues, and organs are but channels along which the 

 flood of life-force flows. Boveri has well said, " There 

 is too much intelligence (Verstand) in nature for any 

 purely mechanical theory to be possible." 



Each theory contains important truth. Nageli's 

 view of the importance of initial tendencies, inherent 

 in the original living substance, is too often under- 

 valued. My own conviction, at least, is steadily 

 strengthening that, without some such original ten- 

 dency or aim, evolution would never have reached its 

 present culmination in man. His error lies in em- 

 phasizing this factor too exclusively. The funda- 

 mental proposition of Weismann's theory, that hered- 

 ity is due to continuity of germ-plasm, seems to contain 

 important truth. But we need not therefore accept 

 his theory of a germ-plasm so isolated and indepen- 

 dent as to be beyond control or influence by the habits 

 of the body. The importance of use and disuse, and 

 the transmissibility of their effects, would seem to 

 supply a factor essential to evolution. AVeismann has 

 done good service in emphasizing the stability of the 



