THE QUEST OF A CRITERION 59 



the strongest argument against mechanism. So we 

 must at once indicate in a general way, before passing 

 to the detail, why explanations from &quot;adaptation &quot; seem 

 to us insufficient. 



Let us first remark that, of the two hypotheses 

 just described, the latter is the only one which is not 

 equivocal. The Darwinian idea of adaptation by auto 

 matic elimination of the unadapted is a simple and clear 

 idea. But, just because it attributes to the outer cause 

 which controls evolution a merely negative influence, 

 it has great difficulty in accounting for the progressive 

 and, so to say, rectilinear development of complex 

 apparatus such as we are about to examine. How 

 much greater will this difficulty be in the case of the 

 similar structure of two extremely complex organs 

 on two entirely different lines of evolution ! An 

 accidental variation, however minute, implies the 

 working of a great number of small physical and 

 chemical causes. An accumulation of accidental varia 

 tions, such as would be necessary to produce a com 

 plex structure, requires therefore the concurrence 

 of an almost infinite number of infinitesimal causes. 

 Why should these causes, entirely accidental, recur the 

 same, and in the same order, at different points of space 

 and time ? No one will hold that this is the case, 

 and the Darwinian himself will probably merely main 

 tain that identical effects may arise from different causes, 

 that more than one road leads to the same spot. But 

 let us not be fooled by a metaphor. The place reached 

 does not give the form of the road that leads there ; 

 while an organic structure is just the accumulation of 

 those small differences which evolution has had to go 

 through in order to achieve it. The struggle for life 

 and natural selection can be of no use to us in solving 



