THE MECHANISTIC CONCEPTION 23 



tadpole, the animal soon forms a new one, thus mak- 

 ing the eye apparently as efficient as before. That 

 there should be a regeneration of the lens at all is, of 

 course, in itself a notable thing. But in this case 

 there is something more to consider than the fact of 

 regeneration something which presents the embry- 

 ologist with an extremely puzzling problem. For 

 the new lens is not made in the same way or from the 

 same kind of material as the old one. In normal 

 development the lens is formed from the outer skin, 

 or ectoderm, of the head. When the lens is regen- 

 erated, it is built up by cells from the iris, which 

 forms the edge of the optic cup, and this originates 

 in the embryo as an outgrowth from the brain itself, 

 and not from the outer skin. Neither the animal 

 itself nor any of its ancestors can reasonably be 

 supposed to have become habituated to such a 

 method of regeneration. It is therefore difficult 

 to see how such a singular power as this can have 

 been acquired and how it can be a potential quality 

 in the cells of the organism. Some naturalists are 

 disposed to attribute these properties to some sort 

 of intelligent purposive action. But if this is really 

 the case, why do the higher animals and man lack the 

 help of similar favoring activities? The truth is 

 that the supporter of the mechanistic view of life is 

 brought to a standstill by questions of this nature 

 in the sense that he can see no hope of solving them 

 in the near future on the basis of any known physico- 

 chemical forces. Writers who have felt this diffi- 



