1.1 RESULT OF THE DISCUSSION 87 



in the vegetable kingdom. Even in animals, it will act 

 only on points which are under the direct or indirect con- 

 trol of the will. And even where it does act, it is not clear 

 how it could compass a change so profound as an increase 

 of complexity: at most this would be conceivable if the 

 acquired characters were regularly transmitted so as to 

 be added together; but this transmission seems to be 

 the exception rather than the rule. A hereditary change 

 in a definite direction, which continues to accumulate 

 and add to itself so as to build up a more and more complex 

 machine, must certainly be related to some sort of effort, 

 but to an effort of far greater depth than the individual 

 effort, far more independent of circumstances, an effort 

 common to most representatives of the same species, 

 inherent in the germs they bear rather than in their sub- 

 stance alone, an effort thereby assured of being passed on 

 to their descendants. 



So we come back, by a somewhat roundabout way, 

 to the idea we started from, that of an original impetus 

 of life, passing from one generation of germs to the fol- 

 lowing generation of germs through the developed organ- 

 isms which bridge the interval between the generations. 

 This impetus, sustained right along the lines of evolution 

 among which it gets divided, is the fundamental cause 

 of variations, at least of those that are regularly passed 

 on, that accumulate and create new species. In general, 

 when species have begun to diverge from a common stock, 

 they accentuate their divergence as they progress in their 

 evolution. Yet, in certain definite points, they may evolve 

 identically; in fact, they must do so if the hypothesis of a 

 common impetus be accepted. This is just what we shall 

 have to show now in a more precise way, by the same 

 example we have chosen, the formation of the eye in 



