t ORTHOGENESIS 73 



changes, as before to be assured of the continuity of 

 direction of successive variations. But in neither case can 

 parallel development of the same complex structures 

 on independent lines of evolution be due to a mere 

 accumulation of accidental variations. So we come 

 to the second of the two great hypotheses we have 

 to examine. Suppose the variations are due, not to 

 accidental and inner causes, but to the direct influence 

 of outer circumstances. Let us see what line we 

 should have to take, on this hypothesis, to account 

 for the resemblance of eye-structure in two series that 

 are independent of each other from the phylogenetic 

 point of view. 



Though molluscs and vertebrates have evolved 

 separately, both have remained exposed to the influence 

 of light. And light is a physical cause bringing forth 

 certain definite effects. Acting in a continuous way, 

 it has been able to produce a continuous variation 

 in a constant direction. Of course it is unlikely 

 that the eye of the vertebrate and that of the mollusc 

 have been built up by a series of variations due to 

 simple chance. Admitting even that light enters into 

 the case as an instrument of selection, in order to 

 allow only useful variations to persist, there is no 

 possibility that the play of chance, even thus supervised 

 from without, should bring about in both cases the 

 same juxtaposition of elements coordinated in the same 

 way. But it would be different supposing that light 

 acted directly on the organized matter so as to change 

 its structure and somehow adapt this structure to its 

 own form. The resemblance of the two effects would 

 then be explained by the identity of the cause. The 

 more and more complex eye would be something like 

 the deeper and deeper imprint of light on a matter 



