i.) ORTHOGENESIS 69 



changes, as before to be assured of the continuity of di- 

 rection 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 accu- 

 mulation 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 hypothe- 

 sis, 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 with- 

 out, should bring about in both cases the same juxta- 

 position 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 some- 

 how 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 which, being organized, possesses a special aptitude 

 for receiving it. 



But can an organic structure be likened to an imprint? 



