62 CREATIVE EVOLUTION [chap. 



rectly between the function and the organ, as is done in 

 the doctrine of finality, as also mechanism itself does. For 

 organ and function are terms of different nature, and each 

 conditions the other so closely that it is impossible to say 

 a priori whether in expressing their relation we should 

 begin with the first, as does mechanism, or with the second, 

 as finalism requires. But the discussion would take an 

 entirely different turn, we think, if we began by comparing 

 together two terms of the same nature, an organ with 

 an organ, instead of an organ with its function. In this 

 case, it would be possible to proceed little by little to a 

 solution more and more plausible, and there would be 

 the more chance of a successful issue the more resolutely 

 we assumed the evolutionist hypothesis. 



Let us place side by side the eye of a vertebrate and 

 that of a mollusc such as the common Pecten. We find 

 the same essential parts in each, composed of analogous 

 elements. The eye of the Pecten presents a retina, a 

 cornea, a lens of cellular structure like our own. There 

 is even that peculiar inversion of retinal elements which 

 is not met with, in general, in the retina of the inverte- 

 brates. Now, the origin of molluscs may be a debated 

 question, but, whatever opinion we hold, all are agreed 

 that molluscs and vertebrates separated from their common 

 parent-stem long before the appearance of an eye so com- 

 plex as that of the Pecten. Whence, then, the structural 

 analogy? 



Let us question on this point the two opposed systems 

 of evolutionist explanation in turn — the hypothesis of 

 purely accidental variations, and that of a variation di- 

 rected in a definite way under the influence of external 

 conditions. 



The first, as is well known, is presented to-day in two 

 quite different forms. Darwin spoke of very slight vari- 



