U ORTHOGENESIS 71 



cumstances to good account than by adapting itself to 

 them passively at the outset. Where it has to direct a 

 movement, it begins by adopting it. Life proceeds by 

 insinuation. The intermediate degrees between a pig- 

 ment-spot and an eye are nothing to the point: however 

 numerous the degrees, there will still be the same interval 

 between the pigment-spot and the eye as between a photo- 

 graph and a photographic apparatus. Certainly the photo- 

 graph has been gradually turned into a photographic 

 apparatus; but could light alone, a physical force, ever 

 have provoked this change, and converted an impression 

 left by it into a machine capable of using it? 



It may be claimed that considerations of utility are 

 out of place here; that the eye is not made to see, but that 

 we see because we have eyes; that the organ is what it is, 

 and " utility " is a word by which we designate the functional 

 effects of the structure. But when I say that the eye 

 "makes use of" light, I do not merely mean that the eye 

 is capable of seeing; I allude to the very precise relations 

 that exist between this organ and the apparatus of lo- 

 comotion. The retina of vertebrates is prolonged in an 

 optic nerve, which, again, is continued by cerebral centres 

 connected with motor mechanisms. Our eye makes use 

 of light in that it enables us to utilize, by movements of 

 reaction, the objects that we see to be advantageous, and 

 to avoid those which we see to be injurious. Now, of 

 course, as light may have produced a pigment-spot by 

 physical means, so it can physially determine the move- 

 ments of certain organisms; ciliated Infusoria, for in- 

 stance, react to light. But no one would hold that the 

 influence of light has physically caused the formation of 

 a nervous system, of a muscular system, of an osseous 

 system, all things which are continuous with the apparatus 

 of vision in vertebrate animals. The truth is, when one 



