88 CREATIVE EVOLUTION [chap. 



molluscs and vertebrates. The idea of an "original 

 impetus, " moreover, will thus be made clearer. 



Two points are equally striking in an organ like the 

 eye: the complexity of its structure and the simplicity 

 of its function. The eye is composed of distinct parts, 

 such as the sclerotic, the cornea, the retina, the crystalline 

 lens, etc. In each of these parts the detail is infinite. The 

 retina alone comprises three layers of nervous elements — 

 multipolar cells, bipolar cells, visual cells — each of which 

 has its individuality and is undoubtedly a very compli- 

 cated organism: so complicated, indeed, is the retinal 

 membrane in its intimate structure, that no simple de- 

 scription can give an adequate idea of it. The mechanism 

 of the eye is, in short, composed of an infinity of mechan- 

 isms, all of extreme complexity. Yet vision is one simple 

 fact. As soon as the eye opens, the visual act is effected. 

 Just because the act is simple, the slightest negligence 

 on the part of nature in the building of the infinitely com- 

 plex machine would have made vision impossible. This 

 contrast between the complexity of the organ and the 

 unity of the function is what gives us pause. 



A mechanistic theory is one which means to show us 

 the gradual building-up of the machine under the influence 

 of external circumstances intervening either directly by 

 action on the tissues or indirectly by the selection of better- 

 adapted ones. But, whatever form this theory may take, 

 supposing it avails at all to explain the detail of the parts, 

 it throws no light on their correlation. 



Then comes the doctrine of finality, which says that 

 the parts have been brought together on a preconceived 

 plan with a view to a certain end. In this it likens the 

 labor of nature to that of the workman, who also pro- 

 ceeds by the assemblage of parts with a view to the real- 

 ization of an idea or the imitation of a model. Mechanism, 



