THE VITAL IMPETUS 



93 



diverge from a common stock, they accentuate their 

 divergence as they progress in their evolution. Yet, in 

 certain definite points, they may evolve identically ; in 

 fact, they must do so if the hypothesis of a common 

 impetus be accepted. This is just what we shall have 

 to show now in a more precise way, by the same 

 example we have chosen, the formation of the eye in 

 molluscs and vertebrates. The idea of an &quot; original 

 impetus,&quot; 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 complicated organism : so complicated, indeed, 

 is the retinal membrane in its intimate structure, that 

 no simple description can give an adequate idea of it. 

 The mechanism of the eye is, in short, composed of 

 an infinity of mechanisms, 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 complex 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 



