234 THE PHILOSOPHY OF BIOLOGY 



nerve-endings are on the surface of the retina, which is 

 turned away from the hght, that is, the optic nerve 

 runs towards the anterior surface of the retina, and 

 then its fibres turn backwards. This " inversion of 

 the retinal layers " occurs in all vertebrate animals, but 

 it is exceptional in the invertebrates. The above 

 general description applies equally well to the eye of 

 the vertebrate and to that of Pecten. 



Let us admit that these mantle organs in Pecten 

 are eyes, for there is no conclusive experimental evi- 

 dence that they really are visual organs, and plausible 

 reasoning suggests that they may subserve other 

 functions. Let us assume that the minute structure 

 of the Pecten eye is similar to that of the vertebrate, 

 and that its development is also similar : as a matter 

 of fact both histology and embryology are different. 

 Then we have to explain, on the principles of natural 

 selection, the parallel evolution of similar structures 

 along independent lines of descent ; for mollusc and 

 vertebrate have certainly been evolved from some very 

 remote common ancestor in which the eye could not 

 have been more than a simple pigment spot with a 

 special nerve termination behind it. In each case the 

 organ was formed by a very great number of serially 

 occurring variations, yet these two sets of variations 

 must have been the same at each stage in two in- 

 dependently occurring processes. On any reasonable 

 assumption as to the number of co-ordinated variations 

 required, and their chances of occurrence, the mathe- 

 matical improbability that these two series of varia- 

 tions did occur is so great as to amount to impossibility 

 so far as our theory of transformism is concerned. 

 Natural selection could not, therefore, have produced 

 these two organs. 



This argument of Bergson's fails, of course, in the 



