THE THEORY OF LAMARCK 35 



there are other instances in which the Lamarckian ex- 

 planation seems adequate, and where an alternative 

 hypothesis is lacking. Such a case is afforded by the 

 family of the flat fishes, including such well-known 

 species as the sole and plaice. In the adult condition 

 these fishes lie flat on one side ; and during their 

 development from the young condition, that eye which, 

 if it remained in its original position, would look 

 directly downwards travels round the head until it 

 comes to lie quite upon the upper surface. As Darwin 

 pointed out, agreeing in this with Mivart, a sudden 

 spontaneous transformation in the position of the eye 

 is hardly conceivable, and it is equally impossible to 

 explain the origin of this remarkable feature by the 

 action of natural selection, because a slight change 

 in the position of the eye could be of no advantage 

 so long as this organ remained upon the under surface. 

 The very young fish, whilst still symmetrical, are 

 known sometimes to fall upon one side, and when in 

 this position to twist the lower eye forcibly upwards. 

 Darwin himself therefore supposed that the origin of 

 the adult structure is to be attributed to the inherited 

 effect of efforts of this kind. 



The interest of the last case lies in the fact that it 

 relates to a structure, the origin of which does not 

 appear explicable on the theory of natural selection ; 

 its bearing will therefore be better understood when 

 we come to discuss that theory in the next chapter. 



The inherited effects of voluntary striving can clearly 

 have no application to the case of plants. Lamarck 

 therefore supposed that evolution in the vegetable 



32 



