258 Evolution and Adaptation 



the lower side in the metamorphosis is an hereditary charac- 

 ter, and not a change produced in each individual by the 

 withdrawal of the lower side from the action of light. On 

 the other hand, the experiments show that the absence of 

 pigment-cells from the lower side throughout life is due to 

 the fact that light does not act upon that side, for, when it is 

 allowed to act, pigment-cells appear. It seems to me that 

 the only reasonable conclusion from these facts is, that the 

 disappearance of pigment-cells was originally due to the 

 absence of light, and that the change has now become 

 hereditary. The pigment-cells produced by the action of 

 light on the lower side are in all respects similar to those 

 normally present on the upper side of the fish. If the dis- 

 appearance of the pigment-cells were due entirely to a 

 variation of the germ-plasm, no external influence could 

 cause them to reappear, and, on the other hand, if there 

 were no hereditary tendency, the coloration of the lower 

 side of the flatfish when exposed would be rapid and 

 complete." * 



This evidence might be convincing were it not weakened 

 by two or three assumptions. In the first place, it is not 

 shown that if the loss of color on the lower side had been 

 the result of the inheritance of an acquired character that 

 the results seen in Cunningham's experiment would follow 

 as a consequence. Thus one of the starting-points of the 

 argument really begs the whole question. In the second 

 place, it is unproven that, had the loss of color of the lower 

 side been the result of a variation of the germ-plasm, no ex- 

 ternal influence could cause it to reappear. In this connec- 

 tion there is another fact that has a bearing on the point here 

 raised. In some species of flatfish the right side is turned 

 down, and in other species the left. Occasionally an indi- 

 vidual is found in a right-sided species that is left-sided, and 

 in such cases the color is also reversed. Now, to explain this 



1 Natural Science, October, 1893. 



