RECAPITULATION 213 



stimulation assumes that the same stimulus has been 

 acting for many generations. 



It is necessary, however, to consider how far the 

 conclusions drawn from these experiments are con- 

 tradicted by the mutations occurring in nature, some 

 of which have already been mentioned. We will 

 consider first ambicolorate specimens. If the 

 absence of pigment from the lower side in normal 

 Flat-fishes is due to the absence of light, how is it 

 that the pigmentation persists on the lower side of 

 ambicolorate specimens, which is no more exposed to 

 light than in normal specimens ? The answer is that 

 in the mutants the determinants for pigmentation 

 are united with the determinants for the lower side 

 of the fish. My view is that the differentiation of 

 these determinants for the two sides was due in the 

 course of evolution to the different exposure to light, 

 was of somatic origin, but once the congenital 

 factors or determinants were in existence they were 

 liable to mutation, and thus in the ambicolorate 

 specimens there is a congenital tendency to pig- 

 mentation on the lower side, which would only be 

 overcome by exclusion of light for another series of 

 generations. 



Mutations also occur in which part or whole of 

 the upper side is white and unpigmented. Several 

 such specimens are mentioned in the memoir by 

 myself and Dr. MacMunn in the Phil. Trans, already 

 cited, one being a Sole which was entirely white on 

 the lower side, and also on the upper, which was 

 pigmented only over the head region from the free 

 edge of the operculum forwards. Since the upper 

 sides in these specimens are fully exposed to light in 

 the natural state and yet remain unpigmented, it 



