VARIATION 367 



the coloured left side downwards. There was some colour 

 on the upper or right side of the head, so that the condition 

 of the fish would be nearly described by saying that the body 

 was normal, but the head reversed. 



Assuming that these abnormalities are mutations, and not 

 due to any exceptional conditions of life or development, we 

 must regard them as the result of abnormalities in the constitu- 

 tion of the eggs from which the fishes develop. According to 

 a well-known theory of heredity, the egg is supposed to contain 

 elements called determinants, corresponding with the parts of 

 the body into which the egg develops. Thus, in the egg of a 

 flat-fish we may suppose that there is a set of determinants for 

 the right side and another set for the left ; if these two sets 

 have in a given egg changed places, that egg develops into a 

 reversed specimen. In the abnormal turbot last described we 

 may suppose that reversal has taken place in the head only, and 

 so a reversed head is joined to a normal body, and this may be 

 the reason why the anterior extension of the dorsal fin is not 

 attached to the head, the two parts not being in proper relation 

 to each other. This leads to the idea that a similar explana- 

 tion accounts for the malformation of the head in amb'icolorate 

 specimens : we may suppose that in these the head is normal, 

 and the body reversed, so that the upper side would be origin- 

 ally white, but has become coloured from exposure to the light. 

 As an alternative hypothesis we might suppose that in ambi- 

 colorate specimens, two coloured sides were united in the same 

 egg, instead of a left and right as in normal cases. It is un- 

 necessary to discuss these hypothetical explanations more in 

 detail : enough has been said to indicate how such congenital 

 abnormalities may arise from peculiarities in the egg, and to 

 show that their occurrence is not necessarily inconsistent with 

 the view that the normal characters of flat-fishes were originally 

 caused by the habit of lying on one side, and the changes in 

 the use of organs, and in the influence of light which resulted 

 from this habit. If it were true that the peculiarities of flat- 

 fishes were due to the "inheritance of acquired characters," the 

 fact would still remain that the peculiarities are hereditary and 

 therefore liable to mutations like any other hereditary char- 

 acters. Moreover, the nature of these remarkable abnormalities 

 affords no evidence of the possible occurrence of such spon- 



