2i8 COLOUR IN NATURE chap. 



side" (Bateson). This hardly, however, explains 

 the detailed correlation seen in the turbot, and the 

 whole subject is still imperfectly understood. 



As to the meaning of the similarity between the 

 upper and lower surfaces in the ambicolorate speci- 

 mens, both Mr. Bateson and Mr. Cunningham point 

 out that this cannot be due to reversion, for there is 

 no reason to suppose that the upper and lower 

 surfaces of the ancestral Pleuronectidas were identical 

 in colour. Mr. Bateson regards it as a case of 

 what he calls homoeotic variation. " In the flat- 

 fish the right side and the left have been differ- 

 entiated on different lines, as the several appendages 

 of an arthropod have been, but on occasion the one 

 may suddenly take up all or some of the characters, 

 whether colour, tubercles, or otherwise, in the state 

 to which they have been separately evolved in the 

 other " {Materials for the Study of Variation). 



In summarising the points which give their 

 special interest to the coloration of flat-fish, we may 

 note the usual absence of pigment from one side, 

 right or left as the case may be, the occurrence of 

 two forms which normally possess traces of pigment 

 on this side, the facility with which pigmentation of 

 the lower surface may be produced by artificial 

 illumination, the occurrence of ambicolorate forms 

 as a frequent variation in nature, and finally, the 

 extreme difiSculty of accounting for this variation on 

 any hypothesis of reversion. 



