218 
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 Pleuronectidse 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 difficulty of accounting for this variation on 
any hypothesis of reversion. 
