VARIATION 365 



a right-handed spiral, but some species are left-handed, and in 

 right-handed species left-handed specimens occasionally occur. 

 Abnormalities of coloration are also frequent in flat-fishes. 

 In some cases pigment is present on the lower side as well as on 

 the upper in varying degrees from a few small spots to complete 

 pigmentation of the lower side. In these cases the presence of 

 the pigment must be considered to be independent of any in- 

 fluence of light, and at first it seems difficult to reconcile the 

 occurrence of such variations with the conclusions drawn from 

 the experiments described in a subsequent chapter on the in- 

 fluence of light. Consideration of the facts, however, shows 

 that the same effect, pigmentation of the lower side, may be 

 due in different cases to two quite different causes, namely the 

 action of light and spontaneous variation, just as a man may be 

 born blind or may become blind from disease or injury. When 

 the lower side is exposed 10 light the pigment develops gradu- 

 ally, whereas in those specimens which are found with lower 

 sides pigmented in nature we have no reason to suppose that 

 the lower sides have been exposed to light, or that the pigment 

 developed gradually. The congenital nature of the abnormal- 

 ity is still more evident in cases of the opposite condition, when 

 areas of greater or less extent on the upper side are white. 

 When as sometimes happens the whole or nearly the whole of 

 the lower side is coloured like the upper the specimen is said to 

 be ambicolorate, and this condition is associated with a structural 

 similarity of the two sides. In the turbot and flounder there 

 are no regular scales in the skin, but in their place there are 

 spiny, calcified tubercles scattered all over the upper surface in 

 the turbot, confined to the bases of the fins and the region of 

 the lateral line in the flounder. In other cases, as in the sole 

 and dab, the scales are spinulate, or bordered with minute spines. 

 In all such cases the dermal structures are, in normal specimens, 

 less strongly developed on the lower side than on the upper, 

 but in ambicolorate specimens the armature is equally 

 developed on both sides ; the specimen is " ambiarmate " as 

 well as ambicolorate. Even when there is only a well-defined 

 patch of colour on the lower side the armature within this area 

 usually resembles that of the upper side, while in the white part 

 it is less developed. This increased development of the arma- 

 ture could not be due to the influence of light. 



