34 CLASSIFICATION AND ADAPTATION 



the diminution in the amount of Hght falHng on the 

 lower side, so that the upper and lower sides reflect 

 approximately the same amount of light, and con- 

 trast, which would be otherwise conspicuous, is 

 avoided. But the white lower sides of Flat-fishes 

 are either not visible at all, or, if visible, are very 

 conspicuous, so that the utility of the character is 

 very doubtful. 



We may distinguish then between characters 

 which correspond to external conditions, functions, 

 or habits, and those which do not. The word 

 * adaptation,' which we have hitherto used, does not 

 express satisfactorily the peculiarities of all the 

 characters in the former of these two divisions. If 

 we consider three examples — enlarged hind-legs for 

 jumping as in kangaroo or frog, absence of colour 

 from the lower sides of Flat-fishes, and, thirdly, the 

 finlets on the lower side of Zeugopterus — we see that 

 they represent three different kinds of characters, 

 all related to habits or external conditions. We may 

 say that the third kind are correlated with some 

 other character that has a relation to function or 

 external conditions, as the extension of the fins on 

 the under side of Zeugopterus is correlated with the 

 enlargement of the fins, whose function is to cause 

 the adhesion of the fish to a vertical surface. 



With regard to the specific characters of the species 

 of Zeugopterus nothing is known of peculiarities in 

 mode of life which would give an importance in the 

 struggle for existence to the concrescence of the 

 pelvic fins with the ventral in punctatus, to the 

 absence of this character and the elongation of the 

 first dorsal ray in unimaculatus^ or to the absence 

 of both characters in norvegicus. No use is known 



