ADAPTATIONS 375 



mens of which the lower side had been exposed to light for 

 fourteen months. 



In this case then the utility principle, in other words the idea 

 of the theory of natural selection, is not applicable, and the ques- 

 tion arises whether we are to call such a feature an adaptation or 

 not. If we define an adaptation as a peculiarity of structure 

 which is advantageous to the animal the word is not applicable 

 to the absence of colour from the lower side of flat-fishes. 

 But as the latter feature is due to the conditions of life, the 

 others may have been produced in a similar way. The dis- 

 tortion of the eyes may have been caused in evolution by the 

 constant efforts of the fish to direct the lower eye towards the 

 light, that is to the upper side, together with the greater use 

 of the muscles and bones on the lower side in connection 

 with the jaws, but the latter condition does not seem to be 

 necessary, for in many species, e.g. turbot and halibut, the 

 jaws appear to be equally developed on the two sides. The 

 extension of the dorsal and ventral fins may have been due to 

 the greater use made of them. If this were true these two 

 features would be due to the conditions of life acting not 

 directly, as light acts on the skin, but indirectly by stimulat- 

 ing the functional exercise of the parts affected. It seems cer- 

 tain that all three peculiarities are in some way due to the 

 conditions of life, and if we confine the term adaptation to these 

 structural adjustments which have a definite function, a purpose, 

 as it were, we require another term to indicate changes due to 

 the conditions of life whether advantageous, indifferent, or even 

 harmful. The best term for such changes is modifications, and 

 we may define modifications as changes of structure produced 

 in evolution from a more primitive condition, by changes in the 

 conditions of life. 



The close relation between the distribution of pigment on 

 the skin of a fish and the incidence of light is remarkably 

 shown by the peculiar pigmentation and the peculiar habits of 

 various species of Synodontis (Plate XXXI., A), a genus of 

 African Catfishes (Siluridae). Many species of Synodontis 

 which live in the Nile and other African rivers, but especially 

 S. membranaceiis and .S. batensoda, are in the habit of swimming 

 at the surface of the water in an inverted position, with the belly 

 upwards. These same species have the pigmentation also 



