522 FISHES. 



So far some of the more important facts, what of their 

 interpretation ? That these asymmetrical forms have been 

 derived from symmetrical ancestors is plainly suggested by 

 their development. Of the original cause of the asymmetry 

 we are quite ignorant. Did changes in the conditions of life 

 induce the ancestral forms to leave the surface for the 

 bottom ? Or was the change due to certain peculiarities of 

 structure requiring, of course, previous explanation such 

 as the great depth of the body and the degeneration of 

 the swim bladder ? Or did both these causes operate at 

 once ? 



But, supposing we had attained to some clearness in 

 regard to the change of habitat and loss of vertical balance, 

 we should then have to consider the twisting round of the 

 downward turned eye and the absence of pigment cells on 

 the downward side. 



As to the change of the eye, it may be said (i) that this 

 has gradually resulted from the efforts of the fish to continue 

 to use the lower eye, a possible interpretation if acquired 

 characters can be transmitted. (2) It may be said by those 

 who do not believe in "use inheritance" that the twisting 

 round of the lower eye is not a result of a transmitted growth 

 tendency at all, but is wrought out by effort in each genera- 

 tion de novo. But young turbot and brill have nearly 

 completed the twisting round of the lower eye long before 

 they have abandoned their pelagic habit. (3) It may be 

 said that the twisting round of the lower eye arose as a 

 germinal variation, apart from any direct influence of 

 function or environment, and that it has been retained and 

 strengthened in the usual course of natural selection. 



Again, as to the absence of chromatophores, it may be 

 supposed that this also is a useful adaptive character per- 

 sistent as the result of selection. But, apart perhaps from 

 economy, it is not evident in what the advantage consists. 

 It seems more likely that the under surface is unpigmented 

 because it is shaded, and Mr. J. T. Cunningham, who has 

 devoted special attention to the problem of flat fishes, has 

 proved experimentally that artificial illumination of the lower 

 sides by means of a mirror induces the development of 

 pigment cells. It must be noted, however, that pigmenta- 

 tion of both sides occurs also as a natural variation. 



