CLASSIFICATION AND ADAPTATION 29 



nothing to do with protective resem])lancc, variable 

 or permanent. The red spots of the Plaice are 

 specific and diagnostic, but they confer no advan- 

 tage over the Dab or the Lemon-dab, in which they 

 are absent, nor can any relation be discovered 

 between these spots and mode of life or habits. 



The function of the lateral line organs is still some- 

 what obscure. The theory that they are sensitive 

 to differences of hydrostatic pressure as the fish 

 moves from one depth to another rests on no foun- 

 dation, since it has yet to be shown how a change of 

 pressure within the limits of the incompressibility 

 of water can produce a sensation in an organ per- 

 meated throughout with water. It is more probable 

 that the organs are affected by vibrations in the 

 water, but we are unable to understand how a differ- 

 ence in the anterior curvature of the lateral line 

 would make a difference in the function in any way 

 related to the difference in conditions of life between 

 Plaice and Dab. There is, however, reason to con- 

 clude that the organs, especially on the head, are 

 more important and larger in deeper water, and 

 thus the enlargement of the sensory canals in the 

 head of the Witch, which lives in deeper water than 

 other species, may be an adaptive character. 



Another genus of whose characters I once made a 

 special study is that named Zeugopteru<s. The 

 name was originally given by Gottsche to the largest 

 species Z. puiictatus, from the fact that the pelvic 

 fins are united to the ventral, but this character does 

 not occur in other species now included in the 

 genus. There are three species, occurring only in 

 European waters, which form this genus and agree 

 in the following characters. The outline of the 



