30 THE EVOLUTION OF SEX. 



is here desirable to emphasise, that just as we admit the 

 importance of sexual selection as a minor accelerant in the 

 differentiation of the sexes, so we are bound to recognise that 

 natural selection is also continually in operation as a check to 

 a divergence of the sexes which would otherwise tend to 

 become extreme. If this retarding influence of natural selec- 

 tion on the evolutionary process were not continually present, 

 we should find cases like bonellia and the rotifers much 

 commoner than they are among animals. But it is an error to 

 exaggerate this limiting action into an explanation of the 

 process itself. It should also be noted, that both the retarding 

 action of natural selection, and the accelerant action of sexual 

 selection, become of increasing importance as we ascend the 

 series. And thus, indeed, we are impelled towards a heresy 

 which, as we shall see later, has bearings against the theory of 

 natural selection, which overpass the limits of our present 

 theme. 



Postscript,— Y)xT. W. I'ulton, Naturalist to the Scottish Fishery Board, 

 has been good enough to furnish us with some of his results on the size 

 and numerical proportions of male and female fishes, (i.) The females 

 are usually considerably more numerous than the males, and never less 

 numerous except in the angler and the cat-fish. The proportions of females 

 to males among flat-fishes ranges from about i : i in the flounder, to about 

 12 : 1 in the long rough dab. Among " round " fishes the same proportion 

 varies from about 3 : 2 in the cod, to 9 : 2 in the common gurnard. (2.) The 

 female is longer and larger among all the flat-fishes, sometimes by as much as 

 30 per cent. In cod, haddock, angler, and cat-fish, the males are larger, 

 while in the whiting the females are slightly larger, and in the common 

 gurnard decidedly so. The subject is being worked up by the above- 

 named naturalist, and cannot fail to yield very valuable results. 



