270 Preponderance of Males and Females in Fish 



many of Hertwig's experiments, where a shorter delay in fertilization 

 gave a less preponderance of males. 



The sex-chromosome formula for fish has not been definitely estab- 

 lished, either cytologically or genetically. There is some evidence, 

 however, that they are of the type % XX, (fXY. For instance Gerschler 

 ('14 ; see pp. 77, 78, 83) made crosses between two genera of Cyprino- 

 donts, Xiphophorus and Platypoecilius (related to Girardinus). The 

 former has, in both sexes, a zig-zag red band along its side ; in the 

 latter this is absent. When Xiphophorus was the female parent, the 

 resultant F^ fish showed this stripe in both sexes ; but when Platypoecilius 

 was the mother, only the F^ females possessed the stripe. This would 

 follow if Xiphophorus were of constitution (fXY, $ XX, with the factor 

 for red zig-zag in the X-chromosome. Unfortunately the latter cross 

 was only made with one individual, and not carried beyond F^. 



It will be seen later that if our hypothesis of the modifiability of 

 sex-chromosome constitution by external factors prove to be correct, the 

 case of Girardinus affords confirmatory evidence in favour of the XF, 

 XX formula. 



If then we assume that this formula does hold for Girardinus, the 

 few male fish present during the first period would be normal, of XF 

 constitution. At the close of the first period, when the disturbing factor 

 ceased its action, these males mated to somatic (XF) females would 

 give IXX ( ? ) : 2X Y{f^) : 1 YY (non-viable). 



The sex-ratio during the first period was given as 3 $ : 1 </". If this 

 period had only lasted one generation, two out of every three females 

 would be normal (XX) and one a feminized XF. It can thus easily be 

 calculated that a swing-back after this would give a sex-ratio of 5 $ : Gf/*. 



Mr J. B. S. Haldane, of New College, Oxford, has pointed out to me 

 that if the modifying influence were to continue for several generations, 

 two efiects would be produced. In the first place, the preponderance of 

 females during the period would gradually decrease, but, on the other 

 hand, among the females the proportion of somatic females (X F females 

 or transformed males) would increase. This would result in a greater 

 preponderance of males in the generation after the end of the first 

 period (swing-back generation). 



An example will make this clear. We must assume that the strength 

 of the modifying factor remains constant — i.e. that it converts the same 

 percentage of males into somatic females in each generation. Let us 

 assume that the factor causes a modification of 50 °/^ of the X F 

 individuals in each generation. This will give us our first-generation 



