540 GENETICS IN RELATION TO AGRICULTURE 



equality of males and females in every generation. Extensive statistical in- 

 ve^tigations of the sex-ratio in a number of animals demonstrate, however, 

 that there are usually small but significant deviations from the numerical 

 equality of the sexes. The data in Table LXX collected by Morgan 

 illustrate this point. 



In man the sex-ratio varies among different peoples, but almost in- 

 variably the proportion of males is slightly in excess of that of females. 



10J2510.75 11.25 11.75 12.25 12.75 13.25 13.75 14.25 14.75 15.25 15.75 



Fio. 213. Frequency polygon of head lengths of spermatozoa in the pig. (Data from 



Wodsedalek.) 



In dealing with sex-ratios it must be emphasized that a study of the 

 deviations which are obtained may point to conditions which alter the 

 sex-ratio without disturbing the mode of determination of sex. The 

 mechanism of sex-determination is not such a one as would lead invari- 

 ably to numerical equality of the sexes. This may easily be seen by 

 a consideration of several disturbing factors which may enter into it. 



The first of these has to do with the relative sizes of male and female 

 producing gametes, particularly in those animals in which the male is the 

 heterozygous sex. Thus Wodsedalek has shown for the pig and the horse 

 that there are two intergrading classes of spermatozoa as respects size 

 (Figs. 213 and 214). Unquestionably in both cases the larger class 



