540 GENETICS IN RELATION TO AGRICULTURE 
equality of males and femalesinevery generation. Extensivestatistical in- 
vestigations of thesex-ratioin 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. 
10.2510.75 11.25 11.75 12.25 12.75 13.25 13.75 14.25 14.75 15.25 15.75 
Fie. 213.—Frequency polygon of head lengths of spermatozoa in the pig. (Data from 
W odsedalek.) 
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 
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