It is seen, then, that there exists a very considerable prenatal sexually selective mortality. Far more males than females are conceived in those forms in which the male is the digametic sex, and far more males than females perish prenatally so that the secondary sex-ratio approaches equality.
It is highly probable that much of this sexually selective mor tality is due to the action of hereditary sex-linked factors, lethal or semi-lethal in their effects in that they lead to the development of characters which render the further development or the con tinued existence of the individual impossible. Since the male is digametic any recessive factors borne in the X-chromosome will take effect in all males carrying them whereas in females both X-chromosomes must carry the factor before the corresponding character can appear. It is well known that many recessive characters in animals are harmful to their exhibitors : when markedly so they may not permit the organism to live ; when slightly so they may yield but a slight reduction in viability. If such recessive characters are sex-linked then the male embryo and foetus will suffer far more often than will the female. It is reasonable to assume that such recessive sex-linked factors should in man, as in other forms, frequently reduced viability. If so, a complete formal explanation of the excess of male infant mortality is provided.
This selective elimination is also affected by favourable and unfavourable conditions during pregnancy. Savorgnan concluded from an examination of his data concerning the sex-ratio and the war that it was not privation but absence of husbands which determined the rise in the sex-ratio. He explains the relatively low sex-ratio of still-births by suggesting that long absences of husbands imply less frequent pregnancies and therefore healthier mothers. It is not improbable that the lower male ratio in still births and the increase in still-births are but reflections of pri vation during the latter part of pregnancy.
The same explanation can be applied to the fact that the secondary sex-ratio among illegitimate children is lower than that among legitimate—the lack of prenatal care and hygiene re sulting in an intensification of the forces that always make it relatively difficult for the mammal to beget male offspring and for the bird to produce as many females as males. The secondary sex-ratio is highest among those people and those herds in which the highest degree of prenatal hygiene is practised. It is prob able that amongst the coloured people there is a higher prenatal mortality and that this is the case also among the hybrids of coloured races and of white and coloured, whereas the mortality of hybrids of two white races is relatively very low in conse quence of the hybrid vigour or heterosis that results from such a mating.
But in certain forms the two forms of gamete elaborated by the digametic sex are not produced in equal numbers. In the case of bird and moth the egg contains the X- and Y-chromosomes in conjugation before the polar bodies are formed. Into the first polar body goes either the X or the Y. If it is but a matter of chance which way this chromosome pair lies on the spindle, then equal numbers of X-bearing and Y-bearing eggs will result. But if in a particular line this pair should habitually be so orientated on the spindle that the X passes into the polar body more often than the Y, then in this line a preponderance of female offspring (XY) would be observed. Such a differential production can be obtained experimentally, as is shown by the work of Seiler (1920) on the Psychid Talaeporia tubulosa in which the female is the heterogametic sex. Seiler was able to show that the ratio of the eggs in which the X-chromosome passed into the polar body to those in which it remained in the egg, was exactly the same as the sex-ratio. Moreover, since in the course of these observations it was possible to detect the moment of the dis junction of the sex-chromosomes, it became possible to attempt to influence this disjunction experimentally and so to disturb the sex-ratio. Seiler by varying the temperature during the period of maturation division obtained the following highly interesting and significant results : The fact that there is competition between sperm is illustrated in the results of an experiment by Cole and Davies (1914). A rabbit was served by two bucks, and it was found that the majority of the offspring claimed one of these bucks as their sire. In repeated matings this was always so. But when the sperm of this buck was alcoholised it could not compete with that of the other buck, though it was shown that when employed alone it could and did fertilise ova. It is reasonable to assume that if differences in the size of the sperm are associated with differences in motility, activity or resistance to unfavourable conditions within the genital passages of the female, then chance would favour fertilisation by one rather than by the other kind of sperm.