The secondary sex-ratio varies with the parity, i.e., the chrono logical number of the pregnancy. In the case of the human, the dog, and the mouse, it has many times been noted that there is a continuous drop in the sex-ratio at each succeeding pregnancy (Wilckens, Punnett, Bidder, Copeman and Parsons). King and Stotsenburg found that the same rule obtained in the rat.
Parkes points out, however, that most of the second and higher births occurred at the end of the breeding season when the sex ratio is at its highest, and that too much must not be inferred from his figures until the experiment has been repeated.
It has been suggested that the sex-ratio varies with the time relation of successive conceptions. Rumley Dawson (1921), for instance, maintained that in the case of the female producing one young at a time the right ovary elaborated only male-pro ducing ova, the left only female-producing, and that the ovaries function alternately, ovulation occurring in one ovary at one oestrous period, in the other at the next. This view cannot be brought into harmony with established facts, and therefore can not be accepted on its present-day evidence. The theory is sup ported by a collection of selected statistical data applied without proper statistical treatment, and those cases which do not fit into the scheme are airily dismissed, whilst the great body of estab lished facts which supports other theories and cannot support this particular one is neglected. Variations of this theory are numerous, and like it are based upon the conception that in the human, horse, and cattle, the female is digametic. Many believe that offspring of either sex can be obtained at will by persuading the semen of the male to flow to the right or the left of the body of the female, towards the left ovary or to the right, that is. This is ensured by the female lying on one side or the other after coitus, or standing on a slope. The matter has been tested experimentally and found wanting. Doncaster and Marshall o) have shown that unilateral ovariotomy in the rat does not result in the production of offspring of one sex only, and the cogency of these experimental results cannot be dismissed by the statement that it is too far a cry from the rat to the human female. If the breeder really desires to have this theory tested, the way is simple, for it can readily be shown that unilateral ovariotomy in the horse or in the cow is not followed by the pro duction of offspring of one sex only, and that the production of both male and female progeny is not to be explained by any regeneration of the imperfectly removed ovary.
Statistical evidence has been presented, sometimes supporting, at other times contradicting, the suggestion that the sex-ratio is affected by the relative ages of the parents, the offspring being mostly of the same sex (or of the opposite sex) as the older (or as the younger) parent. Hofacker (1828) and Sadler's (I83o) law—that the sex of the offspring is that of the older parent— finds no support in the result of critical inquiry : it is contradicted by the work of Schultze (1903) on mice, for example. According to some data, the age of the mother has a relation to the sex of the offspring, younger mothers producing a preponderance of males (or of females). In the majority of cases the data upon which these theories are based have not been collected by biometrical experts, nor can their genuineness be absolutely guaranteed. Other data seem to suggest that the sex of the off spring tends to be that of the more (or the less) vigorous parent. For example, the theory of Starkweather (1883) suggested that the "superior" parent tended to beget offspring of the opposite sex, but since it is impossible as yet to define "vigour" and "superior ity" in accurate physiological terms, such theories are not suitable for scientific discussion. There is no experimental basis for such conceptions.
It has been suggested that the sex-ratio varies with the time of service during the oestrous period. This is not supported by the elaborate data of Pearl (1917).
It has often been stated that as a result of war more males are born. Savorgnan (1921) and others have supported this con tention. (See SEX-RATIO AT BIRTH AND DEATH.) Causes of Inequality of Secondary Sex-ratio.—The sec ondary sex-ratio is probably always above too, whereas the ter tiary is almost invariably below too. If the sex-ratio of still births is examined it will be found to be higher than that of live births. If the sex-ratio of abortions is next examined it will be found to be considerably higher than that of still-births. The empirical data only extend back to about the third month of pregnancy, but it is well established that the relative male mortality increases the nearer conception is approached so that the primary sex-ratio must be considerably higher than that encountered in the latter part of pregnancy. Gunther (1923) has calculated that the early wastage of males must be very considerable indeed.