The problem of the determination of sex in the human species is one
which has occupied natural philosophers from the very earliest times,
and has always greatly interested all classes of the population.
The interest awakened by the subject depends principally on the fact
that female children have usually been less desired than male in all
periods of history and among almost all races. In the uppermost circles
of society the truth of this statement is manifested by the fact that
the birth of a prince is announced by a salute of 101 guns, that of a
princess by a salute of 35 guns only.
It would serve no useful purpose to transcribe here the opinions, or
rather guesses, which were ventured on this topic in earlier days when
the very nature of the reproductive process was still entirely unknown,
and we shall merely mention that the curious will find various
references to the determination of sex in the works of _Hippocrates_,
_Aristotle_, _Plutarch_, _Soranus_, _Susruta_ and _Galen_.
Broadly speaking, the earlier theories may be said to diverge in two
main directions, some holding that the sex of the infant was in some way
determined by the mode of intermixture of the male and the female
elements in the act of generation, and others maintaining that sex was
already inalterably predetermined at the time of intercourse either in
the male or in the female sexual elements.
_Pari passu_ with the modern development of the theory of evolution, and
with the enormous increase in recent days in anatomical and
physiological knowledge, the theory of the determination of sex has been
very widely extended. The rival views may be briefly arranged in the
four following categories:
I. That sex is already inalterably predetermined in the ovum, upon the
constitution of which it solely depends; there are therefore male and
female ova, and the process of fertilization exercises no influence
whatever upon the determination of sex. The alternative theory to the
above, that sex is determined solely by the constitution of the
fertilizing spermatozoon—_i. e._, that the spermatozoa, and not the ova,
are male and female, respectively—is one which in recent years has
tended more and more to disappear from the field.
II. That sex is determined in the moment of fertilization by the
reciprocal interaction of male and female, of zoösperm and ovum. One
variant of this theory maintains that each reproductive element strives
for the reproduction of its own sex; that a struggle takes place and
that the victor in the contest stamps its own sex-likeness upon the
fertilized product. According to another view, however, sex is not
directly transmitted in this manner; it is supposed that the more
powerful the proper reproductive element (according to this theory the
ovum) the more strongly does it tend to determine the reproduction of a
stronger, _i. e._, a male organism; thus the greater potency of the
female element in the act of reproduction tends to favour the
determination of the male sex.
III. That sex is not determined until after fertilization, during the
early stages of the development of the embryo; the determining causes
are supposed to be various factors capable of influencing the developing
organism during this period, and more particularly the nutritive
conditions of the mother.
IV. That the determination of sex is not dependent solely upon the
action of any single one of the factors above enumerated, but arises as
a resultant effect of the operation on the germ of all three of these
acting in temporal succession.
Modern physiology has endeavoured to solve this problem by statistical
investigations, by anatomical demonstration, and finally by experiment.
_I. Statistical Investigations._
Statistical data have been collected showing the ratios between male and
female births in the most varied conditions possible, and from these
data the attempt has been made to draw valid conclusions regarding the
causes of the determination of sex. Now in the first place it is above
all necessary to bear in mind that such statistical data cannot possibly
have any value unless they relate to very numerous instances, and even
then they are liable to be invalidated by various sources of fallacy. We
may with advantage quote in this connection the remarks of _Hensen_ in
his work on the “Physiology of Reproduction:” “Each individual instance
is rendered unique in kind by the interaction of certain incommensurable
elements; for instance the state of health of the individual organs in
their innumerable combinations, variations in the general health of the
parents, the frequency of coitus and the time at which it took place,
the desire of the parents to have a son and then no more children, their
social position—in these ways innumerable complications are introduced
into the problem, and the difficulty of drawing valid conclusions is
rendered almost insuperable, unless the number of instances dealt with
is enormously large.”
One fact definitely established is that more boys are born than girls,
the proportion between the two, known as the sexual ratio, being
106 : 100. Statistics relating to the half of Europe (_Oesterlen_) and
dealing with 59,350,000 births, showed a ratio of 106.3 male to 100
female births; in individual countries variations from this mean are
found to occur, but these are not very extensive, the highest ratio
being 107.2 : 100, and the lowest ratio 105.2 : 100.
From the works of _Hofacker_ (“Ueber die Eigenschaften welche sich
bei Menschen und Tieren von den Eltern auf die Nachkommen
vererben”—Concerning the Qualities transmitted from Parents to
Offspring in Men and Animals—Tuebingen, 1828) and _Sadler_ (“Law of
Population,” London, 1830) conclusions have been drawn regarding the
effect of a variation in the age ratio of the parents on the
determination of sex. The deductions in question, known as
_Hofacker_ and _Sadler’s_ law, are as follows: