The Determination of Sex.

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:

1. If the husband is older than the wife more boys are born than girls.

2. If husband and wife are the same age somewhat fewer boys are born than girls.

3. If the wife is older than the husband the excess of female births is larger still.

Hofacker’s actual figures were the following:

Father younger than mother	90.1 boys to 100 girls.
Father same age as mother	93.3 boys to 100 girls.
Father 4 to 6 years older than mother	108.8 boys to 100 girls.
Father 6 to 9 years older than mother	124.7 boys to 100 girls.
Father 9 to 12 years older than mother	143.7 boys to 100 girls.

Sadler’s results were closely similar:

Father younger than mother	86 boys to 100 girls.
Father same age as mother	94 boys to 100 girls.
Father 1 to 6 years older than mother	103 boys to 100 girls.
Father 6 to 11 years older than mother	126 boys to 100 girls.
Father 11 to 16 years older than mother	147 boys to 100 girls.
Father 16 years and more older than mother	163 boys to 100 girls.

Goehlert found that the offspring of marriages in which the husband was younger than the wife were 71 boys and 86 girls; of marriages in which husband and wife were of the same age, 263 boys and 282 girls; and of marriages in which the husband was older than the wife, 2,017 boys and 1,865 girls.

Wappaeus, combining the data supplied by these three investigators, Hofacker, Sadler and Goehlert, obtained the following sexual ratios for the entire 8,000 cases (i. e., the number of boys born to each 100 girls born): When the husband was younger than the wife, 88.2; when husband and wife were of the same age, 93.5; when the husband was older than the wife, 113.0.

It will be observed that the mean sexual ratio of these 8,000 cases is 109.6; whilst, as we saw above, when a sufficiently large number of instances is taken, the sexual ratio always closely approximates to 106.3. From this it appears that the numbers dealt with by Hofacker, Sadler and Goehlert in their investigations were too small for the deduction of trustworthy averages.

The same criticism is applicable to the observations of Ahlfeld, Breslau and Noirot, whose results conflict with those just given. According to Ahlfeld, in the case of 1,376 births where the father was at least 10 years older than the mother, the sexual ratio was only 98.2. According to Noirot’s data, in cases in which the father was older than the mother, this ratio was 99.7.

Oesterlen gives the following brief summary of the researches made for the establishment and confirmation of the law of Hofacker and Sadler:

Author.	Father older than mother.	Father and mother same age.	Mother older than father.	Sexual ratio.	Number of instances.
Hofacker	117.8	92.0	90.6	107.5	1,996
Sadler	121.4	94.8	86.5	114.7	2,008
Goehlert	108.2	93.3	82.6	105.3	4,584
Noirot	99.7		116.0	103.5	4,000
Legoyt (Calais)	109.9	107.9	101.6	107.9	6,006
Legoyt (Paris)	104.4	102.1	97.5	102.9	52,311
Breslau	103.9	103.1	117.6	106.6	8,084

The law of Hofacker and Sadler cannot be regarded as possessing universal validity, although the figures on which it is based seem to show pretty clearly that we are justified in regarding the mutual interaction of the male and female reproductive elements at the moment of fertilization as effective in the determination of sex. In the investigations to which we have hitherto alluded it is only the relative ages of husband and wife that have been taken into account; but other researches have shown that the absolute age alike of the husband and of the wife has an influence in the determination of sex.

The influence of the absolute age of the mother in the determination of sex has been very clearly established. Ahlfeld was the first to draw attention to the fact that among the children of elderly primiparæ there was always to be found an excess of boys, and that there was an increase in this excess proportionate to the greater age of the mother. Among 102 children born to primiparæ over 32 years of age the sexual ratio was 137 : 100; and a later investigation made by the same author in conjunction with Schramm showed that among 1,038 children born to primiparæ over 28 years of age the sexual ratio was 124 : 100.

Hecker obtained similar results. Among 432 children born to primiparæ over thirty years of age the sexual ratio was 133 : 100. Winckel, dealing with primiparæ of the same ages, found a sexual ratio of 136.8 : 100.

Düsing, examining the records of the lying-in hospitals of Leipzig, Dresden and Jena and thus obtaining a very large number of instances whereon to base his conclusions, confirmed the view that elderly primiparæ give birth to an excess of boys, and further that the older they are the larger the excess of male births. He drew up the following table:

Age of primiparæ.	Leipzig. Boys. Girls.	Dresden. Boys. Girls.	Jena. Boys. Girls.	Total Nos. Boys. Girls.	Sexual ratio.
15	1 : −	1 : 2	1 : −	3 : 2	549 : 494 = 111.1
16	4 : 4	6 : 10	2 : 2	12 : 16
17	23 : 13	20 : 15	9 : 7	52 : 35
18	67 : 55	103 : 100	17 : 13	187 : 168
19	110 : 103	152 : 141	33 : 29	295 : 273
20	148 : 147	187 : 185	32 : 45	367 : 377	807 : 781 = 103.3
21	157 : 145	241 : 201	42 : 57	440 : 404	807 : 781 = 103.3
22	120 : 133	191 : 207	48 : 53	359 : 393	903 : 962 = 93.9
23	106 : 108	168 : 149	51 : 51	325 : 308
24	71 : 105	111 : 118	37 : 38	219 : 261
25	79 : 57	73 : 72	35 : 27	187 : 156	531 : 469 = 113.2
26	45 : 35	30 : 43	20 : 20	125 : 98
27	31 : 35	52 : 55	10 : 12	93 : 102
28	32 : 33	26 : 33	19 : 16	77 : 72
29	19 : 10	26 : 18	4 : 13	49 : 41
30	9 : 15	30 : 13	9 : 6	48 : 34	155 : 104 = 150.0
31	3 : 8	15 : 11	3 : 3	21 : 22
32	5 : 6	12 : 9	7 : 3	24 : 18
33	2 : 2	5 : 5	5 : 2	12 : 9
34	4 : −	8 : 5	2 : −	14 : 5
35	2 : −	9 : 3	2 : 1	13 : 4
36	1 : −	3 : 3	1 : 1	5 : 4
37	4 : 1	4 : 3	1 : −	9 : 4
38	− : −	− : 1	1 : −	1 : 1
39	− : −	4 : −	1 : −	5 : −
40	1 : 1	2 : 1	1 : −	4 : 2
41	− : −	− : 1	− : −	− : 1

As an explanation of this statistically proved fact, that elderly primiparæ gave birth to a large excess of boys, Düsing suggests that these women who conceive for the first time comparatively late in life, are, prior to the conception, in a state corresponding with that of a lower animal species suffering from a deficiency of males, and for this reason exhibit a tendency to procreate a larger number of individuals of the deficient sex. In multiparæ also it is possible to trace the influence of a deficiency of male individuals. When there is such a deficiency the interval between successive births is unduly protracted. Düsing found (once more from the records of the lying-in hospitals of Dresden, Leipzig and Jena) that the longer the interval between one parturition and the next the longer, that is to say, the mother has had to wait for her next conception, the greater is the excess of male births. Düsing therefore lays down the law: “Delayed impregnation gives rise to an excess of male births.”

Age of primiparæ.	Leipzig. Boys. Girls.	Dresden. Boys. Girls.	Jena. Boys. Girls.	Total Nos. Boys. Girls.	Sexual ratio.
1	162 : 158	194 : 178	58 : 45	414 : 381	108.6
2	366 : 307	374 : 361	168 : 145	908 : 813	111.6
3	198 : 196	207 : 194	116 : 94	521 : 484	107.7
4	127 : 109	132 : 106	59 : 45	318 : 260	115.7
5	59 : 54	55 : 54	38 : 38	152 : 146	115.7
6	61 : 62	52 : 49	49 : 24	162 : 135	121.9
8, 9 and 10	18 : 16	41 : 23	16 : 24	75 : 63
11 and more	5 : 15	12 : 9	4 : 6	41 : 30
Totals: 4,903 births, 2,591 m.; 2,312 f.; sexual ratio = 112.06.

Bidder considers that his own observations entitle him to modify Ahlfeld’s dictum regarding the influence of age in primiparæ in giving rise to an excess of male births. He tabulates his results as follows:

Age of Mother.	Number of cases.	Sexual ratio.
17 to 20	80	122.2
20 to 22	405	130.1
22 to 24	369	109.9
24 to 26	1,138	104.6
26 to 30	2,049	105.5
30 to 32	878	112.5
32 to 36	1,120	119.6
36 to 39	676	123.1
40 and upward	215	131.5

and formulates the following thesis: Very young primiparæ give birth to an excess of boys; primiparæ in the first bloom of womanhood give birth to an excess of girls; later, however, as the age of the primiparæ increases the excess of male births soon reappears and rapidly increases.

Hofacker’s data and the researches of Hampe agree with those of Bidder in showing that to very young primiparæ, as well as to elderly primiparæ, an excess of boys is born. Among the offspring of 363 mothers, at ages varying from 16 to 26 years, Hofacker found the sexual ratio to be 121; among the offspring of 1,056 mothers, at ages 26 to 36, the ratio was 101; and among the offspring of 567 mothers at ages 36 to 46, the ratio was 111. Hampe tabulated 5,992 instances as follows:

Age of Mother.	Number of instances.	Sexual ratio.
Below 20 years	56	107.7
20 to 25 years	871	90.6
25 to 30 years	1,633	114.9
30 to 35 years	1,631	108.3
35 to 40 years	1,185	117.1
Over 40 years	616	124.0

We learn, therefore, that if the age of the progenitors is to be regarded as one of the causes by which sex is determined, we must take into consideration not only the relative ages of husband and wife but, in addition, the absolute age of the wife.

Goehlert undertook a statistical investigation in which the absolute age of the husband was taken into consideration as well as that of the wife. From this it appeared that the maximum sexual ratio was exhibited when the father was between the ages of 30 and 35 years. When the age of the mother is treated as the determining influence, we find the maximum sexual ratio in the offspring of mothers between the ages of 25 and 30 years. Goehlert believes, however, that the paternal influence is more powerful than the maternal in the determination of sex. The respective influences are compared in the following table:

Age of Father.	Age of Mother.
Age of Father.	20 to 30 years.	30 to 40 years.	Over 40 years.	Totals.
25 to 35 years	105.76	107.87	109.14	106.6
35 to 45 years	102.8	105.1	105.3	104.7
Over 45 years		104.3	103.9	109.1
In general	105.25	105.97	104.9	105.5

Geissler, studying the data obtainable regarding the sexual ratio during a 10–year period in the Kingdom of Saxony, ascertained that in families possessing two children or more there was a very definite distribution of the possible sex-combinations. Where there was an even number of children those families were in the majority in which the number of boys and girls was identical. If the number of children in the family is an unequal one, those combinations are most frequent in which the number of boys exceeds the number of girls by one; next in frequency are those combinations reversed to this, i. e., in which the number of girls exceeds the number of boys by one. All other combinations are comparatively infrequent in proportion as the discrepancy in number between boys and girls is larger. Rarest of all are families in which the children are all of the same sex; and among these, again, the most unusual are those consisting of boys only.

This distribution of the sex-combination is believed by Geissler to depend upon the fact that in the first birth and all the subsequent births there is generally speaking a slight advantage in favour of the male sex. It has not been proved that the sex of the first-born exercises a determining influence on the sex of the subsequent children. It does, however, seem clear that in the case of parents who have given birth in succession to several children of one sex only, there exists some definite obstacle to the procreation of children of the opposite sex. Putting these exceptions out of consideration, there seems to exist a tendency in the later births of a series toward the procreation, more especially of that sex which has been absent or deficient in the earlier births of the series. The strength of this equalizing tendency increases as soon as it has for the first time manifest itself. It is always greater when the sex deficient in the earlier births of the series has been the male.

I have myself undertaken a statistical investigation of the births occurring in the reigning families of Europe and in the families of the leading members of the aristocracy. The necessary particulars are to be obtained from the genealogical court calendars; and it is my belief that the data obtained regarding these uppermost strata of society are comparatively free from many sources of error by which the ordinary statistics of the subject are apt to be invalidated. For 556 marriages there were 1,972 births, comprising 1,023 boys and 949 girls, and thus exhibiting a sexual ratio of 107.7.

In relation to the relative ages of the parents, the following table was drawn up:

	Boys.	Girls.	Sexual ratio.

Husband older than wife by one to five years	294	283	103.8
Husband older than wife by more than five and less than ten years	327	306	106.8
Husband older than wife by more than ten and less than fifteen years	190	167	113.7
Husband older than wife by more than fifteen years	138	113	122.1
Husband same age as wife	34	42	80.9
Husband younger than wife	40	38	105.2

From these figures we may deduce the following conclusions, which are not wholly concordant with the law of Hofacker and Sadler: When the husband is older than the wife the excess of male births among the offspring is greater than it is in the case of an average drawn from the offspring of all marriages (in my cases the difference was 111.8 as compared with 107.7). But a closer analysis shows the difference to be less simple than at first sight appears. If the husband is older than the wife by one to five years, the excess of male births among their offspring (103.8) is less than in the average of all marriages (107.7); the same is true of the offspring of marriages in which the husband is more than five and less than ten years older than the wife, though the difference here is very trifling (106.8 as compared with 107.7). It is not till we come to the offspring of marriages in which the husband is from ten to fifteen years older than the wife that the increase in the excess of male births becomes notable (113.7 as compared with 107.7); and when the husband is more than fifteen years older than the wife the excess of males is higher still (122.1).

If we arrange these data so as to show, in cases in which the husband is older than the wife, the additional influence of the absolute age of the wife, we obtain results which partially conflict with those of Bidder, as follows:

Husband Older than Wife.	Boys.	Girls.	Sexual ratio.
Wife’s age, 15 to 20 years	280	287	97.6
Wife’s age, 20 to 26 years	595	513	116.0
Wife’s age, 26 to 33 years	74	69	110.1

Thus we see that when the wife is very young, i e., less than twenty years of age, even though the husband is older than the wife, there is among their offspring no excess of male births, but the contrary—a sexual ratio of 97.6 only. Most marked is the excess of boys in cases in which the husband is older than the wife, and the age of the wife is from twenty to twenty-five years. When the husband is older than the wife, and the wife’s age lies between twenty-five and thirty-two years, the excess of male births is not so great, though still considerable.

Hence it appears that the law of Hofacker and Sadler, which cannot be regarded as fully valid in the terms in which it was originally expressed, must be modified as follows: If the husband is at least 10 years older than the wife, and the latter is at an age when her reproductive capacity is at its maximum (twenty to twenty-five years), the offspring exhibit a notable excess of male births. There is still a considerable excess of male births in the offspring of marriages in which the husband is at least ten years older than the wife, and the wife is more than twenty-five years of age. On the other hand fewer boys are born than girls as the offspring of marriages in which, although the husband is older than the wife, the wife has not yet attained the age of maximum reproductive capacity—i. e., is less than twenty years of age. The excess of female births is most marked when the husband and wife are of the same age. When the wife is older than the husband there is a moderate excess of male births.

I admit, however, that the figures upon which I have based these conclusions are, like those of Hofacker, too few in number for the foundation of trustworthy inferences. The instances in my computation number 1,972; those in that of Hofacker, 1,996; but, as I have already remarked, there are reasons for believing that the data I have employed admit of the introduction of fewer sources of error.

The influence of the absolute age of the mother in the determination of sex having been statistically proved, many have inferred that this determination is not effected during the instant of fertilization, but occurs at a later stage of intra-uterine life, and is influenced by the manner in which the embryo is nourished by the maternal organism. It is suggested that elderly and immature mothers are unable to furnish the embryo with nutriment so well as those mothers who are at the age of maximum reproductive capacity, and that upon this fact depends the excess of male births in the latter case. (We shall return to this matter—the influence of deficient nutrition in relation to the excess of male births). But the proof of the fact that the absolute age of the father has also an influence in the determination of the sex of the offspring offers a ground for opposing this assumption that the sex of the embryo is determined during intra-uterine life subsequent to fertilization, and suggests that the father also exercises a determining influence in the origination of sex during the act of fertilization.

The absolute age of the husband seems also to have some influence upon the sexual ratio. The absolute age, like the relative age, of the father appears favourable to the procreation of a greater excess of boys. Thus, Hofacker found in 1,193 cases, in which the age of the father was from twenty-four to thirty-six years, that the sexual ratio was 100; in 683 cases in which the age of the father was from thirty-six to forty-eight years, the sexual ratio was 114; and in 105 cases, in which the age of the father was from forty-eight to sixty years, the sexual ratio was 169.

In investigations based upon larger collections of cases Schumann and Düsing have endeavoured to determine the variation in the sexual ratio according to the absolute age of the father.

Düsing examined the statistics of births in Norway, Alsace-Lorraine and Berlin, and from the data thus obtained he compiled the following table:

Age of Mother.	30 to 35 Years.			25 to 30 Years.			20 to 25 Years.
Age of Father.	Boys.	Girls.	Sexual ratio.	Boys.	Girls.	Sexual ratio.	Boys.	Girls.	Sexual ratio.
15 to 30 years	8,525	7,887	108.1	27,389	25,843	106.0	21,560	20,330	106.0
30 to 35 years	23,283	21,823	106.9	23,394	23,486	103.9	7,954	7,469	106.5
35 to 40 years	17,885	17,070	104.7	10,272	9,838	104.2	2,426	2,416	100.4
40 to 45 years	7,972	7,681	103.8	3,165	3,058	103.5	1,154	1,100	105.0
Over 45 years	4,220	3,997	105.6	1,734	1,525	113.8	1,154	1,100	105.0

In this table we find the births arranged in relation to varying ages of the fathers and in relation to mothers whose ages are tabulated in three classes, the ages of the latter being those at which they are most prolific. The table shows clearly that the excess of boys is larger at the beginning and at the end of each column. Thus, the age of the mother remaining constant, young fathers and elderly fathers procreate a larger proportion of boys than do fathers of intermediate age.

But I find in this table, which is based upon a very large number of instances indeed, a confirmation of the thesis which I stated above, for the highest sexual ratio of 113.8 is in this table found in the case of fathers over forty-five years of age who are married to mothers of ages twenty-five to thirty years; this is, as I said, the case in which “the husband is at least ten years older than the wife, and the latter is at the age at which her reproductive capacity is at its maximum.”

From such figures as these, which seem to show the influence of the absolute age of the father upon the determination of sex, it has by many been inferred that the man exercises a preponderating influence upon the determination of the sex of the embryo, impressing his own sex upon it, and that the greater the sexual potency of the begetter the more powerful also is the influence exercised by the latter; that the point of first importance in this connection is the sexual capacity of the man; and that the excess of male births increases pari passu with the increase in the potency of the procreating male.

Two additional considerations have been adduced to demonstrate the influence of the father in determining the sex of the offspring. The first of these is a comparison of the ratio between male and female births in towns and in rural districts, respectively; and the second is a comparison between the ratio of males to females in the offspring of married and of unmarried parents, respectively.

In towns the excess of male births is smaller than it is in the country. The average sexual ratio in Prussia during the five-year period, 1875 to 1880, is given by Düsing as follows:

In Berlin	105.70
In other large towns	105.72
In medium sized towns	105.44
In small towns	106.17
In rural districts	106.62

The indisputable fact that in towns more girls are born than in rural districts is referred to the fact that in the country the husband usually enters on marriage with his virile powers completely unimpaired, whereas in towns many men only marry after they have for many years expended their best forces in irregular sexual intercourse, and thus reserve for their wives only the dregs. But, as we shall show presently, the difference between town and country in this respect is susceptible of a different interpretation.

It has also been asserted (Horn) that extra-conjugal sexual intercourse is favourable to the procreation of boys, the suggestion being that the greater sexual vigour in the former case determines a preponderance of males in the offspring; but this assumption is invalidated by the statistical evidence which now accumulates, that among illegitimate offspring there is a smaller excess of boys than among legitimate offspring. Babbage, for instance, came to this conclusion as a result of the comparison of 1,000,000 illegitimate births with 14,000,000 legitimate births. On the other hand, the following utterance of Ploss appears extremely artificial: “In a country in which illegitimate births are very numerous, in which the illegitimate children are for the most part begotten by enervated debauchees, the excess of male births is smaller; but in a country in which the illegitimate births are less numerous, and in which the illegitimate infants are for the most part the fruit of love and are begotten by youthful lovers, the excess of male offspring is larger.”

Next to the age of the progenitors, their nutritive condition is shown by statistical investigations to exercise an important influence in the determination of sex. The following proposition has, in fact, been established: Where the supply of nutriment is deficient, the offspring contains an excessive proportion of boys.

Ploss, in an article on “The Causes of Variations in the Sexual Ratio,” published in twelfth volume of the “Berliner geburtsh. Monatsheft,” has collected a number of statistical data to demonstrate that the determination of sex is principally dependent upon the nutritive condition of the mother. In his opinion the determination of the sex of the embryo depends neither upon the quality of the ovum nor upon that of the spermatozoon, nor again upon the reciprocal influences exerted by ovum and spermatozoon on one another. During the earliest time after fertilization the embryo is sexually neutral and only later, as a result of some new influence acting upon it, does it receive an impulse toward the formation either of the male or of the female sex. Among all the external conditions which are capable during this period of exercising a determinative influence the most important are the nutritive conditions, for nutrition is the most important factor in determining the form of the young animal, and most of the other outward conditions, such as light, heat, and chemical changes, affect the embryo in a mediate manner only, through the changes they are respectively capable of inducing in its nutrition and metabolism. But Ploss is not content with asserting that the nutritive condition of the mother is of great importance in determining the sex of the embryo; he goes further than this, and declares that it is scanty nutrition of the mother which especially gives rise to the birth of males, whereas abundant nutrition of the mother tends to give rise to the birth of females. He refers to observations made in respect of other animals than man. St. Hilaire observed that among the underfed animals in menageries there was an excess of male births. Hofacker and Girou de Bazarcingues have noted similar phenomena in the case of domestic animals; they found that when these were richly fed they gave birth to an excess of females, but that when they were poorly nourished they gave birth, on the contrary, to an excess of males. Among sheep, with whom the number of male and of female births is approximately identical, Martegoute found that those animals which had given birth to female lambs had on the average a greater weight than those ewes which had given birth to male lambs. During the period of lactation, also, the former lost weight more than the latter.

When Ploss had further ascertained that among those engaged in the fur-trade it is the established belief that from fruitful regions the skins chiefly of female animals will be obtained, but from barren regions, on the other hand, among the skins obtained those of male animals will preponderate, it appeared to him that he was justified in drawing the conclusion that among the mammalia a well-nourished mother, one capable of providing abundant nutriment for her offspring, is, on the whole, more likely to give birth to a female than to a male, and conversely. He then endeavoured, by reference to the statistics of population, to show that in the case of human beings also, when the mother is exceptionally well nourished, there is especial likelihood of a girl being born; whereas when the mother is ill nourished she is more likely to give birth to a boy—odd as it may seem, at first sight, that a rich diet should determine the production of the so-called less powerful sex, the female; and conversely. (Ploss, be it noted however, denies that male new-born infants are more powerful than female.) The fact that in the country, comparatively speaking, many more boys are born than in towns (vide supra) is explained by Ploss as dependent on the fact that townswomen are on the whole better nourished than the countrywomen, owing to the greater consumption of meat by the former.

The circumstance again (likewise alluded to above), that in the case of illegitimate births almost always the excess of male births is less marked than in the case of the offspring of legal unions, is regarded by Ploss as offering further proof of the influence of nutritive conditions upon the determination of sex. Illegitimate mothers are on the average women at the very best age for child-bearing, women who have worked vigorously, are themselves well nourished, and are therefore better able to nourish their unborn offspring than (on the average) married women. Further, it is a remarkable fact that in Saxony, regions of which the elevation above the sea-level is less than 1,000 feet, produce comparatively more girls than regions at an altitude of 1,000 to 2,000 feet above the sea. This Ploss refers to the worse nutrition of the women living at the higher altitude, for the fact is well known that the higher we go above the sea the less fertile is the soil and the more limited is its produce.

Starting from the view that the lower classes of the population are in good years better nourished than they are in bad years, Ploss compared statistically the rise and fall of the prices of foodstuffs, in Saxony, with the variations of the sexual ratio in the same kingdom; and he found, in correspondence with his theory, that in bad times there was a greater excess of male births than there was during periods in which food was cheaper. Moreover, the consumption of meat appeared to have more influence upon the variations in the sexual ratio than was exercised by the consumption of vegetable foodstuffs. Ploss endeavoured to show graphically that with an increase in the price of provisions there corresponded an increase in the excess of male births, and vice versa.

That after times of great wars, pestilence, and emigration, in which the male portion of the population has been decimated, there occurs a notable increase in the excess of male births, is explained by Ploss by the assumption that in consequence of the deficiency of male labour, the general supply of provisions is deficient, just as it is after years of bad harvests; hence, in consequence of the scanty nutrition of the mothers, the number of female births is depressed and the number of male births increases. Those countries which in general possess a more thriving population, such as Saxony, England, Belgium, Prussia, etc., exhibit a smaller excess of male births when compared with those countries in which a more widespread poverty exists, such as Russia, Lombardy, Bohemia, and even France. With increasing prosperity, fruitfulness increases, and there is an increase also in the proportion of female births.

These views of Ploss’s are, however, rightly contested by Breslau and Wappaeus.

Breslau (“The Causes of the Determination of Sex”) offers in the first place theoretical objections to the view that nutritive conditions in the mother are determinative in the production of the sex of the offspring; and he then proceeds to quote statistical data which are opposed to any such theory. He rightly insists, in the first place, that, inasmuch as it is unquestionable that in the act of fertilization the spermatozoon induces in the ovum certain changes, the effect of which is manifested in days far later than those of intra-uterine life, by the production of the most manifold and various somatic and psychical resemblances to the father, it is obvious that this single occurrence of the fusion of the spermatozoon, the bearer of the paternal influences, with the ovum is competent to induce in the ovum a molecular arrangement upon which the determination of sex may be supposed to depend—it is not necessary to suppose that for this determination, repeated actions, or a prolonged period of time, is requisite.

In this connection a reference to birds is instructive, for in this division of the animal kingdom we seem absolutely compelled to assume that the sex of the offspring is irrevocably determined in the moment of fertilization. “In birds, who lay an egg every day, eggs which are kept all at the same temperature during the period of incubation, and some of which develop into cock and others into hen birds—how is it possible to imagine that the nutritive condition of the parent is determinative of the sex of the offspring, since this nutritive condition cannot reasonably be supposed to change to any notable extent from one day to the next and then back again?”

The paternal influence in the determination of sex is regarded by Breslau as likely to be at least as important as that exercised by the mother. Unquestionably the semen is subject to certain variations in quality and in quantity, and it is possible that these variations may make themselves felt in the subsequent development of the embryo. Of the semen, as of all secretions, we can assume with some probability that in certain conditions (conditions which are, indeed, but little understood), it may exhibit a greater or less intensity in its stimulating qualities, and that in this way it may be subject to changes, according to which it will tend to influence the development of the embryo, now in the female, and now in the male direction. Again, in view of the continued interchange of nutritive materials between the embryo and the maternal organism, it is probable enough that the constitution of the maternal organism and likewise that of the ovum, at the moment of fertilization, are not matters of indifference to the determination of sex. But to attribute to the nutritive condition of the maternal organism the sole or even the principal influence in the determination of the sex is an unjustifiable assumption.

The proofs alleged by Ploss from the animal world in support of his thesis, viz., that in the case of poorly nourished animals there is a marked excess of male offspring, are justly controverted by Breslau with the remark that the observations on animals are but few in number, and further that we know nothing whatever regarding the normal sexual ratios among the offspring of such animals as lions, tigers, hyænas, bears, etc., in the free state.

If, again, domestic animals when well nourished give birth to an excess of female offspring, and when ill nourished and overworked to an excess of male offspring, in the former case we can only regard the excess of female births as a pathological phenomenon, inasmuch as a superabundant supply of nutriment is not that which furnishes the highest powers or is most suited to the preservation of life. Again, the accounts given by fur-dealers are quite untrustworthy, since these men commonly obtain their goods at third and fourth hand; moreover, most of the animals whose pelts form articles of commerce, are beasts of prey, such as the otter, the lynx, the bear, the wolf, the fox, etc.—and these animals have no “pasturage.” Among vegetable-feeding animals the females are no doubt found on the pasturage more often than the males, for the reason that the females have to nourish both themselves and their young, whereas the males, which have themselves only to provide for, are contented with less food and inhabit chiefly more retired and less fertile regions; they are also shyer and are less easily shot and trapped. Regarding statistical data relating to the influence of nutrition on the production of boys, Breslau shows, from the figures of the Canton of Zurich, that with few exceptions, the years in which the price of corn was lowest were the years in which the production of boys was greatest, and vice versa—the exact opposite of the results obtained by Ploss from his examination of the figures relating to births and the price of provisions in Saxony.

Wappaeus contests the conclusions of Ploss even more vigorously, and adduces the statistics of Sweden in the 20–year period 1770 to 1790, in which period this kingdom repeatedly suffered from famine in consequence of failure of the crops, so that the births and deaths were largely affected. It appeared, however, that the more abundant or more scanty nutrition of the inhabitants of Sweden during this period had no marked influence upon the sexual ratio.

If scanty nutrition of the mother had, in fact, any influence upon the determination of the sex of the embryo in the direction alleged by Ploss, viz., so as to bring about the birth of a greater excess of boys, this influence should be manifested with exceptional clearness in the case of twins, for it is obvious that the nutriment that is insufficient for one embryo would be still scantier for two. Hence various statistical investigations have been undertaken to determine the sexual ratio in the case of twin births. Ploss found that in Saxony, in the case of 23,420 twin births, the sexual ratio was 106.7 boys to 100 girls; Moser gives the sexual ratio in the case of twin births as 106; Meckel von Hemsbach gives it as 105.4; Hecker, 116 and 122; Sickel, 112.3. Düsing combined the figures relating to twin births in various lying-in hospitals, published by Hecker, Sickel, Baillarger, Siebold, Elsässer, and Levy, respectively, and thus obtained a sexual ratio of 121.5 boys to 100 girls. This excess of boys is notably greater than among births in general.

But other statistical data are available which show a reversed condition, viz., that in the case of twin births the sexual ratio is lower than usual. According to von Frick’s collection of twin births in Prussia, the sexual ratio was 104.7 boys to 100 girls; whereas in the case of single births in the same country the sexual ratio was 106.35 to 100. Riecke, in Württemberg, in the case of 60 twin births found 58 boys and 62 girls. In the case of twin births observed by Braun, Chiari, and Späth, 94 in all, the children were in 64 of these of identical sexes, namely, in 30 instances boys, and in 34 instances girls. According to Breslau, in the case of twin births in the Canton Zurich, the sexual ratio was 104.1 to 100; whereas in the case of single births the sexual ratio was 106.2 to 100.

As regards triplets the reports of the sexual ratio vary from 111.76 : 100 (Meckel von Hemsbach) to 104.55 : 100 (Neefe).

It is obvious that the statistical data at present available regarding the sexual ratio in the case of multiple births are far too variable for it to be possible to base upon them any valid conclusions as to the influence of the nutritive condition of the mother upon the determination of the sex of the offspring. And taken as a whole the statistical data hitherto available do not enable us to infer with confidence that nutritive conditions, and more especially the nutritive state of the maternal organism, exercise any effect upon the determination of the sex of the offspring.

Another attempt at the utilization of statistics has been to ascertain whether the time of fertilization in relation to menstruation, (in the first days after the flow, or later in the intermenstrual interval) has any influence upon the determination of sex. The starting point in this investigation was the earlier view that sex depends upon the state of the ovum, and the belief that a very favourable state of ovum and sperm favours the production of the female sex. The ovum, after its discharge from the ovary, like the spermatozoon after its discharge from the testicle, tends sooner or later toward death, and the only thing that can save either from this ultimate fate, is for the two to unite to form a new organism. Precisely what moment in the history of the detached ovum is the most favourable, is a matter regarding which we have no exact information, but it is probable that at the moment of its discharge from the ruptured follicle, it is at the zenith of its vital powers. On this theory the determination of sex depends upon the period at which, after its liberation from the follicle, the ovum encounters a spermatozoon; the ovum which is fertilized early in its career becomes a female embryo; the ovum, on the other hand, which is not fertilized until it has become comparatively old, becomes a male embryo. But, as Hensen points out, a spermatozoon, according to its condition, may either fail sufficiently to fortify an ovum which itself is in good condition; or, on the other hand, a powerful spermatozoon may fortify an ovum of deficient vitality. But it is difficult to say precisely on what considerations the greater or less vitality of the spermatozoon depends—or, to speak more in accordance with the terms of the theory, we do not know exactly what makes it a good spermatozoon or the reverse. It may be “bad,” either because it has remained too long in the testicle or because it has been discharged too quickly, is too recently secreted; moreover, a long sojourn of the spermatozoon in the uterus will doubtless suffice to lower its vitality.

The Jews, more especially, whose religious ordinances forbid them to have sexual intercourse either during or shortly after menstruation, and among whom there is a great excess of male births, have been adduced as a proof of the thesis that sexual intercourse during the later part of the intermenstrual interval tends to favour the procreation of boys. In Leviticus xv. 19, we read: “And if a woman have an issue, and her issue in her flesh be blood, she shall be put apart seven days.” From the commentary in the Talmud it appears that these seven days are to be reckoned from the commencement of the flow. Statistical reports from various countries show that among the Jews there is a greater excess of male births than among the other inhabitants of the respective countries. The overplus of male births exhibited by the Jews varies from 1 to 15%, the difference probably depending on the fact that the number of instances under consideration is too small for uniform results to be possible. In Prussia, during the period 1820 to 1834, the sexual ratio among the Jewish births was 111 : 100; during the period 1849 to 1852 it was 106 : 100; in Hungary during the period 1835 to 1855 it was 117.1 : 100; in Sweden, 1851 to 1855, it was 108 : 100. Among illegitimate children of the Jewish community the sexual ratio was, in Austria, 123.9, in Prussia, 118.6.

Fürst, examining the cases in von Braun’s clinic, and making a statistical collection of the days of conception and of delivery, has endeavoured to prove that “there is an excess of boys when conception occurs during the period of post-menstrual anæmia.” His figures show, in fact, a very notable excess of boys in cases in which conception has occurred during the five days immediately following the cessation the menstrual flow; and an excess of girls when conception has occurred during the later part of the intermenstrual interval. Thus, in the former case the excess of boys is represented by the figures 37 : 12; in the latter case the excess of girls is represented by the figures 79 (girls) : 65 (boys). These figures are explained by Fürst on the theory that in the human species conception during the period of post-menstrual anæmia probably leads to an enormous excess of male conceptions (owing to the fact that the unfertilized ovum is badly nourished during the days just before conception); whereas in the later part of the intermenstrual interval the better nourishment of the ovum probably leads to the procreation of an excess of girls—hypotheses which are based on a quite inadequate number of instances. Fürst gives the following table:

Number of Days Between Reputed End of Menstruation and Reputed Occurrence of Conception.	Number of boys born.	Number of girls born.	Boys and girls together.

1	7	1	8
2	6	3	9
3	9	5	14
4	15	3	18
5	3	6	9
6	5	6	11
7	6	5	11
8	2	8	10
9	4	5	9
10	6	4	10
11	6	5	11
12	3	6	9
13	4	5	9
14	6	5	13
15	1	2	3
16	2	5	7
17	2	3	5
18	3		3
19	1	3	4
20	1	2	2
21	1	1	2
22	4	1	5
23		1	1
24	1		1
25		2	2
26	2		2
27		1	1
28	1	1	2
31	1		1
	102	91	193

Baust gives a report, based, he says, upon “thoroughly trustworthy data,” furnished by personal friends, regarding 14 cases, which show, in his opinion, that every conception occurring in a week after the cessation of the menstrual flow leads to the birth of a girl; but that when conception occurs on the fifth or sixth day after menstruation the result varies as regard sex. Swift, from a report of 20 cases, draws the conclusion that in the intermenstrual interval, boys, in the latter part of the interval girls are conceived.

The influence of the strength of menstruation upon the determination of sex has also been studied by the statistical method, starting with the idea that menstruation, in proportion to its strength, affords on the average a measure for the subsequent nutrition of the embryo, and this nutrition is further supposed to determine the sex. Düsing therefore arranged the births occurring in the lying-in hospitals of Dresden, Leipzig and Jena, according to the information given regarding menstruation in the clinical history of each case; it appeared that when menstruation was comparatively scanty, there was a greater excess of boys than when menstruation was comparatively abundant. The actual figures were the following:

	Abundant menstruation.		Scanty menstruation.
Dresden	902	847	495	431
Jena	66	69	56	45
Leipzig	21	22	239	211
Totals	989	938	790	687
Sexual ratio	105.4		114.7

Here also we may append the figures obtained by Düsing regarding the births of foals at the Prussian stud-farms, which he regards as supporting his view that by natural selection all animals have acquired the faculty, whenever stronger demands are made upon their sexual capacity, of procreating a larger number of individuals of their own sex. In the tables we learn how many mares on an average a stallion had covered in each year, that is, we learn how great were the demands made upon the sexual capacity of the stallion in that particular breeding stable in that year. The figures relating to the years 1859 to 1892 were tabulated and averages were drawn with the following results:

Number of Mares Served.	Number of Foals Born.		Sexual ratio.
Number of Mares Served.	Colts.	Fillies.	Sexual ratio.
60–70	42,445	41,933	101.22
55–59	56,511	66,226	100.49
50–54	59,940	61,096	98.18
45–49	57,077	59,216	96.39
40–44	59,967	62,007	96.71
35–39	38,348	40,181	95.44
20–34	26,354	27,069	97.35

From these figures, which relate a very large number of instances indeed, we learn that when greater demands are made upon the stallion, more males are procreated. In fact, except for two slight divergencies, the rise in the sexual ratio proceeds strictly pari passu with the increase in the number of mares covered.

I must, however, draw attention to the fact that this assumption when applied to the human species, that the man on whom whose sexual capacity especially extensive demands are made, procreates an especially large number of male children, is not confirmed by the sexual ratio among the offspring of polygamous marriages in which unquestionably greater demands are made upon the husband’s sexual powers than is the case in monogamic unions.

The reports of travellers of earlier days, to the effect that in Oriental countries more girls are born than boys, have recently been confirmed by several observers. Campbell states that in the harems of Siam the number of boys and girls born is equal. Clarke states that among the Mohammedan Indians more girls are born than boys. According to McLennan Indian experience teaches us that where polyandry prevails male offspring predominate in numbers; but where polygamy prevails there is, on the contrary, an excess of female infants. The following data collected by Goehlert from historical reports and from genealogical writings, regarding the progeny of notable persons living in polygamous unions, show certainly a large excess of female offspring over male:

	Children.		Sexual ratio.
	Male.	Female.	Sexual ratio.
Morocco: Muley Scherif	24	124	19.4
Palestine: Rehoboam, King of the Jews	28	60	46.6
Arabia: Imon of Sana	14	74	18.9
Turkey: nine sultans	110	128	85.9

According to Tousenel, love marriages give rise to more daughters than sons, whereas among the offspring of conventional or compulsory marriages, male children predominate. Further, among the offspring of legitimate unions, the excess of males is greater than among the offspring of illegitimate unions. A physician, V. J. Cook, maintains that boys are procreated in the evenings (before midnight), but girls during the early morning hours—at which latter time women are less “impressionable” than during the evening hours.

Düsing, starting from the common belief that all animals have the faculty, when there is a lack of individuals of one sex, of procreating an excess of offspring of the sex which is deficient and thus of restoring the balance between the sexes, maintains that numerous factors, through the co-operation of which the sexual ratio is regulated, act in temporal succession. He shows that the individuality of the mother has an influence upon the sex. But this finds expression through the qualitative constitution of the ovum; hence already before fertilization there must exist a tendency toward the development of one sex or the other—for example, younger ova tend to become females; older ova, on the contrary, to become males. It has further been shown that the individuality of the father, that is to say, the qualitative constitution of the sperm, has an influence in the determination of sex. Thus, by means of the influence of the sperm, the already-mentioned pre-existing tendency of the ovum can in some instances be counteracted and overpowered. The influences in the personality of the father and of the mother, which during fertilization find expression in the qualitative constitution of the sperm and of the ovum, respectively, can thus bring about a resultant tendency, acting in one direction or the other with varying force. Thus, after fertilization, we have this resultant tendency toward the formation of a male or female embryo.

But, Düsing continues, at this time the sex is not definitely determined. The influence of the nutritive condition in which the fertilized ovum finds itself has yet to make itself felt. This influence on the determination of sex through the maternal nutrition, continues (in the human embryo) for as long as three months, but even when the reproductive organs of the embryo have definitely begun to diverge in the direction of the masculine or the feminine type, as the case may be, some nutritive influence, if it is sufficiently powerful, may yet turn the balance in the other direction, so that a partial or complete hermaphrodite results, a being uniting the characteristics of both sexes.

Actual inheritance of sex, of which people used to speak, cannot, in Düsing’s opinion, possibly occur. The mode in which one sex or the other develops is indeed inherited; but the decision which sex shall develop does not depend upon inheritance, but is determined by the co-operation of several outward influences. The qualities by which this is effected are acquired by adaptation to general or special vital conditions.

Wilckens (“A study of the Sexual Ratio and of the Causes of the Determination of Sex in Domestic Animals”) opposes the views of Düsing, on account of the results of his own investigations, relating to the births of 30,000 domestic animals. He formed the following conclusions:

1. Locality (soil and climate) has an influence upon the sexual ratio and upon the determination of sex in domestic animals, but this influence is probably indirect only, being exerted through the intermediation of the nutrition of the embryo in utero.

2. The season in which the domestic animal is conceived affects the sex; the hot season favours the production of males, the cold season that of females; in the hot season, in general, the appetite and nutrition of domestic animals diminish, whereas in the cold season these increase.

3. Regarding the male progenitor, neither his age, nor his sexual energy, nor the demand made upon that energy, nor the age of the semen, has any influence upon the sexual ratio or the determination of the sex of the offspring.

4. The age of the female progenitor influences the sexual ratio and the determination of the sex of the offspring in this way, that in general, primiparæ and young mothers conceive a larger number of female offspring. This influence of age may be referred to the fact that in general young mothers nourish their offspring in utero better than older mothers.

5. The nutrition of the fruit in utero influences the determination of sex, speaking generally, in the following way, that better nutrition favours the determination of the female sex, worse nutrition favours the determination of the male sex.

6. In addition to the influence of nutrition of the fruit upon the determination of sex, other influences, whose nature still remains obscure, must also co-operate, because one and the same progenitor in similar nutritive conditions does not always procreate offspring of the same sex.

7. Owing to the operation of these unknown influences, prediction of the sex of the offspring, and voluntary determination of the sex of the offspring, remain impossible. All we can say is that there is some probability that young and well-nourished mothers will procreate a comparatively larger number of female offspring, whilst elderly and ill-nourished mothers will procreate a comparatively larger number of male offspring.

II. Anatomical Investigations.

Of anatomical investigations and discoveries, those more especially relating to the sex-relationships of twins and triplets have been applied to the elucidation of the problem of the determination of sex.

The first and most important fact in this connection, one that is not merely a rule confirmed again and again by anatomists and gynecologists, but is further, as Mayrhofer has demonstrated as a result of his researches in von Braun’s clinic, a “natural law,” is this, that twins and triplets enclosed in a common chorion are invariably of the same sex. The sex-identity of such twins has been referred to the similarity of their nutritive conditions (Leuckart, Ploss), and more especially to the communication between their bloodvessels; and an intimate connection between these relationships and the determination of sex has been believed to exist.

Mayrhofer, however, opposes this assumption by the following deductions (“The Determination of Sex in the Human Species”); “Fœtuses enclosed within a single chorion always possess a common placenta, in which the blood-channels from both umbilical cords frequently, in the case of twins perhaps invariably, communicate. It might therefore be supposed that the sex-identity of embryos enclosed within a single chorion is due to the intermixture of their blood in the placenta. Hyrtl, however, describes a triplet’s placenta, in which, though all three fœtuses were enclosed within a single chorion, the vessels passing to the umbilical cord of one of the fœtuses were entirely distinct from the vascular area common to the two other fœtuses; it is therefore probable that in the case also of twins enclosed within a single chorion there is not necessarily any communication between their bloodvessels in the common placenta. But even if it were proved that in the case of twins enclosed within a single chorion their bloodvessels always do communicate in the common placenta, we could not therefore infer that the intermixture of the blood of the two fœtuses is the cause of their sexual identity.

“For the intermixture of the blood of the two fœtuses in the common placenta could never lead to a complete identity in the composition of the blood of the two; it could only lead to a diminution of the differences which would exist between the bloods if their placental circulations were entirely distinct, and the similarity in the bloods thus established could not be expected to do more than make it a general rule that such twins or triplets should be of the same sex; but to this rule exceptions might be expected to occur in certain cases, as when hæmatopoiesis in the two (or three) fœtuses was very different, or when the circulation through the intercommunicating bloodvessels was interfered with through the pressure of fibrinous deposits—differences between the bloods would then arise sufficient to cause differences in sex (if identical composition of the bloods is presumed to be the cause of the sexual identity).

“Experience teaches us that the existence of communicating vessels in the placenta does not suffice to induce a close similarity of growth and of the formation of the organs in twin fœtuses; nor does it prevent the illness and death of one fœtus leaving the health of the other undisturbed, although the communicating channels remain open; so that, if we except acardiac monsters, it is correct to say that each fœtus pursues a secluded life, uninfluenced by the life of its neighbour. Acardiac monsters, on the other hand, always receive blood which has already served for the nutrition of the normal fœtus, and the result of this is an arrested development and a striking preponderance of connective tissue in the acardiac twin. But notwithstanding the fact that the failure of its own proper circulation (which is indeed rendered possible by the existence of the communicating vessels in the placenta) leads to its defective nutrition with a blood inferior to that supplied to the normal fœtus, the acardiac monster is always of the same sex as the normal twin.”

From these considerations Mayrhofer rightly infers that the identity of sex of two fœtuses contained in a single chorion, since it does not depend upon the existence of communicating vessels in the placenta, must arise from a developmental tendency already existing in the two germs at the time of conception—or, in other words, that at the time of conception their sex is already inalterably determined.

This conclusion with respect to the sex of twins contained in a single chorion may very readily be extended to the inference that in the case of all human fœtuses the sex is already determined at the time of conception.

Another anatomical fact is that many twins are contained in a single chorion for this reason, that they originate from two germinal vesicles within a single ovum. It is an open question whether it is not possible for two embryos contained in separate chorions to come to lie in a single chorion through atrophy of the intermediate wall. If this is indeed possible, the invariable identity of sex in the case of fœtuses lying in a single chorion must lead us to agree with Mayrhofer in inferring that two ova lying within a single follicle, simultaneously fertilized, give rise to embryos of identical sex.

B. S. Schultze and Ahlfeld, as a result of the investigations regarding twins, also came to the conclusion that the sexual identity of twins depends upon their derivation from a single ovum. If, in accordance with what has been said above, an explanation of the sexual identity of certain twins is to be found in the fact that for such twins there has been a single conception only, and hence the influence, whatever it may be, by which sex is determined acts on both germs at the same time—still the sexual identity of twins in general is remarkable and has not yet been fully explained. The sexual identity in fact occurs much more frequently than appears to correspond to the percentage of twins derived from a single ovum. Von Fricks examined the data relating to multiple births in Prussia during the period of 1826 to 1879 and compiled the following table:

Per Cent.	Twins.	Triplets.	Quadruplets.	Quintuplets.
Boys only	32.6	24.5	14.3	33.3
Girls only	30.3	22.5	19.4
Boys and girls	37.1	53.0	66.3	66.7
	2 B. 1 G.	28.5
	1 B. 2 G.	24.5
		2 B. 2 G.	23.4
		3 B. 1 G.	19.5
		1 B. 3 G.	23.4
			4 B. 1 G.	33.3
			3 B. 2 G.	33.3

According to Ahlfeld, of twin births in general, the ratio of those with a common chorion to those with separate chorions is 1 : 8.15. If, however, we wished to explain the frequency with which twins are of identical sex from the occurrence of such twins derived from a single ovum, we should expect to find a very different ratio, namely, 1 : 3.84; that is to say, twins with a common chorion would have to be nearly three times as common as they actually are. From these facts Düsing endeavours to draw the conclusion that external conditions have an influence upon the determination of sex, for very many external conditions are identical in the case of twins; as, for instance, the age of the father, the age of the mother, the nutritive conditions of the ova and of the spermatozoa, the nutrition of the embryo, etc.—all of these would influence both the twins in the same direction. A difference in the sex of twins, on the other hand, might be due to two successive fertilizations.

B. S. Schultze has been led by his embryological studies to the conclusion that there are male and female ova. Thus he believes that sexually identical twins originate from a single ovum with two germinal vesicles, thus assuming that a double fertilization of such ova is possible. But since in such cases the sexes of the resulting twins are always identical the spermatozoon can have no influence upon the determination of sex, but the conditions leading to the development of one sex or the other must pre-exist in the ovum—i. e., there must be male and female ova.

That as a matter of actual fact in some cases the female progenitor exercises an overwhelming influence on the determination of sex, and that the opinion held by so many that this determination depends upon the mother alone is sometimes supported by facts, is shown by the incident recorded by Darwin (“Descent of Man,” Vol. I.), that an Arab mare was delivered seven times successively of a filly, never of a colt, although she was covered by seven different stallions. On the other hand, the circumstance, if it is not to be attributed to pure chance, can also be explained by the constitutional vigour of this mare (Hensen) without adopting the above hypothesis.

Recently Upjohn has maintained that there are two kinds of spermatozoa, male and female; the latter are the commonest, but the former are the more energetic.

An interesting anatomical fact bearing upon this question has been discovered by M. Nussbaum, namely, that in ascaris megalocephala the reproductive glands are indicated already before the separation of the germinal layers; and Nussbaum suggests that this is also the case in all animals, although the proof cannot be obtained in every instance.

Semper has shown also in the case of some of the plagiostomata that long before the beginning of the definitive development of the reproductive organs, the apparently hermaphrodite embryo already possesses at least the tendency toward the constitution of one sex or the other. For in these animals, at a time so early that as yet the reproductive glands exhibit no sexual differentiation whatever, the sexes can nevertheless be distinguished by what appears to be a secondary sexual character. In the female, namely, one ovary only is developed; and very early indeed in the embryos destined to become females we can observe an asymmetrical development of the two germinal furrows. By this characteristic the two sexes can be distinguished far earlier than it is possible to do so by the recognition of a histological differentiation of the reproductive glands.

According to Mayrhofer the prepotency of the male gives rise to the procreation of an excess of males in this way, that physical prosperity of the male probably leads to the generation of boys, whereas prosperity of the female tends to give rise to the generation of girls. Moreover, economizing of the semen by infrequent sexual intercourse tends to originate offspring of the male sex. In this connection Hensen remarks, apropos of the greater excess of male offspring among the Jews: “We might ask whether, in consequence of the comparative sexual continence of the Jews which demands a definite power of endurance on the part of the ova, there may not be effected a certain selection of the ova; and thus we may perhaps explain how it is that in this race an exceptional vigour is somewhat more often to be observed than in other races.” Bock assumes that “thinner semen,” such as results from more frequent sexual intercourse, favours the procreation of the female sex, whereas when intercourse is less frequent a larger number of boys is likely to be born. Janke indicates as two important fundamental principles of scientific physiology that, (1) sexual intercourse represents as it were a contest between the two parties to the sexual act as to which shall transmit his or her sexual influence to the child, the victor in the contest determining the sex of the offspring, and (2) that a crossed inheritance occurs, inasmuch as whichever progenitor proves stronger in this contest transmits to the child the sex other than his or her own. He therefore advises women who long to have a boy to drink a glass of champagne before fulfilling their conjugal duties in order to gain increased sexual vigour.

Fiquet also expresses the opinion that when a vigorous, passionate, and sanguine male progenitor has intercourse with a frigid and phlegmatic woman there will be a preponderant tendency for the offspring to be female; whereas, when the conditions are the opposite of these, the male progenitor being phlegmatic and cold, the female on the contrary sanguine, passionate, and ardent, the offspring will probably be of the male sex.

In opposition to these opinions of Fiquet and Janke, to the effect that the temperament and the sexual vigour of the progenitors have a determining influence in the origination of the sex of the offspring, Düsing insists that the quality of the reproductive products are alone influential. Düsing lays down the following proposition: “The greater the scarcity of individuals of one sex is, the more extensive consequently the demands made upon the sexual capacities of the individuals of that sex, the more rapidly their reproductive products are employed; and the younger these products therefore are when employed the more individuals of their own sex will appear among the offspring.”

Richarz believes, on the contrary, that the prepotency of the male gives rise to the procreation of more girls; a mother of high reproductive capacity will have more boys, one of less reproductive capacity, on the other hand, more girls.

Starkweather (“The Law of Sex,” London, 1883) states his view in the proposition: “The superior parent produces the opposite sex;” and he holds the quaint view that this superiority is displayed in certain anatomical characters of the face of the progenitor. He endeavours from the shape of the head and from the facial expression to deduce the superiority of the male and the female progenitor respectively. A high, square forehead, with prominent supra-ciliary ridges, constitutes, according to Starkweather, one of the principal symptoms of this superiority; important also are a strongly developed middle third to the nose, narrow lips, etc. He declares that in families known to him the possession of this Roman nose in the father is signalized by the possession also of a large number of daughters, while a Roman-nosed mother has many sons. The more the parents’ noses resemble each other the more equal will be the distribution of the sexes among the offspring. He connects this fact (!) also with the fact that the possessor of the aquiline nose is the ruler of the family. Men of great strength of character procreate chiefly daughters; women, on the other hand, with a powerful character and a firm will bring into the world a notable excess of boys. In the Southern States of the American union Starkweather found confirmation of his theory, since he observed there that among the offspring of white fathers and coloured mothers there was an excess of girls amounting to 12 to 15 per cent. Among the half-castes of Java, the so-called Lipplapps, in the third generation girls only are born, and these are sterile. The excess of girls in these cases depends upon the superiority of the white father; this superiority is transmitted to the few sons of the second generation, and these therefore procreate girls only to constitute the third generation; the latter are not powerful enough to bear children at all.

Roth has revived the old view that one ovary provides the germs for the male offspring, the other those for the female offspring. He believes further that in the process of cohabitation the mechanical impressions and stimuli received by one-half of the external genital organs, reinforced by contact with and pulling on the pubic hair, are transmitted through the pudic nerve and the hypogastric plexus to the corresponding half of the vagina and the uterus, to the Fallopian tube of that side and to the corresponding ovary. If, now, we can consider it as established (?), that in the human female one ovary discharges male ova only and the other female ova only, it seems to him that it is the corresponding half of the external genital organs, when specifically stimulated in sexual intercourse, and the consequently increased vital activity in the pudic nerve and its connections with the hypogastric plexus, that must be regarded as the organ by means of which sex is determined. Roth has also been informed by laymen that when for some time they have procreated daughters only they subsequently procreated sons, “when, having been accustomed to sleep on one side of their wife, they adopted the practice of sleeping on the other.”

Ricardi reports that in Modena the peasants say that a man whose wife has hitherto had daughters only, must, if he wishes to have a son, assume some other posture than usual in the performance of coitus.

III. Experimental Investigations.

A considerable number of years ago, Thury attacked the problem of the determination of sex by the experimental method. In his series of experiments he ascertained, using 29 cows, that in the case of 22 of these, which were served early in their heat, the calves were without exception heifers, while in the case of the remaining 7, which were served late in their heat, the calves were equally without exception bull-calves (Thury, “The Law of the Determination of the Sexes,” Leipzig, 1863); in the record of these experiments no mention is made of the age of the cows. Thury concluded that the sex was determined according to the fertilization of the ovum soon or late after its liberation from the ovary; namely, that an ovum fertilized soon after its discharge produced a female, whilst an ovum which had become comparatively old before it was fertilized became a male.

Thury’s sensational experiments gave rise to a succession of similar experiments, made mostly by cattle-breeders, above all in agricultural colleges and in stud-farms.

Some of these experiments were made in the agricultural colleges at Proskau and Eldera. The cows, which according to Thury’s views should have been delivered of heifers, were served as soon as their heat was observed (the heat lasted as a rule from 24 to 30 hours); these were delivered of 5 heifers and 5 bull-calves (in Proskau) and of 3 heifers and 5 bull-calves (in Eldera); the sexual ratio in these cases was therefore normal. On the other hand, cows which were not served until their heat had lasted for 20 hours were delivered (in Proskau) of 1 heifer and 4 bull-calves.

Further experiments (in Waldau) gave the result that cows served early were delivered of 1 heifer and 1 bull-calf. In another series (in Eldera) 9 cows served as soon as heat was observed (or, speaking strictly, in from ½ to 1½ hours of this), gave birth to 7 heifers and 2 bull-calves.

Experiments made at the Royal Friedrich Wilhelm Stud-Farm gave the following results: In the case of 20 mares, which if Thury’s theory had been correct should all have given birth to fillies, 11 only fulfilled this expectation; but 10, on the other hand, were colts. Touchon, in his experiments at Hohenhau, obtained 11 calves and 2 foals, exhibiting the sex expected in accordance with Thury’s theory.

Düsing made a compilation of the figures given in all the experiments made to test Thury’s theory, with the following results: Cows fertilized early were delivered of 13 bull-calves and 29 heifers; mares fertilized early were delivered of 10 colts and 13 fillies; cows fertilized late were delivered of 5 bull-calves and 2 heifers.

Gerbe made experiments on rabbits, putting the buck to some doe-rabbits soon after the beginning of their heat, but to others as late as possible in their heat. On examining the young in the horns of the uterus, from the ovary downward, the distribution of the sexes was found to be approximately equal.

Whilst Coste’s experiments on a hen gave results contradictory to Thury’s theory, Albini’s experiments, made also on the common fowl, gave results in agreement with that theory. He found that the hens began again to lay fertilized eggs 3 to 6 days after intercourse with the cock (from which they had previously been kept separate); on the average, the distribution of the sexes in the chickens hatched from these eggs was approximately equal, with, however, a slight preponderance of cock birds. On the ninth and tenth days after separation from the cock the eggs laid were half fertilized and half unfertilized; on the twelfth day after separation from the cock the unfertilized eggs were in a great majority; but even as late as the eighteenth day after separation some of the eggs laid were still fertilized. The fertilized eggs laid from the tenth to the fifteenth day after separation when incubated produced a great preponderance of hen birds.

We have to thank breeders for a large number of experiments, such as those made by the breeder Fiquet, at Houston in Texas, who found that a bull upon whose sexual capacities excessive demands were made, procreated bull-calves exclusively; whereas in herds containing numerous bulls there were found among the calves born a preponderance of heifers. In thirty experiments on cattle Fiquet always found that the larger the number of cows a bull had to serve, and the longer they were kept in service, the larger was the proportion of bull-calves among their offspring. On the other hand, if certain cows had their sexual desires first satisfied by a gelded animal and were then served by a vigorous and lusty bull, an excessive proportion of heifers was born to these animals.

Janke obtained similar results in the breeding of sheep. In a report made to Düsing he states that in the early part of the lambing season more ewe-lambs are born than rams; in the latter part of the lambing season, on the contrary, more rams than ewes. The explanation he gives is that at the commencement of the pairing the rams are fresh and lusty, whilst later their potency is comparatively exhausted. In stud-farms, according to the same observer, it is a familiar experience that the most vigorous stallions serving a mare in the morning commonly procreate a filly; but if later in the day they serve a second mare they almost always procreate a colt. This, he thinks, finds its explanation in the fact that the stallion, when he serves the second mare, is in a condition of comparative sexual exhaustion, the more so because he usually covers the first mare twice.

Maritegoute’s breeding experiments at the sheep-farm of Blanc (Haut-Garonne), on the other hand, gave divergent results. In the early part of the pairing season, as long as the ram’s sexual powers were completely unimpaired, he procreated more male than female lambs. But when a few days later a great number of the ewes were simultaneously on heat and the ram, owing to very frequent acts of intercourse, began to be sexually exhausted, the procreation of female lambs was in excess. But when, finally, this period of maximum demands upon the ram’s powers was past, and the number of ewes on heat became once more small, the procreation of male lambs in preference to female was again observed.

The data obtained by Düsing from the Prussian stud-farms, in which, when greater sexual demands were made on the stallions, more males were procreated, have been already mentioned.

Fiquet made interesting experiments on cows and believed that in this way he was able to demonstrate the influence of nutrition upon the determination of sex—to such a degree, indeed, that he believed it was possible to breed calves of either sex at will. The following method gave him positive results in more than thirty instances. He never had the cow served by the bull at the first heat, but only at the second (if a cow is left unserved when on heat, the heat recurs after an interval of three weeks). The interval of three weeks was utilized in the preparation of cow and bull for the copulatory act. If a bull-calf was wanted the cow was supplied with the most invigorating fodder and was kept on the richest pasture available. The bull, on the other hand, that was to serve this cow was turned out to graze on the poorest pasture and was given poor fodder. At the end of the three weeks, when the cow came on heat for the second time, its sexual appetite was as intense as possible, whereas the bull showed but slight inclination to the sexual act. If the bull now served the cow a bull-calf was procreated. The opposite procedure led to the procreation of a heifer. For this purpose Fiquet kept the cow on low diet during the interval between the first and second heats, and had her first served by a castrated animal. When in this way, and by the low diet, the sexual appetite of the cow had been sufficiently diminished, it was served by a lusty bull, which for a long time had not been put to any cow, and the sexual potency of which had been increased to the uttermost by feeding it for several weeks on the most invigorating fodder.

The results of these experiments, according to which the nutrition of the parent-animals before the copulatory act has an influence upon the determination of sex, is explained by Düsing in this way, that nutrition influences also the quality of the reproductive products. “Poor nutrition gives rise to diminished functional capacity of the genital apparatus. Thus, for example, the production of semen is lessened. It can, in fact, hardly be replaced as quickly as it is used up. This occurs when there is a lack of adequate means of subsistence, and also when there is a lack of comparatively young males. In both cases alike we trace the effects in the birth of an excess of males. Converse conditions give rise to an excess of female offspring.”

Passing now to consider investigations made by physiologists, Born, at the anatomical institute at Breslau, has endeavoured to solve the problem of the determination of sex by means of experiment. He employed for this purpose rana fusca, an animal with which positive results can be obtained in a comparatively short period of time. He examined the sex both of the frogs in the free state (165), and also of larvæ which he had bred in specially arranged aquaria. Whereas among the frogs developing in the open, the numbers of the sexes appeared to be approximately equal (there was an excess of females amounting to 2 or 3 per cent.); among those bred in the aquaria there was an enormous preponderance of females (96 per cent.). This remarkable result is referred by Born to the inadequate supply of nutriment in the case of the larvæ bred by him (he fed them on hydræ and on putrefying frog and tadpole meat); from an examination of the alimentary canal of tadpoles caught in the open Born ascertained that their normal food was the mud of the pools in which they were hatched, containing infusoria, radiolariæ, diatoms, algæ, etc. The accuracy of this explanation appeared to Born to be more convincingly shown by the results in the case of one of his twenty-one aquaria. In this one alone the percentage of males was as high as 28 per cent., and the tadpoles in this attained the same size as those developed under natural conditions in the open, whilst in all the other aquaria the tadpoles remained abnormally small. This particular aquarium, owing to an oversight, had pond-mud on its floor, whilst all the other aquaria were floored with clean sand.

A. von Griesheim disputes Born’s results and believes that the latter, determining the sex of the tadpoles by means of a hand lens, must have mistaken a great many female tadpoles for males. He himself, by repeated enumerations of a large number of tadpoles (685), part caught in the open and part taken from a large aquarium, found that the ratio between the sexes in the case of rana fusca was regularly 36.7 males to 63.3 per cent. females.

E. Pflüger refers the divergence between Born’s results and his own and those of von Griesheim not to any error made by Born in the diagnosis of the sex of the tadpoles, but to the fact that in the latter’s aquaria the mortality of the male tadpoles was probably greater than that of the females. Pflüger endeavoured to ascertain whether the concentration of the semen might have an influence in the determination of sex. A quantity of frog-spawn was fertilized with concentrated semen, taken direct from the seminal vesicles, and another quantity of spawn was fertilized with diluted semen, obtained by making an aqueous extract of the incised testicles. The ratio between the sexes in the case of the two lots of tadpoles, which were kept in separate aquaria, proved, however, to be mathematically identical. But another experiment showed that the number of males was very different, according to the kind or race from which the animals were derived. He therefore believes that for the character of the development of the reproductive organs, the race of the parent animals is determinative. There is very little likelihood of being able to modify this inherited sexual ratio by means of outward influences affecting the ova and the ripe semen prior to fertilization, and just as little by means of a number of abnormal influences (change of climate, of water, of nutriment, etc.) acting on the fertilized ova.

Düsing,—who in his work on “The Regulation of the Sexual Ratio” (Jena, 1884) most ingeniously advocates the thesis that all animals have the power, when there is a lack of individuals of one sex, of procreating an excess of individuals of this deficient sex, or, to put it in another way, that an excess of one sex determines the procreation of an excess of the other sex,—instituted experimental investigations regarding the determination of sex in the following manner (in accordance with a suggestion made by Pflüger): About ninety guinea-pigs were distributed in two pens in such a way that in one pen there was a great deficiency of males and a great excess of females, whilst in the other there was a deficiency of females and an excess of males. Thus the sexual ratios in the two stalls were opposed. In accordance with Düsing’s theory, therefore, more males should have been born in the first pen and more females in the second pen. Every week each pen was examined once or twice, the sex of the new-born young was ascertained, and they were distinguished by small incisions in the margin of the ear. A week later, when the young animals had developed a little further, they were re-examined to make sure that no mistake had been made.

At first, in the pen containing the original excess of females, there occurred a quite remarkable excess of male births. This, however, was merely the result of chance, for soon the relationships of the sexes among the new born was reversed, and thenceforward many more females were born than males. But if all the births occurring in this experiment are taken into consideration the number is still far too small to allow trustworthy conclusions to be drawn.

Düsing emphasizes the fact that such an experiment as this, in order to furnish results worth consideration, must be continued until the sexual ratio has become constant, so that it is no longer subject to alteration by chance variations. If we assume that Düsing’s theory is false, the results obtained would be the following: In both the pens, in that in which there was originally an excess of females and in that in which there was originally an excess of males, the births, if observed through a sufficient period, would present a definite sexual ratio which would be the same in both the pens. But if the theory is well founded the sexual ratio of the new born would vary in the two pens: in the stall in which there had originally been a deficiency of males there would be an excess of births of males over females; whereas in the pen in which there had originally been a deficiency of females there would on the contrary be an excess of births of females over males. Düsing recommends that for such experiments even more fruitful animals, such as rats and mice, should be utilized.

Institutes for pisciculture would also be extremely suitable for such experiments in breeding for the determination of the matter under discussion because, owing to the fact that in these animals fertilization is effected outside of the body of the parents, a direct examination of the ova and the semen used in the experiments can be undertaken, and the fertilization can be made to occur under conditions subjected to various alterations; also we can employ the roe and the sperm of fishes whose age, life history, weight and size are accurately known.

Much attention has recently been paid to the theory of Schenk, based, as he states, upon numerous experiments regarding the influences by which sex is determined. This observer also starts from the principle that ovulation is not independent of the influences of nutrition and metabolism. He believes that in the cases in which combustion in the body is effected in such a manner that remnants of unconsumed substances, still capable of heat-production, make their appearance in the urine, the ovum of the human female in process of formation is not so far advanced in its development as it is in cases in which the urine is entirely free from sugar, or at any rate is free from any demonstrable traces of the presence of this body. In the former case we shall find that the ovum is not only less mature, but also that it is presumably less well nourished. In his view such an ovum is less completely endowed in respect of the indwelling qualities and forces of its protoplasm, and it appears for this reason to be adapted only for the development of a female individual. But when, on the contrary, in the maternal individual, all the substances formed in and assimilated by the organism have undergone combustion so completely that there is no sugar in the urine, not even in the minutest discernible traces, the maternal body is in a condition suitable for the development of an ovum adapted to become a male individual. From these inferences, weak though the chain of argument is, Schenk draws the conclusion, that by the regulation of the nutritive material supplied to the organism, and by the suitable choice of that material, we are to a considerable extent enabled to support an ovum in its process of maturation in such a manner as to cause it to develop into a male individual.

The nutritive material selected for this purpose must be of such a nature that the elimination in the urine of even the minutest quantities of sugar may be prevented; the urine must appear free from sugar even when the phenyl-hydrazine test is employed. Thus in every case in which we wish to influence a woman’s nutrition in such a way as to lead to the procreation of a male individual we must above all ascertain whether, in the woman in question, the normal quantity of sugar is present in the urine. If after the most careful examination no trace of sugar can be found in the urine, and if reducing substances are present in this excretion in abundance, no change need be made in the diet, and all we have to do is to recommend that the requisite fertilization should be effected as soon as possible, since there is every probability that in this condition the embryo will prove to be of the male sex. But when, on the other hand the “normal” quantity of sugar is present in the urine, or when even traces only of that substance can be detected, it is necessary by changes in the diet to cause the disappearance from the urine of every trace of sugar, and at the same time to bring about the appearance in that fluid of an abundance of reducing substances. Schenk claims by the experiments he has made along these lines to have obtained results which show that it is possible in this way to influence the determination of sex.

His method is to nourish the mother mainly on nitrogenous materials and fat, and to give in addition only so much carbohydrate as is necessary to prevent the absence of this from being seriously felt. This diet should be continued for a considerable period, at best for two or three months before the fertilization is effected. After conception also, the same diet should be continued. In such a manner we are able in certain cases to bring about the procreation of male offspring. On the other hand, the desire for the procreation of female offspring remains one which as yet we have no direct means of fulfilling.

These vague experiments and ill-grounded theories of Schenk’s do not, as a matter of fact, constitute an important advance in the theory of the voluntary determination of the sex of the human offspring. What in reality are the decisive influences in the determination of sex, and how the final impulsion in one direction or the other is actually effected, remain altogether obscure. Prediction of the sex of the offspring, and the voluntary procreation of male or female infants, remain problems for the solution of which the most essential data are still lacking.

Ernest Hæckel writes regarding Schenk’s theory: “This important ‘discovery,’ which at the time of its first announcement attracted throughout the world an attention rarely given to true scientific advances, has now dwindled to the incomplete demonstration that the nutritive condition of the mother exercises a certain influence upon the determination of the sex of the child. But we knew this much a long time ago. Düsing and others, partly by physiological experiments and partly by statistical demonstrations, had shown that changes in the quantity and the quality of the nutriment supplied to either parent is capable of influencing the procreation of boys or girls. But if what Professor Schenk maintains were really true peoples living chiefly upon meat (as, for instance, in the pampas of South America) should have an exceptionally large proportion of male offspring; whereas those living mainly on a proteid-free diet (on meal, sugar, and other carbohydrates), should have an exceptionally large proportion of female offspring (as, for example, the rice-eating Indian and Mongolian nations). But this is by no means the case. And many other well-known facts are likewise opposed to the ‘epoch-making’ theory of Schenk. Whether the fertilized ovum develops into a boy or a girl, depends, I am convinced, upon far more complex, and to a large extent still entirely unknown, physiological causes. The final judgment upon the ‘Schenk theory’ must be, ‘Much Ado About Nothing.’”

Our exposition of the present standpoint of the doctrine of the origination of sex in the human species, has, in fact, shown that hitherto by statistical work, nor by anatomical investigations, nor, finally, by the experimental method, have results been obtained which render it possible to predict the sex of the unborn infant. And even in respect of the study of those influences which exercise a determining influence upon the origination of sex, no positive, indisputable conclusions have been reached. We can only say it appears probable that there exist several causes of the determination of sex the co-operative action of which proves effectual. Not in the ovum alone, nor in the spermatozoon alone, but in the reciprocal influence they exert one upon the other in the act of conception is sex determined. In the latter connection the relative and absolute ages of the progenitors appear to have a certain influence in the determination of the sex of the embryo; of importance also is the greater or less demand made upon the sexual capacity of the begetter; of influence too is the time at which the ovum is fertilized after its discharge from the ovary. It appears to be fairly well established that when the husband is at least ten years older than the wife, while the latter is at the age at which a woman’s reproductive powers are at a maximum, more boys are conceived than girls (Kisch); also that one of the progenitors upon whose sexual capacities the greater demands are made, tends to procreate an excess of individuals of his or her own sex (Piquet, Düsing); and, finally, that intercourse a considerable time after the cessation of the menstrual flow (in the second week of the intermenstrual interval or later) is favourable to the procreation of a male infant (Thury, Hensen). The influence of nutritive conditions in the determination of sex is less clearly established.

Statistical evidence has proved beyond dispute that given a sufficiently large number of instances in varying conditions the sexual ratio is 106, and this fact suggests that the determination of sex is dependent upon the interaction of two influences operating in opposite directions within narrow limits, in such a manner that the chances of the birth of a male infant preponderate over the chances of the birth of a female infant in the proportion of 106 to 100. In elucidation of this fact Hensen makes the following comparison: “Let us imagine a balance the beam of which has two arms of equal length; from the two extremities of this beam two balls of nearly equal weight begin to roll toward one another; if one ball rolls more quickly than the other, if one is lighter than the other, or if one starts to roll before the other, the opposite end of the beam will sink. The three influences are variously distributed; one influence may reinforce another, or may counteract another; but a decisive sinking of one end of the beam will always ultimately ensue. A minimal shortening or lightening of one arm of the balance will make the chance that the other arm will descend correspondingly greater.”

Number of Days Between Reputed End of Menstruation and Reputed Occurrence of Conception.	Number of boys born.	Number of girls born.	Boys and girls together.

1	7	1	8
2	6	3	9
3	9	5	14
4	15	3	18
5	3	6	9
6	5	6	11
7	6	5	11
8	2	8	10
9	4	5	9
10	6	4	10
11	6	5	11
12	3	6	9
13	4	5	9
14	6	5	13
15	1	2	3
16	2	5	7
17	2	3	5
18	3		3
19	1	3	4
20	1	2	2
21	1	1	2
22	4	1	5
23		1	1
24	1		1
25		2	2
26	2		2
27		1	1
28	1	1	2
31	1		1
	102	91	193

Number of Days Between Reputed End of Menstruation and Reputed Occurrence of Conception.	Number of boys born.	Number of girls born.	Boys and girls together.

1	7	1	8
2	6	3	9
3	9	5	14
4	15	3	18
5	3	6	9
6	5	6	11
7	6	5	11
8	2	8	10
9	4	5	9
10	6	4	10
11	6	5	11
12	3	6	9
13	4	5	9
14	6	5	13
15	1	2	3
16	2	5	7
17	2	3	5
18	3		3
19	1	3	4
20	1	2	2
21	1	1	2
22	4	1	5
23		1	1
24	1		1
25		2	2
26	2		2
27		1	1
28	1	1	2
31	1		1
	102	91	193

Number of Days Between Reputed End of Menstruation and Reputed Occurrence of Conception.	Number of boys born.	Number of girls born.	Boys and girls together.

1	7	1	8
2	6	3	9
3	9	5	14
4	15	3	18
5	3	6	9
6	5	6	11
7	6	5	11
8	2	8	10
9	4	5	9
10	6	4	10
11	6	5	11
12	3	6	9
13	4	5	9
14	6	5	13
15	1	2	3
16	2	5	7
17	2	3	5
18	3		3
19	1	3	4
20	1	2	2
21	1	1	2
22	4	1	5
23		1	1
24	1		1
25		2	2
26	2		2
27		1	1
28	1	1	2
31	1		1
	102	91	193