Popular Science Monthly/Volume 26/January 1885/Influences Determining Sex
By Professor W. K. BROOKS,
OF JOHNS HOPKINS UNIVERSITY.
OF the many writers upon this subject, some have approached it with such an imperfect and narrow acquaintance with the facts that their contributions are of no interest to the scientific student; while other writers have allowed some minor generalizations to assume such prominence that their papers are of little value in themselves. The subject has an especial fascination for many minds, apart from its very great scientific interest, and we can, therefore, understand that the fugitive literature is somewhat in disrepute among students.
In Düring's papers, however, we find a remarkable combination of the two elements of scientific research: thorough observation and accumulation of evidence, and reflection upon its hidden significance. Although they contain comparatively little original observation, they are remarkable for the encyclopedic information which the author has collected from all sources. In many cases his generalizations are based upon observations which run up into the millions, and, even where the reader is not prepared to accept his conclusions, he will find in the papers a mine of recorded facts.
Having shown that there is in each species of animals or plants a constant average ratio between the sexes adapted to the conditions of life, he gives his reasons for believing that this ratio is not determined by any inner law, but by the influence of external conditions, which act sometimes upon the parent organism, sometimes upon the egg before fertilization, sometimes upon the developing egg or embryo, and sometimes, as in the case of many plants, upon the mature organism. He also believes that the character of the influence thus exerted by external conditions has been determined for the good of each species—by natural selection.
He treats, in the first part of his paper, of those conditions which act upon the two parents in opposite ways, and he summarizes his conclusions as follows: "Each species has acquired, through natural selection, the useful property, in virtue of which any deviation from the average ratio between the sexes is corrected by an increased number of births of the deficient sex, or a decreased number of births of the sex which is in excess."
We have space for only two of the many illustrations which he quotes to prove the existence of this law, and for further proof must refer the reader to the long tables of statistics in the original paper.
As the result of nearly a million observations of the birth of colts, he shows that, as the number of mares put to a stallion in a year is increased, there is a corresponding and regular increase in the number of male colts as compared with the female colts, and he gives the following summary:
In three cases where the power of parthenogenetic reproduction has been acquired as a compensation for the absence of males, the parthenogenetic eggs give rise either universally, or in the vast majority of cases, to males.
For instance, as bees destroy the males after they have been rendered unnecessary by the fertilization of the queen, they are exposed to the danger that when males are needed none may exist, and there can be no doubt that the power of parthenogenetic reproduction has been acquired by bees as a compensating adjustment.
When the nuptial flight of the queen is delayed by accident, or by the intervention of the breeder, the effect is, of course, equivalent to a scarcity of males, and in such a case more male larvæ than usual are produced; while early fertilization, which is a sign of the abundance of males, results, according to Huber, in an excess of female births.
Any influence which is equivalent to a lack of individuals of one sex acts, according to Düring, to produce an excess of births of that sex, although there may be an actual deficiency.
Thus, when the queen-bee is restrained by confinement, or by the lack of wings, from the nuptial flight, or when the seminal receptacle has been removed by accident or by an operation, or when the contained semen has been killed by frost or exhausted, only males are produced.
Something of the same kind has been observed in man, and the fact that a war, which carries most of the men away from their homes, is followed by an unusually great number of male births, has been re corded by many observers.
The second part of the paper, which will be found by far the most interesting to the scientific student, treats of those influences which act in the same way upon both parents, and the author's conclusion may be summarized as follows:
The power to regulate fertility according to the means of subsistence would be of use to the organism, and since the female has gradually acquired, through division of labor, the function of providing the material for the growth of the young, an excess of females is a condition of rapid multiplication. We might therefore expect, what we actually find to be the case, that organisms have gradually acquired, through natural selection, the power to produce an excess of females in time of plenty, and in a season of scarcity of food an excess of males.
I think, however, that careful examination of the evidence which Düring has brought together will show that he has stated his generalization in too narrow terms, and I think his facts will prove the following: A favorable environment causes an excess of female births, and an unfavorable environment an excess of male births.
It is true that abundance or scarcity of food is one of the most important elements of that whole which makes up the environment of an organism, and in most of the cases which Düring quotes it is the controlling factor; but he gives many cases, some of which will be noted further on, where a variation in other conditions of life has produced the same effect, causing an excess of male births when unfavorable, and an excess of female births when favorable.
In the case of man, the conditions of life are so much under control that it is difficult to state just what constitutes a favorable environment, but I think we may conclude that, as a general rule, an environment which produces a high birth-rate is favorable, and vice versa. Now, During gives many tables to show that, among mankind, the number of female births, as compared with the number of male births, increases as the birth-rate increases.
At the Cape of Good Hope the Boers are very prolific—six or seven is a small family, and from twelve to twenty children are not unusual; while the badly nourished and overworked Hottentots seldom have more than three children, and many of the women are barren, and Quetelet says that in 1813-'20 the free whites gave birth to 6,604 boys and 6,789 girls, or 97·2 boys to every 100 girls; while during the same time the Hottentot slaves produced 2,936 boys and 2,826 girls, or 103·9 boys to each 100 girls.
The birth-rate is higher in towns than in the country, and more boys are born for each hundred girls in the country than in the towns.
Thus, in Prussia, in 1881, the number of boy-births for each 100 girls was 106·36.
This table shows that in all the towns the ratio of boys was below the average for the whole of Prussia, and that in Berlin it was very much below the average.
Ploss was the first to point out that there is an excess of female births in time of prosperity, and he found that in Saxony the ratio of boy-births rose and fell with the price of food, and that the variation was most marked in the country.
It is well known that the number of conceptions among mankind is greater at some seasons of the year than at others, and from a record of nearly 10,000,000 births During has compiled the following table, which shows that the ratio of boy-births is greatest in three months when the birth-rate is smallest:
From this table it will be seen that in June, the month when the birth-rate was smallest, the ratio of boys to each 100 girls was highest, and very much above the average for the whole year; while in March, the month when the birth-rate was greatest, the ratio of boys was smallest.
More than 0,000,000 births took place in the seven months when the ratio of boys was below the average for the year, and only 4,000,000 in the five months when it was above the average; and the table shows clearly that an increase in prosperity, as measured by the birth-rate, is accompanied by a decrease in the ratio of boy-births, and vice versa.
Among the lower animals, satisfactory statistics are wanting; but During states that, while domesticated animals are much more prolific than their wild allies, there is also a much greater preponderance of female births; that when animals are taken from a warm to a cold climate, the ratio of male births increases; and that leather-dealers say that they obtain most female skins from fertile countries where the pastures are rich, and most male skins from more barren regions; and he thinks we may safely conclude that the lower animals, as well as man, give birth to the greatest number of females when placed in a favorable environment, and to most males in an unfavorable environment.
An extreme instance is furnished by those animals which, during the seasons when food is abundant, lose the power to copulate and multiply parthenogenetically at a marvelous rate of increase, giving birth to generation after generation of parthenogenetic females, so long as the environment remains favorable, but giving birth, as soon as the conditions of life become less favorable, to males and to females which require fertilization.
The cladocera and aphides furnish the most striking instances of this kind of parthenogenesis, which has apparently been acquired, not to secure fertilization, but to enable the animals to utilize to the utmost the conditions which are most favorable to them, and to expand and contract their numbers in conformity to changes in their environment.
Among the parthenogeneticboth males and females are to be found in the fall, and a few males are found in the early spring; but during the warm months of spring and summer only females are found. These multiply very rapidly through the summer by parthenogenesis, generation after generation, and they differ from the females which are fertilized by a male in many features, all of which are of such a character as to render the parthenogenetic females unusually fertile.
They produce small eggs, which are discharged from the ovary while immature, and are nourished in a vascular broad pouch. They have little or no yolk; they "are not protected by a hard shell, and they develop immediately into parthenogenetic females, which mature very rapidly, and in some cases, as in produce eggs before they themselves are born. All their peculiarities are of such a character as to secure the greatest possible fertility; and thus to enable the animals to avail themselves, to the utmost, of the abundant supply of food.
Ramdohr found that a single isolated female daphnia produced 190 young in nineteen days, and he computed the number of descendents, at the end of sixty days, to be 1,291,370,075.
As the supply of food begins to fail in the fall, males are born, and the females produce the so-called winter eggs, which do not develop unless they are fertilized. These are few in numbers, much larger than the summer eggs, and they are incased in protecting shells. Their purpose is not to multiply the race, but to carry a few individuals through the winter, and over to the next season of plenty. They are slowly matured in the ovary, and contain an abundant supply of food yolk. They are not nourished in a broad chamber, and in many cases they have, in addition to the proper shell, an extra covering or ephipium, formed out of part of the integument of the parent. In daphnella three summer eggs are matured, at one time, in each ovary; but the animal produces only one winter egg, which is seven tenths as long as the whole body.
While the abundance or lack of food is a very important factor in determining the absence or presence of males, it is not the only one. Kurg found a few males in mid summer, but only in pools which were nearly dried up; and he was thus induced to attempt the artificial production of males. He was so successful that he obtained the males of forty species, in all of which the males had previously been unknown. He proved that any unfavorable change in the water causes the production of males, which appear as it dries up, as its chemical constitution changes, when it acquires an unfavorable temperature, or in general when there is a decrease in prosperity.
From these observations and from many others quoted by During, I think we may safely conclude that among animals and plants, as well as in mankind, a favorable environment causes an excess of female births, and an unfavorable environment an excess of male births.
Now, what is the reason for this law? If the welfare of the species can be secured, under a favorable environment, by females alone, why are males needed when the environment becomes unfavorable?
I have tried to show, in another place, from evidence of another kind, that the female is the conservative factor in reproduction, and that new variations are caused by the influence of the male. While the environment remains favorable no change is needed, but, as the conditions of life become unfavorable, variation becomes necessary to restore the adjustment, and I believe that we have, in Düring's results, an exhibition of one of the most wonderful and far-reaching of all the adaptations of Nature—an adaptation in virtue of which each organism tends to remain stationary as long as no change is needed, and to vary when variation is demanded.
That this is the true view is shown, I think, by the contrast between domesticated animals and captive animals. The fact that an animal has become domesticated shows that it finds in captivity a favorable environment, and Düring says that domesticated animals are unusually fertile, and that they produce an excess of females. Animals which are kept as captives in menageries and gardens have, as a rule, no fitness for domestication, and their conditions of life are unfavorable. Geoffroy Saint-Hilaire says that individuals born in menageries are usually male, while skins sent to museums are usually female, and that the attempt to domesticate a wild animal increases the number of male births. Düring states that captive birds of prey and carnivorous mammals are very infertile, and that the young are nearly always males.
The wild races of Oceania and America have been suddenly brought into contact with the civilization which has been, in Europe, the slow growth of thousands of years. Food and climate have not changed, but a new element has been introduced into their environment. The New-Zealanders are very infertile, and nearly all the children are boys, and the census of 1872 for the Sandwich Islands gave a ration of 125 male births to 100 female births.
I believe that we may see, in these instances, the last effort of Nature to save the race from extinction, by securing a favorable variation.
It is no more than right, however, to point out that Düring himself gives a different explanation, and attributes the excess of male births under unfavorable conditions to the need for preventing close interbreeding. He shows that close interbreeding causes sterility, small size, and lack of general vigor and vitality; and he also shows that these effects are most marked when the other conditions of life are unfavorable, and that no evil effect follows close interbreeding when food is very abundant and the environment in general conducive to prosperity. As the evil effects of interbreeding are most marked when the environment is unfavorable, and as male births are then in excess, he believes that the excessive production of males is an adaptation, which has been gradually acquired for the purpose of preventing close interbreeding at the time when it is injurious.
I believe that a little examination will show that this explanation is imperfect, although true in a certain sense. As natural selection can not act in such a way as to establish an injurious property, the evil effects of interbreeding can not be primary. The thing which is advantageous and which has been secured by natural selection is crossing, or the sexual union of organisms which are not closely related.
As the object of crossing is to secure variability, it is most necessary when variation is needed, that is, when the conditions of life are unfavorable. Natural selection has accordingly acted to secure this, by rendering the offspring of a cross more able to resist an unfavorable change in the conditions of life than the offspring of closely related parents or the parthenogenetic children of unfertilized females, and the excessive birth of males, under unfavorable conditions, is for the purpose of securing variation, rather than the prevention of interbreeding.
In conclusion, I wish to again call the attention of the reader to Düring's papers, as they are filled with interesting reflections and suggestive observations which have received no notice in this short review.
They not only contain a treasury of facts, but they also show that in many parts of the field there is a great lack of recorded observations, and as some of our readers may be able to contribute something toward filling these gaps, and thus to extend our knowledge of the subject, the writer of this review takes this occasion to ask all who have made any observations upon the number of male and female births of wild or domesticated animals to make their results known. If they are sent to him, he will take pleasure in tabulating them, and will give proper credit for them.