It is often a matter of the utmost interest in families to have a child of a particular sex. There is always a disappointment in having a number of children all either boys or girls. The father, as a rule, takes greater interest in his daughters, the mother in her sons. The ideal family is composed of some of each sex.

Now we believe that we are not asserting prematurely a scientific discovery, when we confidently say that the law which governs the production of the sexes has been ascertained ; and that, with a due allowance for certain elements of uncertainty, and they few in number, persons can have either a daughter or a son as they prefer.

What is more, this law is not confined to the human race, but extends throughout all those species of animals technically known as oviparous, or which reproduce by means of an egg, whether this egg is deposited without the body or matured within it. And as stock-raisers, bird fanciers. bee merchants, and all engaged in the breeding of the various kinds of domestic animals, often would give a great deal to have it in their power to breed either sex at will, we shall give such details of the extent and workings of this law as to put it in their power, in the large majority of cases, to obtain either males or ismales as they prefer.

The discoverer of this law was a French veterinary surgeon, Prof. Thury, of the Academy of Geneva. He studied with particular care the sex of the offspring with reference to the date of conception. Of course, in mares, bitches, cows, and the other domesticated animals, this could be ascertained without any doubt. He found that when the male was given at the first signs of heat in the female, the result was a female ; but when the male was given at the end of the heat, the result was male offspring. With hens the eggs first laid after the tread gave females, those laid subsequently, males. The eggs first laid by the queen bee yielded females, those laid later, males.

A certified report to the Agricultural Society of Canton de Vaud, Switzerland, made in 1867. reports the result of a careful testing of Professor Thury's law in the following Words :-

" On twenty-two successive occasions I desired to have heifers. My cows were of the Schurtz breed, and my bull a pure Durham. I succeeded in these cases. Having bought me a pure Durham cow, it was very important to me to have a new bull, to supersede the one I had bought at great expense, without leaving to chance the production of a male. Accordingly I followed the advice of Prof. Thury, and the success has proven once more the correctness of his law. I have obtained from my Durham bull six more bulls for field-work ; and having chosen cows of the same color and height, I obtained perfect matches of oxen.

"In short, I have made in all twenty-nine experiments after the new method, and in every one I succeeded in the production of what I was looking for - male or female. I had not one single failure. All the experiments have been made by myself without any other person's intervention; conse-quently I do declare the law discovered by Professor Thury to be real and accurate."

Much other evidence from recent writings on the rearing of domestic animals could be adduced to justify the opinion of this reporter. On a number of stock farms in France, England, and this country, experiments have been conducted which show that there is much that we can depend upon in Professor Thury's law. Certain exceptions and apparent contradictions have also been noted, and some objections on theoretical grounds have been urged. For instance, Dr. Waldeyer, of Breslau, in his recent able work on the "Ovary and Ovum," opposes Thury's hypothesis on the ground that the ovum, for 6ome time after fecundation, is in a certain sense a hermaphrodite; in other words, the elements which go toward the formation of the sexual organs are alike in all. But as neither Waldeyer nor any one else has been able to say what mysterious something it is that finally decides the development of these elements into the peculiar organs of the one or the other sex, his objection falls to the ground. It is quite likely to be something in the ovum itself, dependent upon the length of time it has left the ovisac, as Thury asserts.

Some curious facts may be explained by this theory. We referred on an earlier page to the statistical observation that more male than female children are born. This would seem to be because the time when the ovum can produce a female is limited to a few days of its earlier independent existence ; while all the rest of its life it can lead to a male. If we take a large number of observations, it will be seen that when the husband is from fifteen to twenty years older than his wife, most of the children will be boys. This again is because the conjugal rights are more rarely exercised by men of advanced years, and the limited time just referred to, when the ovum can become a female, is skipped more frequently.

Observations in the human subject on this point are of course vastly more uncertain and liable to error than in the lower animals. Nevertheless, a sufficient number have been recorded to remove any reasonable doubt that it holds good with man, as it does with the inferior animals.

Physicians constantly observe that if labor comes a few days before " full term," or just at term, the child is more likely to be a female ; but if labor is delayed beyond term, which is the same as saying if the conception took place quite a number of days after the cessation of menstruation, then it is more likely to be a boy.

Several physicians, interested in satisfying themselves on this important topic, have noted the occurrences in their own families, and published the results in medical journals. So far as these have come to our notice, they are uniformly in support of Thury's law.

There remains an uncertainty as to the precise time at which the human ovum loses its power of producing the female sex in the foetus. For reasons very readily under, stood, the study of this subject is surrounded with difficulties. Moreover, it may well be that a difference in this respect exists in ova and in individuals.

There is also a liability to error from a want of exact knowledge on our part as to how long the male element remains active after it is removed from the body and before it comes into contact with the ovum of the female. Should this indeterminate period extend over several days, as it is highly probable that it does, it will readily be understood that an error in the application of the rule might result.

A third possibility of error arises from some uncertainty as to whether the act of menstruation in the human female is strictly analogous to and coincident with the process of ovulation. While there is no question that the external sign, and the general congestion arise from the maturation of an ovum, it is not yet known whether this ovum is discharged from the sac in which it has been ripening, at the commencement, during the course, at the termination, or immediately subsequent to the presence of the monthly symptoms. Authorities differ on this, and it is most probable that their disagreement is to be explained by supposing that there is no fixed time for the discharge of the ovum. Consequently we are at a loss to estimate exactly the age of the ovum at any given period after menstruation.

We have been careful to note all these elements of error in adopting Thury's law, because we believe his discovery to be one of vast importance, and well established in the inferior species; and in its application to the human race, it were to be regretted if a few disappointments, which may readily be explained, should lead to its rejection. As a general rule, we consider ourselves perfectly safe in saying that the earlier conception takes place after the menstrual flow has ceased, the greater is the probability that the offspring will be female; and the further removed from that period (always omitting four or five days anterior to the following monthly illness), the more likely is it that the child will be a male.

Before leaving this subject we will glance at one obstacle which has stood in the way of its reception. Some have imagined that the theory of M. Thury is overthrown by the fact that twin children are sometimes of different sex. But this is an argument founded on our ignorance. We do not know at all positively that the conception of both these beings took place at the same time. It is not merely possible, but for various reasons highly probable, that days intervened between the commencement of life in the one and in the other. So this fact, too, fails to militate against the general law.

[Authors and Works referred to in this section. - Mar - shall, Outlines of Physiology; Dalton, Human Physiology ; Dr. Seguin, On the Causes of Idiocy, N. Y. Medical Journal, 1870; Dr. Edward Smith, Cyclical Changes in Health and Disease; Acton, Disorders of the Reproductive Organs, p. 105; Hufeland, Art of Prolonging Life ; De Brantome, Vies des Dames Galantes, Discours I. p. 35; Raciborski, L'Age Critique chez la Femme, p. 484 ; Philadelphia Medical and Surgical Reporter, vol. xix. p. 305 ; Rev. John Todd, The Cloud with a Dark Lining; Bergeret, Les Fraudes dans Accomplissement de I'Acte Generatrice; Dr. Hodge, Criminal Abortion, 1869; Storer, Criminal Abortion' Waldeyer, Eierstock und Ei, p. 152, etc.]