The first necessity of human health is food; and it is in the precise constitution of food materials that modern nutrition studies have made remarkable discoveries of great theoretical and practical value. It has been shown that ordinary foodstuffs, to be sufficient for bodily welfare, must contain certain substances, very powerful in effect but very small in quantity, in addition to the proteins, carbohydrates, and fats which make up more than 99 per cent of their bulk. These substances are the vitamines, and no amount of food material, however good it may appear, will keep a mouse or a human being in normal condition, if it lacks any of these peculiar elements.

The vitamines are mysterious. Their exact nature and chemical constitution are by no means understood, but some of their properties have been detected by means of extremely ingenious experimentation on living animals. In many zoological laboratories investigators who specialize on the physiology of nutrition are actively engaged in this work, rearing white rats, birds, and other animals on diets made up with reference to vitamine content; and new information about nutrition in general and about the causes of such diseases as scurvy, beri beri, certain eye troubles, and of failures in development and growth is rapidly accumulating. All this is set forth in detail in a later chapter; but it is mentioned here because it is a branch of zoological research which has a very direct bearing on human welfare.

Perhaps no department of zoological investigation has afforded more striking results than the investigation of the endocrine system. This is a group of peculiar organs in the vertebrate body popularly referred to as "glands" and more exactly known as ductless glands, glands of internal secretion, or endocrines. They have been recognized, in part, from early times; but only recently has their real importance come to be fully appreciated. The study of these structures is now being actively pursued, and, indeed, it constitutes one of the most exciting and difficult frontiers of biology, as well as one of the most promising in practical value. The endocrines, such as the thyroid, the parathyroids, the adrenals, and the sex glands, exert a powerful influence on the organism as a whole, through subtle substances called hormones, which enter the blood directly from the glands and are carried by it to various parts of the body.

It has long been known that removal of the testes in early life results in profound changes in the appearance and in the psychology of the individual thus treated, producing what is called the eunuch in the human species and, in other vertebrates, such types as the gelding, ox, and capon.

In general the result of this operation is to prevent the normal development of the usual male characteristics and to produce an individual which is mild, strong, and heavy, but incapable of reproduction. Such traits are obviously of value in the Eastern harem and in the use of animals for heavy work and for food. And where male sopranos are required for sacred music in religious worship, the choirs have been maintained (until very recently, at least) by surgical dedication of boys with musical talent. These facts led a few curious persons to experiment with various animals and with various organs, until Bayliss and Starling showed how an extract of the duodenum injected into the blood stream causes the pancreas to secrete, owing to the effect of a hypothetical substance of very specific and powerful nature to which they gave the name of hormone. The idea thus established, many zoologists applied themselves to exact and well-directed experimentation, and in a few years the modern science of endocrinology grew up, with a new terminology, a new technique, and a new knowledge and control of physiological processes which had hitherto been mysterious.

The thyroid gland, for example, produces a hormone which tends to maintain normal chemical activity and hence normal growth of body and mind. If this organ is defective, the lack of the hormone produces sluggishness and abnormal development that may result in various diseases, such as myxoedema, ordinary goitre, and cretinism - all more or less fully remediable by supplying thyroid substance from animals. If the organ is over-developed, exophthalmic goitre, great increase in basal metabolism, over-activity of the heart, loss of fat, and other changes occur, which can be advanced or retarded by various experimental means. Much of all this was discovered by research on tadpoles and other lower animals; and gradually it is leading to the scientific treatment of human beings, as the technique develops through experimentation on the higher mammals.

A most remarkable series of discoveries has resulted from recent investigation of the gonads - the testes in the male and the ovaries in the female; and here we have most decidedly an advancing frontier of zoology, for the facts already found point toward hitherto unsuspected complexities and bear directly upon functions of the greatest practical interest. The sex hormones certainly control the development of the secondary sexual characteristics, such as bodily size, strength, and shape; the distribution and length of hair; the quality of the voice; and probably certain psychological traits. The differences which naturally distinguish the sexes and play such an important part in sex attraction, mating, and reproduction are absent or imperfectly developed if the sex hormone is not produced at the normal time. There is little doubt that some cases, at least, of perverted and otherwise abnormal sex behavior depend upon endocrine disturbances, and it is equally likely that further knowledge of these conditions will lead to practical methods of cure. But there is much more than this.

In the female the ovary produces several hormones. One, as we have seen, governs sexual development; another, produced by the corpus luteum (a group of cells formed on the surface of the ovary as each egg is extruded), controls menstruation and affects the condition of the breasts most strikingly when pregnancy occurs. If the egg fails to develop in the uterus, the corpus luteum soon degenerates, menstrual bleeding follows, and the mammary glands are little affected. But when the fertilized egg is implanted and forms an embryo, the corpus luteum persists and grows larger, probably preventing menstruation and giving out the hormone that stimulates growth and activity in the breasts. There are obscure relations here between at least three parts of the ovary - the interstitial tissue or framework, the follicles in which the eggs develop, and the corpora lutea - not to mention the lining of the uterus; and exact knowledge about these relations has not been reached. It is in such matters that experimentation on higher animals must be relied upon to form a background for the rare opportunities that permit direct work on human beings in the course of surgical practice. It has been found, in a few cases, that grafting a piece of normal ovary so that it grows in the body of the patient will cure abnormal and painful menstruation; but the significance of such facts in detail can rarely be made out unless the operation can be repeated with various modifications on animals. Zoologists are actively engaged in these studies in certain laboratories where the necessary equipment is provided; and they have developed a new branch of technique, biological surgery, which enables them to transfer living tissues from one animal into another, perform operations on embryo mammals, observe the condition of the reproductive organs at various times and after specific treatments, remove single glands without otherwise disturbing the animal, and in general carry out ingenious and dangerous manipulations with a percentage of success far greater than has been possible in the past. This has made possible certain investigations on the control of reproductive activity in the female, which are now being quietly but actively pursued and which have already yielded results that will, in all probability, have great significance for human welfare. By the use of the female hormones mentioned above, it is already possible to render certain female animals either fertile or sterile at will for given periods; and the successful application of such methods to the human species can hardly be considered more than a mere matter of technique. With the hormone material concentrated or isolated and thus readily available and the effects thoroughly understood, birth control will become not only sure and easy, but also biologically sound in the sense that ovulation and menstruation will be started and stopped by the natural agency - but at such times as seem desirable to the persons concerned. Contraception is already well-nigh universal in spite of the imperfection of the commonly known methods; and endocrine zoology promises to make it safe and certain. If and when this happens, the dwindling forces of opposition may as well abandon the struggle and contribute their enthusiasm and altruism to the important task of seeing that reproduction is reduced and regulated in such manner as to benefit all concerned.