P. SNOW By C.
THE sciences of living things are still in an unsettled condition. No one can say—as it is reasonable to say of physics—that the fundamental ideas have been sketched out. A great deal of time is being spent on attempts to construct a " theoretical " biology ; but at present, and probably for many years, the real interest in biology will be its special triumphs in restricted fields—genetics, Pavlov's reflexes, the structure of proteins, and so on. For a combina- tion of vital importance and pure scientific interest, these are among the most fascinating problems of the day ; and the further they are removed from chemistry and physics—as in the case of Pavlov's work and the subject of this article—the more they call on a quality which is nothing like so necessary in sciences where the rails are already laid down. I should like to call this quality " steadiness of vision " ; it is the cardinal feature of Pavlov's researches, and one becomes vividly aware of it in F. H. A. Marshall's work on reproduc- tion, sexual selection and periodicity—a subject so complex and so obstructed by our own inevitable prejudices that the importance and excitement of these new discoveries are in danger of passing us by.
It is easy for most of us to be fairly detached on physics and astronomy. A spiral nebula is a long way away, and even its philosophical significance can be endured without too much discomfort. But it is nothing like so easy to remain detached about work which may ultimately have the closest connexion with our own conception of ourselves, and even the adjustment of our own lives. For that reason among others, Marshall and his colleagues have more than ever needed this steadiness and consistency of vision. Obviously the scientific study of reproduction is bound to be important to human society. That fact has not been stressed. These researches have been concentrated on animals, where objectivity and definiteness of result are at present far more within our power. (There is a good scientific reason for leaving man to the last ; his sexual life is far more complicated than any animal's.) Obviously also, there are many methods of approach to these phenomena —with animals as well as men. To make any progress at all, it was necessary to select one and keep rigidly to it. The whole of Marshall's advance, just as with Pavlov's, has only been possible through passing a self-denying ordinance ; he has limited himself to the physiological events in the animal's sexual life.
You can study an animal's life in many other ways ; you can pay attention to its psychology, or you can regard it as part of a species and reason statistically. To get anywhere, however, you must not mix your different visions. There is a synthesis, of course, but we know nothing like enough to achieve it. And so Marshall has confined himself to one kind of insight—looking at the animal as a physiological individual. Its behaviour in sexual periodicity must be explained in terms of its own physiology ; the scheme must be self-consistent and complete within the limits imposed by the investigator. It is impossible to convey the subtlety and ingenuity of the arguments, but some of the recent results are understandable without any special knowledge.
The great majority of animals have a definite' breeding season. Thus in England sheep, deer and most other ruminants breed for a few weeks in the autumn ; many animals, e.g., ferrets and other rodents, in the spring. For an individual animal living without interference in its native environment, this breeding season will occur rhythmically all its adult life. There is a certain inherent rhythm— which is often marked externally by a change of appearance, as in the sexual plumage of many birds. Internally, the rhythm corresponds to an increase of the quantity of some substances in the blood : these substances are now called " sex hormones," belong to the chemical family of sterols (perhaps the most important group in the chemistry of life) and are secreted from glands known as the gonads. Thus left to itself, the animal goes through an inherent periodic change, which begins with the increased activity of the gonads, is followed by the excess of hormones in the blood and then by the external manifestations of the breeding season. In most animals this happens once a year and constitutes the natural sexual periodicity. But we have so far given no explanation at all of why the gonads suddenly produce their hormones at a specific time.
Well, the final answer is not yet certain. A number of clues, however, of greater and greater weight have been looked for and found. The first is, that many animals immediately change their breeding season on being taken in to the southern hemisphere. After an English breeding season in October sheep have been transported to South Africa and promptly had their rhythm revolutionised by another season in March (the ordinary breeding season for ruminants in southern latitudes). There are many similar examples ; the evidence is not always clear-cut, but it begins to seem probable that the essential factor in changing the animal's rhythm was the change in daylight, i.e., animals are affected by the lengthening or shortening of the days.
This view was tested by a series of ingenious experiments. One of the most striking was as follows : a number of ferrets (which are particularly suitable animals for the purpose, because of a well-marked periodicity) were subjected to strong " light baths " in various coloured lights. That is, through an autumn and winter the ferrets lived, so to speak, in a slightly curious Riviera ; one was continuously exposed to red light, another to green, another to ultra-violet, and so on throughout the spectrum. The temperature of their cage3 was kept normal, they ate no more food than usual. The only difference from their habitual circumstances rested in bright arti- ficial illumination instead of the dusk of a Cambridge winter. These conditions have been repeated for many ferrets over several years, and the results are positive and remarkable. Normally the breeding season for ferrets begins in March. Those treated with infra-red rays showed no change ; they behaved exactly like the ordinary ferrets with an undisturbed rhythm. But all the other ferrets which had been exposed to light began to breed weeks and months earlier than the normal—in the middle of winter instead of spring. Ultra- violet light was much the most effective ; it both accelerated the breeding season most sharply, and also prolonged it more than any other light. Similar results are true of most animals. We can alter their normal rhythm by means of light ; and by adjusting the kind and intensity of light, we can often control it.
That can be accepted as a fact. The mechanism by which the light affects the internal rhythm is not quite established. In all probability, the process consists of two stages (t) the light is collected by the eyes and travels by nervous paths to the gland known as the anterior pituitary (2) the anterior pituitary is stimulated to produce another hormone which acts on the gonads and brings them to their full activity.
In any case, there is now proof that the animal's sexual periodicity is controlled by two factors—the inherent rhythm, the ebb and flow of certain hormones, on the one hand, and on the other disturbances caused by the outside world and carried by the nervous system. A change in light is a broad and typical example of these nervous disturbances. This clear recognition and separation is a cardinal advance. The next step will probably be biochemical : why do the hormones act ? how far can they be controlled ? The answers may be important before long in human life.