which are rife to-day. But we must always bear in mind that it has not yet occurred.

The last decade of the 19th century added greatly to our knowledge of the physiology of the ductless glands, while in 1902 Bayliss and Starling's discovery of secretin opened out the vista of a chemical control of the body, in which the nervous system merely acted as a trigger, firing off a series of reactions. This conception was elaborated until it bid fair to dethrone the nervous system from its supreme position, although Langley had already provided the antidote by his generalisation that the action of adrenalin on any part was the same as that obtained by stimulating the sympathetic. But the far-reaching significance of that generalisation was not yet appreciated. "The race is not to the swift nor to the strong but to the wise." "The key to evolution lies in the continuous development of the nervous system"-said Gaskell. Or, as Elliot-Smith put it: "Man has developed and become the greatest of the primates because of his faithful dependance upon development of the brain." Considering the struggle the central nervous system has had to obtain control, it is not likely it would abdicate its suzerainty in favour of the more ancient dynasty of chemo-tropism.

We ordinarily speak of the evolution of the nervous system as peacefully accomplished. A struggle for supremacy between two animals or between two species, we recognise. But it would appear that a similar struggle accompanies the integration of the multicellular individual. Many apparently peaceful events in nature prove on closer analysis to involve a concealed struggle. Man is a gregarious animal but he has not found it an easy task to adjust the desires of the individual to the needs of the community. Philosophers may lament this, theologians may attribute it to original sin, but it should lead the biologist to enquire whether the cells of which he is composed have always found it easy to sink their individuality in that of the organism. The thesis of a hostile symbiosis between the tissues of the body has been skilfully upheld by Morley Roberts. A strong central government is needed to keep order and no high degree of differentiation is possible in the animal body without the control of a centralised nervous system, which has gradually acquired an increasing predominance. It is not too much to say that this control, though for the benefit of the body as a whole, may be resented by the individual tissues. Wilfrid Trotter in a remarkable paper on "The Physiology of Pain" has recently developed the argument of an hostility between nervous and somatic tissues, which is expressed in the way the former insulates itself.

Without defensive mechanisms no animal can survive in the struggle for existence, and no method of defence remains impenetrable, for, if it were, the species possessing it would multiply to the exclusion of others. As evolution proceeds the apparatus for defence comes under the control of a central nervous system. Some defensive mechanisms, such as nettle cells, never become so controlled, but are replaced by other methods as the nervous system evolves. Certain primitive motor apparatus, such as cilia, also never come under such control, though they persist even in the highest animals. In general terms a central nervous system enables very rapid reactions to occur, and the need for such rapidity of response will first be experienced in the struggle for existence, when its advantage is at once manifest. G. H. Parker in his book on The Elementary Nervous System has brought new observations into relationship with previously known facts and has impressed upon us afresh that a nervous system starts as a series of independent receptors and effectors in the deeper layers of the skin, the cells of which sink in deeper and deeper, until they become concentrated into a central nervous system. Between the receptor and effector an adjustor mechanism developes, which is the germ of the whole associative apparatus. In the vertebrates these internuncial adjustor neurons compose the chief mass of the central organs. It is not too fanciful to compare the origin of the nervous system to a group of settlers on the coast, who gradually invade the interior, first singly and then in an organised army, as in the nervous system of vertebrates which arises as a tubular invagination from the surface. Once established the invader assumes control over the indigenous inhabitants, fortifying itself as it goes and maintaining its protectorate by a system of rapid communication throughout the invaded areas. The biological and sociological parallel is remarkably complete.

The sympathetic nervous system retains several primitive structural features its ganglion cells remain peripheral, it operates through a primitive nerve net in such places as the intestine, and it has adjustor neurons lying largely outside the central nervous system. Functionally it recalls primitive methods in its lack of discriminative sensibility, and in the urgent, immediate, widespread and explosive character of its response. The sympathetic nervous system being originally evolved for rapid defensive purposes, the persistence of such primitive methods becomes intelligible.

Hostile symbiosis does not exclude alliances, and the sympathetic nervous system has entered into a close alliance with the endocrine glands, which represent a specialised survival of the old chemiotropic

methods of control. The closeness of this alliance is particularly well seen in the adrenals. The chromaffin cells and the sympathetic ganglion cells migrated out from the sympathetic nervous system together, but the latter gradually became the more abundant, while the connector fibres of the sympathetic end alike around both and stimulate them to action. The medulla of the adrenal and the post-ganglionic elements of the sympathetic nervous system are homologous structures; and there is a gradual predominance of the nervous over the chemical element in the partnership as the higher levels of the nervous system are evolved. Further, their influence is reciprocal-the sympathetic exciting the secretion of adrenalin while adrenalin stimulates the sympathetic endings. Herein we can begin to appreciate the full significance of Langley's generalisation. A similar reciprocity exists between the thyroid and the sympathetic, the sympathetic stimulating thyroid secretion, which in its turn lowers the threshold to sympathetic stimulation. But further, thyroid secretion quickens the whole metabolic activities of the body, and that is precisely how the sympathetic activates the body for flight or fight.

I need hardly stop to remind you how the effects of sympathetic stimulation are designed to this end since it has been done in so masterly a fashion by Canon. I will content myself with a few, perhaps rather dogmatic statements to clear the ground for the next step in my argument.

The autonomic, vegetative or visceral nervous system consists of two great divisions, the sympathetic and the parasympathetic or extended vagus. The former is katabolic, converting potential energy into kinetic and facilitating outward manifestations of that energy, while the latter is anabolic, directing energy inwards where it is stored up. When these two are distributed to the same structure their action is always antagonistic, and when one is stimulated the other is inhibited. The rhythm of life largely depends on the fluctuating balance between these two. The parasympathetic plays the chief part in the digestion and assimilation of food, the sympathetic spends the energy thus derived. In sleep the parasympathetic gains control, and the arrest of external manifestations of energy lasts until the balance is restored in favour of the sympathetic, when the subject awakens ready to expend energy again.

Each of these great divisions co-operates with a group of endocrine glands the sympathetic with the adrenals, thyroid and pituitary, the parasympathetic mainly with the glands of the digestive organs and their annexes. Possibly the parasympathetic co-operates also with the para

thyroids and some lymphoid structures, but this is too controversial a matter to consider now.

My next point is to remind you that as special cells were of necessity set apart for reproduction as soon as the protozoon evolved into a multicellular organism and even before the latter developed a nervous system, it is only to be expected that a close relationship should persist between the gonads and the primitive chemiotactic mechanism now elaborated into the endocrine glands. And since the sympathetic nervous system entered into a defensive and offensive alliance with the endocrine glands, a basic tripod came to be formed which was entrusted with the duty both of the preservation of the individual and the continuity of the species. Their structural association is indicated by such facts as the common origin of the adrenal cortex and the interstitial cells of the gonads from the Wolffian body, and the development of the thyroid from the uterus of the Palaeostracan ancestor. Their close association is shown in disease as well as in health and is reflected in many psychoneuroses. Disease is not likely to manifest itself in one limb of this tripod for long without affecting all three.

To get a general conception of the principles of internal secretion it is necessary to start from the premise that the development of each individual recapitulates, briefly and with modifications, the history of the race. And it is helpful to remember that while-life has been present on the earth for millions of years, man did not appear more than a comparatively few thousand years ago. Every time an individual repeats the history of the race he does it more easily as the result of practice. Thus it happens that the foetus in nine short months of pre-natal life recapitulates the history of millions of years, while his longer infantile life only repeats the history of a few thousand years. That even an adult may speedily revert to the instincts and habits of a cave man the war has abundantly shown. Later veneers of civilisation are easily cast aside.

Now the most primitive forms of life multiplied by simple fission. The unfertilised ovum is the nearest approach to the primordial cell that we know. Like the primordial cell it starts to divide by simple fission, forming two polar bodies. Millions of years ago simple fission was found inadequate for a complex animal, yet every ovum starts on this course till it is abruptly brought up by the consequent exhaustion of its powers of growth. But when fertilisation has remade its nuclear network a tremendous impetus is given to growth. When growth is expressed as a fractional increase of body weight it will be found that its curve continuously diminishes from fertilisation onwards. In other words, the

impetus thus derived gradually wears itself out. A new life may be compared to a projectile travelling with a constantly diminishing acceleration. But unlike an ordinary projectile it is supplied with intrinsic regulators, capable of increasing or diminishing its velocity. These regulators are the two groups of endocrine glands and their associated nervous mechanisms. In the first few years of life the original impetus is so enormous that a brake is needed-which seems to act through the thymus. By seven years of age this brake is needed no longer, and should it continue to be applied from any cause, infantile features persist. With the aid particularly of the thyroid and pituitary, active growth continues until puberty gives another twist to the mechanism from the active development of another set of glands-a new position of equilibrium has to be found. With adult years a part of the energy is diverted from growth of the individual to providing a supply for the next generation; and again the regulators have to provide a new adjustment. Another jolt often a violent one is given to the mechanism by the climateric, and yet a new position of equilibrium has to be acquired. From this point on, the dying down of the initial velocity is marked; the accelerating thyroid seems to die down before the sedative parathyroid; calcium fixation goes on unchecked, therefore; the intercostal cartilages grow rigid, the chest becomes emphysematous, greatly diminishing the bodily activities by restricting respiratory exchange, the arteries become calcareous, diminishing the blood flow to every part of the body, while childish and even infantile characters may reappear as the organism slows down for the terminus.

This may be considered the normal course of events. That everybody should ultimately only die from senile decay is the goal of medicine. But apart from violent death the regulators may become worn out with consequent increase of friction to the mechanism.

Invasion by bacteria and their toxins may prematurely exhaust the endocrine glands, or endogenous poisons may produce a similar effect; metabolism becomes balanced on a razor edge, the sport of every wind that blows. Nervous shocks and strains excite a reaction of the sympathetic nervous system and, through it, of its associated chain of endocrine glands, with consequent exhaustion of them. And these two sources of endocrine exhaustion may interact, so that a psychic conflict may be produced because the body cannot adjust itself to a difficult environment largely because of an endocrine deficiency. Thus the two newest methods in medicine, psychotherapy and endocrinology, become not opposed but different aspects of the same problem. Or a body may start out illequipped with a supply of endocrines, so that its growth curve suffers