an important factor in the scheme of prevention. We shall see later on how such habits have a large influence on the immunity of some modern races of mankind.

The wise choice of foodstuffs which by their consistency tend to assist the cleansing process has been shown by many observers (of late very forcibly by Dr. Sim Wallace) to exercise a very beneficial protective influence.

It is necessary, therefore, to examine minutely the environment, habits, and dietary of various races of mankind, past and present, and to consider them in relation to the comparative immunity or liability of those races. In this connexion I shall be able to give the results of a fairly extensive examination of skulls and to compare my own observations with those of other explorers.

In connexion with the question of cleanliness and the nature of the food, as affecting the teeth of the individual during his lifetime, I have carefully compared the conditions of races inhabiting the hot belt of the earth with those in temperate and frigid zones, as both the habits of personal cleanliness and the dietary are profoundly affected by the temperature of the habitat.

This consideration of the temperature-environment will also enter largely into the discussion of the effects of civilisation, as the tendency of highly civilised races is to inhabit temperate climates.

The effects of variation of the constituents of water used for food upon the prevalence of caries has been, I think, altogether overrated, and in this matter Magitôt came to a similar conclusion.

The second great division of my subject-namely, the strengthening of the natural defences of the teeth themselves-is, I am convinced, by far the more important of the two.

In this connexion it is generally believed that, once the enamel covering of a tooth is completely formed, nothing the individual can do will alter its composition. This at least is the present view of dental anatomists; and though my friend. Dr. Galippe endeavoured to show that the composition of the enamel of teeth does vary during life, his theory has not found many adherents. I shall assume, for the purposes of this essay, that it does not do so.

The actual chemical composition of the tissue called enamel is at present regarded by most authorities as being devoid of organic matter. I have, however, never felt convinced by the experiments upon which Mr. Charles Tomes has rested this opinion. The supposition that a structure like the enamel organ is capable of forming, as part of the living body, a sort of mineral crystalline substance, consisting of a mass of lime-salts and a trace of water, seems to me incredible. There is no analogy in nature (for the

tissue is almost universally held to be the result of a process of conversion of protoplasmic substance, and not of excretion).

Further researches upon this point are being actively prosecuted, but pending their publication I must rest content with expressing my own inability to accept the prevailing theory, whether considered from the point of view of its likelihood or of the convincing nature of the experiments upon which it rests.

Whatever may be the outcome of immediate research touching the development and chemical constitution of enamel, for the uses of my present argument it will be necessary to accept the prevailing teaching. I shall therefore assume that the enamel cap, once formed, does not undergo any further change in its composition; that is to say, that the sufficiency or insufficiency of the defences of the tooth is finally and for life decided when once the enamel covering is formed.

Now this enamel covering, in the case of the permanent teeth, is formed before they erupt. Its strength or weakness does not depend upon any habits acquired during infancy, much less during childhood. The sources of weakness which may at any moment of severe attack lay bare the citadel to the invading bacterial host are active largely before birth, and, it may safely be said, entirely before the cessation of maternal nourishment (with the exception of the aberrant third molar).

It will, therefore, be of the first importance, in prosecuting this inquiry, to examine and compare the varying conditions affecting maternal nourishment in races specially immune and those specially liable to the disease.

The rough outcome of a somewhat extended investigation of the teeth of various races at various periods, of which details will be given presently, amounts to this: that in almost exact proportion to the degree of their civilisation, or, in other words, their artificial life, peoples are liable to caries. The wild races, whatever climate they inhabit, or at whatever period of the world's history they flourished, are always practically immune. A moderate degree of artificiality or civilisation results in a moderate degree of liability to caries. An excessive artificiality or civilisation, such as is found in the environment of successful existing races, is attended by a wholesale and deplorable dominance of the disease.

I shall endeavour to show in detail the elements connected with the nurture of our infants that contribute to this dental degeneracy, but in this introductory section I content myself with summing them up as an unconsciously organised attempt to defy the great laws of survival of the fittest in the physical domain. Throughout wild nature it is the law that the fittest should survive in their environment. It is also the law, often

overlooked, that the mother kind should always act more for the welfare of its offspring than of itself. Civilisation has altered the field of battle physical excellence is no longer the goal; the ultracivilised race forgets the future, and concentrates all its enormous powers upon the present moment. Its genius is possibly misdirected, but one undoubted result is the rampant triumph of the army of micro-organic invasion, and, as I hope to show, dental caries is one of the first of their great victories.

THE INFLUENCE OF CLEANLINESS

If we consider the relative liability of certain teeth and certain parts of those teeth to dental caries, we shall be forced to the conclusion that the habit of cleaning the teeth after meals plays an important part in protecting organs the enamel covering of which is not sufficiently well developed to defy all attacks.

The first molars are no doubt more liable than any other teeth, the lower incisors are certainly least attacked of all. I will consider the various teeth seriatim.

The first molars. The fact that these teeth are erupted between the ages of five and seven years, and are exposed to the bacterial attack during a period when the ordinary disturbances of childhood tend to favour bacterial activity, when, moreover, the other permanent teeth are hidden altogether from the enemy, may be allowed its due importance, but there are other factors.

The habit of cleaning or even rinsing the mouth after meals is extremely rare during childhood, so that these early comers are not only exposed to severe attacks, but, owing to the carelessness or ignorance of parents and nurses, and the indifference of the children themselves, they are feebly, if at all, defended.

The extensive ravages of caries in the milk-teeth are another illustration. The fact that these teeth are presently to be lost leads to a short-sighted neglect of their condition. The permanent nature of the first molar is often not recognised.

The second molars and the premolars are much less liable than the first molar.

The upper incisors are much more liable than the lower incisors. How does this fact bear upon the question of cleanliness?

I believe the answer is this: that the lower incisors are generally in actual contact with their neighbours from the cutting edge to the gum margin, whereas the molars, premolars, and upper front teeth, although normally in contact at the cutting edge, permit, because of their shape, an interval at the gum edge. This wedge-shaped space, after a meal, is filled with debris of food, and, if this is not removed by rinsing the mouth or some such cleaning process, fermentation follows, and the enamel is

roughened. This change once begun, it becomes increasingly difficult to clean the interspaces. It is certainly in these interspaces that caries most often commences, or in the deep pits and crannies to be found in the molar surfaces.

The third molars. These teeth, which are seldom erupted. before puberty, should show a high percentage of exemption, but they certainly do not do so. What explanation is there as regards cleanliness?

The third molars (or wisdom teeth) are disappearing. They are often deformed macroscopically and microscopically, but there is another reason for for their liability. The actual

process of eruption of a wisdom tooth is infinitely slower than that of any other tooth. Instead of appearing like a mushroom in the night it may take many months to finally assume its position. During all this time, while part of the tooth is exposed and the rest covered by a loose flap of gum tissue, the parts so covered, while inaccessible to any tooth-brush or mouthwash, yet offer splendid cover for the undisturbable activity of the enemy. It is generally the posterior portions of the third molars which suffer.

The light thrown upon the cleanliness question by an examination of the skulls of various races, whose habits in this respect are ascertainable, is valuable, but it plainly shows that this aspect of the case is only one important factor, and by no means the whole explanation of immunity and liability.

I will now discuss in this connexion the dental condition of races inhabiting the hot belt of the earth, races not completely Europeanised, but retaining their ancestral religions and customs -native Africans, native Indians, and native Chinese, as illustrated by skulls in our museums.

The African native enjoys practical immunity. The native Indian has on an average one carious tooth for every two skulls (almost immunity). The Chinese, one carious tooth to thirty skulls (practical immunity).

The habits of these peoples as regards tooth-cleaning are very striking.

The African native is taught from early childhood to clean his teeth with small slips of wood, and rinse his mouth freely after every meal. H. M. Stanley notes these habits, as do many other explorers. I have myself obtained minute accounts from negro women, who had no idea why I was asking of these customs, which amount almost to a religion. They frequently use some form of fine powder (generally ashes of some sort, or salt), but the rinsing after meals is never omitted.

The Indian native is equally careful in this matter. I have made very minute inquiries into this. A very large employer

of native labour in India has assured me, from extensive personal observation continued for many years, that the practice of rinsing after meals is universal among them. An Indian prince, who is a personal friend of mine, has assured me that the care from early childhood throughout life in this respect amounts almost to a religious rite in his country.

Quite recently my friend, Dr. Chundra Muthu, himself a native and a medical man, has fully corroborated the truth of the universal prevalence of postcibal rinsing, and has borne out to the full the practical immunity of native races so long as they are unEuropeanised.

The habits of the Chinese natives in this matter I have obtained from Professor Arthur Keith, who assured me from personal observation that they always rinse their mouths after every meal.

These three races inhabit hot countries, which fact of itself tends to encourage habits of personal cleanliness. They are extremely careful about their teeth, and they are almost free from dental caries.

As soon as I reached this stage of my inquiry I felt that a case was almost made out for the toothbrush and the postcibal rinsing. To complete the case, however, I proceeded to examine series of skulls of the inhabitants of Arctic regions (Esquimaux), who never wash at all and never make any attempt to cleanse the oral cavity,' fully expecting to find dental caries rampant. The native Esquimaux showed an average of one carious tooth in twenty-seven skulls (practical immunity). Therefore the dirtiest known inhabitants of the earth were as immune as the cleanest. This was a serious set-back to the triumphal progress of the theory of 'cleanliness' as the whole governing factor.

The examination of inhabitants of a temperate region who were uncontaminated by civilisation (Australian natives) again showed only seven carious teeth to twenty-two skulls, or seven carious in a possible 700 teeth. This too amounts to immunity.

I felt driven to the conclusion that, although cleanliness and protective measures observed during the life of the individual were potent to protect even imperfectly constituted teeth, there existed and had existed races whose teeth were so splendidly constituted that neglect, involving unmolested activity of the enemy, did not succeed in allowing them to break through the first line of defence.

Before leaving the question of cleanliness a word must be said about the conditions prevalent at the present time in ultracivilised communities. It is a question upon which it is extremely difficult to obtain reliable and far-reaching data, but as far as I have been able to obtain them they appear to suggest certain possibilities.