The way in which some diseases seem to confine themselves to particular localities and classes of persons, and the reasons for this have always been favourite studies in medicine; and year by year such questions attract more and more attention. The reason of this is obvious. Those problems have some of the definiteness of pure physical science about them: their study throws a direct light on the nature of disease, while their solution tends to its immediate prevention. Hence the prominence which they have assumed in the new branch of preventive medicine. There is scarcely any word which means so much in this science as the localisation of disease, in its active and in its passive phase. To know why a disease breaks out in a certain place, and to be able to keep it from spreading farther may be said to be the two first aims of public medicine. The first thing to be done is, of course, definitely to connect the disease with its habitat. This can be done far more readily in the case of some diseases than in that of others, but there is no disease that is not more or less localised as to places or the class of persons whom it attacks. The weak points of man's constitution are so many, and the trials to which it is subjected vary so widely with locality, climate, food, work, and circumstances, that this must be so. The infinitely numerous seeds of disease and dissolution are of many species; and while each seed only germinates as it finds fit soil, each species also requires suitable conditions. This is as true in regard to the brain, and the departures from the normal performance of its higher functions, as in regard to every other organ of the body, though as yet but little attention has been directed to this fact. The wide series of diseases which are at present called Insanity prevail more in some places than in others, attack some classes of persons in preference to others, are hurried into actual development, or retarded where latent tendencies to them are in existence, by certain things which have a local prevalence, and they evidently assume one form rather than another through local influences. The extent to which this is the case is as surprising as it is certain. When one comes to look carefully into the reports of lunatic asylums in different parts of the country it is found that there are forms of brain disease (or varieties of insanity as they are called) present in abundance in one place which have almost no existence in another. Diseases of the brain which kill more than a third of all the patients in the asylums of some of our counties do but kill 5 per cent. of them in others. But I shall not anticipate the numerical proof of what I have been stating. This investigation must be very largely conducted on the numerical method, and fortunately the distribution of insanity and its varieties can be more thoroughly made out in this way than that of almost any other disease. When it attacks in a decided form any person in the classes which constitute nineteen-twentieths of the inhabitants of this country its treatment is so difficult and costly that if it is of long duration it almost necessarily must be done at the public expense. This implies that it is publicly recorded in the official documents of the Commissioners in Lunacy and the Poor Law Board. In this way a fairly trustworthy account can be got of the number of persons in every county and district of England and Scotland who are suffering from this disease in any one year. It is true that these numbers include also the persons who are chargeable to public funds on account of idiocy or marked imbecility, dating from birth, and the numbers of the latter cannot be distinguished in these documents from those who labour under insanity. But as congenital brain defect and acquired brain disease certainly have the closest connection hereditarily and in their essential nature, this does not seriously affect an investigation into the local occurrence of insanity founded on the numbers recorded in the official documents I have referred to. The numbers of the insane who are paid for out of their own funds or by their relations, and who appear in those documents as private patients, are left out of the account, because those numbers are comparatively small, and it is impossible to fix correctly the local occurrence of this class of insanity, it being determined in these official records more by the presence or absence of the institutions for its treatment than anything else. This omission affects slightly the scientific accuracy of the results obtained, but does not affect their practical value and medical interest.