purposes of comparison. They are presented in three forms, a "standard" rate, which would be the rate if the age and sex distribution of a community were the same as that of the country of which it forms a part, a "crude" or "recorded" rate, and a "corrected" rate.

TABLE III.

I select two Scottish areas, one purely rural and one purely urban:—

The table shows that while the population of the urban area is three times as numerous as the rural, the insane are less than twice as numerous.

We must look for an explanation of this within the rural area itself. The rural area comprises 59 parishes. In 34 of these the recorded insanity rate is high, in 25 it approximates to the normal. The inhabitants of the two groups are pracFarmers, crofters, fishermen, with the usual

tically similar.

sprinkling of other occupations.

In the group with the high insanity rate the population had been falling for 20 years; in the group with the low insanity rate, the population had been rising by natural increase, alone, for 20 years.

A community which is numerically decreasing is at a disadvantage with respect to the occurrence of insanity, for the normal age distribution of its population is greatly modified,

and there is an insufficiency of new blood to check the tendency of any hereditary affection to appear.

TABLE V.

Age distribution (per cent.) of the two groups:

(2) The Influence of the Limitation of Sexual Selection.

When a community is being drained by emigration, the weak, the infirm and the mentally inferior, as a rule, remain behind, SO that their proportion increases by mechanical ratio.

Exactly the reverse process takes place in the large urban centres which are being constantly reinforced by the addition of young healthy adults.

There is another factor of great importance, namely, the opportunities which a community offers for the marriage of its younger members. It is not possible to verify this statement by statistics, but it may be illustrated by examples.

Delage, in his work, "L'Heredite," says, "The inhabitants of the village of Eycanx had married among themselves from a remote period, and they almost all-men and womenpresented a sixth digit on their hands and feet. Gradually they began to extend their matrimonial alliances beyond their own commune, with the result that the deformity entirely disappeared."

In a small fishing community in the United Kingdom about one in seven of the inhabitants suffered from Huntington's Chorea some forty or fifty years ago. Fishing communities seldom mix with the inland population, and before the recent great extension in the British fishing industry they intermarried within their own communities. With the advent of better boats and better means of communication, the various fishing communities all round the coasts were brought into contact and a much wider marriage selection became possible. As a result, Huntington's Chorea has almost entirely disappeared from this particular community.

About eighteen years ago the Medical Superintendent of the Glamorgan County Asylum wrote as follows: "Glamorgan owes much of its mental healthiness to the fact of its exceedingly mixed population, one third consisting of aliens— that is of people not born within its borders. In this county until it was opened out by the railway, the Vale of Glamorgan was its blackest spot as regards insanity, and its inhabitants were nearly all related and constantly marrying amongst each other."

If we refer again for a moment to Table II, it might at first sight seem that the age distribution of the Scottish rural aleas (except for the greater longevity of the people) was more favourable to a low insanity rate than that of the urban areas. Against that, however, has to be put the evidently large emigration of young marriageable adults from the country to the towns, but for which the towns could not maintain their own populations by natural increase.

This constant stream of emigration reacts unfavourably upon the country districts, and favourably upon the town districts both as regards sexual selection and the tendency of any deleterious variation to appear in excess of the normal

mean.

Insanity comprises probably more than one form of variation from the normal. There is, for instance, congenital mental defect which is largely a teratological variation. There is the variation which constitutes the basis of the confusional insanities which probably comprises a variation in the immunity of the body tissues against infection. There is also the variation responsible for the recurrent insanities and the neuroses. We may, however, from the point of view of vital statistics discount these distinctions owing to the unexplained fact of the transmutability of the heredity in all these diverse forms. That is to say that a neurosis in ascendants may appear as insanity in descendants, and vice versa.

There is no race of mankind that is free from the neuropathic or psycopathic constitution, although, as statistics reveal, some communities may be more favourably constituted than others not only as regards a predisposition to mental and nervous diseases, but also as regards the frequency of the manifestations of these affections.

These favoured communities are those whose populations are increasing by an excess of the birthrate over the deathrate, or by the migration of young adults. The tendency in

all modern countries is towards migration of young adults from the country districts to the towns. So long as the birthrate in the country districts is sufficiently high to maintain a population adequate to the requirements of a normal sexual selection they will continue mentally healthy; but when, either through a fall in the birthrate or a surplus emigration, the population decreases, the outlook is less hopeful.

The large towns depend upon migration because, owing to their limited birthrate and their high infant mortality, they are incapable of maintaining a normal standard of population. Any hereditary deleterious variation from the normal such as the psycopathic constitution upon which insanity depends has a tendency to be fostered by the mating of the units of a population which has grown stale through lack of an infusion of new blood. Conversely it is restricted by the unions of individuals belonging to community groups unrelated by blood and reared in different environments.

The present trend towards urban life makes it imperative in the interests of mental health that there should be, not only a large rural population, but that the birth rate in the country districts should considerably exceed the death rate.

20

THEORY OF RELATIVITY.

THE EVIDENCE FROM PHYSICS.

By

E. F. J. LOVE, M.A., D.Sc., F.R.A.S., University of Melbourne.

TH

HE claims of a new theory of physical phenomena to
acceptance may be summarized under four heads:-
(1) Logical consistency of the fundamental hypo-
theses, including as a special case the truth of
each.

(2) Simplified formulation of the relations between
phenomena known, or assumed, to be inter-
dependent.

(3) Deduction of relationship between phenomena previously regarded as mutually independent. (4) Prediction of new discoveries, the possibility of which is inconsistent with previous theories.

The verification, or disproof, of the theory is attained, as a rule, from observational or experimental tests of the deductions and predictions specified under the last two headings. The simplified formulations are, clearly, not amenable to such tests, nor is the logical consistency of the hypotheses, unless they can be all shown to be true, in which case their consistency becomes a matter of course.

The theory of relativity, in its original, "restricted," or "special" form, was based on two hypotheses. The first of these asserts that natural phenomena run their course, according to the same general laws, for each of two observers, whether they be, relatively to each other, at rest or in uniform rectilinear motion. The second asserts that the ascription of a constant velocity to light, travelling in vacuo, constitutes a general law of the kind contemplated in the enunciation of the first hypothesis.

Neither of these hypotheses is susceptible of direct verification or disproof; the first of them, however, is tacitly assumed in the classical dynamics, which makes no reference to optical phenomena. The second is assumed in the classical electromagnetic theory, so far as the constancy of the velocity is concerned, but without reference to the case of observers in relative motion Each of these theories has proved so valuable within its own domain, that the assumptions of both could be, and were, regarded as well established a posteriori