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hypothesis has more to do with the case than the first. However, having frankly admitted all the
bias in the matter and pronounced the opposing verdicts, it may be well to proceed to consider the
evidence.
Dr. Brownlee's latest paper on the subject was read nearly a year ago before the Epidemiological
Section of the Royal Society of Medicine, and he then gave the results of applying the periodogram
method to the statistics relating to measles deaths, basing his analysis in large measure on London
mortality returns. The conclusions he draws at once arrest attention and call for critical examination;
shortly he finds that there are various strains of measles organisms, each of them prone suddenly to
develope very high infecting power (possibly as the result of some pseudo-sexual process), and he holds
that this power is lost at a rate corresponding to that of a geometrical progression. Each strain manifests
its cycle of activities in a fixed period, the strain commonly encountered on the north side of the Thames
having a 97 weeks', while that predominant on the south side has an 87 weeks' period ; other types
of frequent occurrence have periods of 114, 109½, 105½ and 89½ weeks. It is assumed that attack by
measles of one type protects against measles of another type, and a conclusion is drawn that one strain
spreads geographically outwards from a centre (St. Saviour's, Southwark), and that other types appear
simultaneously all over the town. No mention is made of German measles, and no account is taken
of precautionary measures, or of social influences favouring or preventing the infection of susceptible
persons.
The question which at once arises is that raised in the old story of the knights who, coming from
opposite directions, stopped in sight of a trophy shield, one side of which was gold and the other silver.
Dr. Brownlee has, in a most painstaking manner, described one side of the shield ; but it has another
side. It fortunately happens that this has already been examined by Sir Ronald Ross.
Dr. Brownlee pins his faith upon loss of infectivity of the organism, Sir Ronald Ross studied
periodicity from the point of view of exhaustion of susceptible persons. Both influences, and certainly
many others also, undoubtedly operate, and the behaviour of measles can only be understood by taking
these various factors fully into account. All the same it must be agreed that study of one or other
of them apart from the rest, may throw some light upon the complex problem. In order to simplify
calculation the mathematician sometimes assumes that a very small object, such as a fly or a piece of
string, is weightless. This device is only to be tolerated under due limitations. C. S. Calverley, in his
famous examination paper, made fun of this method by asking about the phenomena produced by a
small elephant whose weight, so the question stated, might be neglected ; but Dr. Brownlee has seriously
approached the problem of the form of the measles curve tacitly assuming that many weighty considerations
may for the time being be entirely disregarded. In an earlier paper (Epidem. Sect., Roy. Soc.
Med., Vol. II., p. 244) he has attempted to justify this attitude ; the measles curve, he said, is symmetrical,
hence, " the want of persons liable to infection is not the cause of the decay of the epidemic. On no
law of infection which I have been able to devise would such a cause permit of epidemic symmetry."
With regard to this the objection may be raised that the measles curve is by no means necessarily
symmetrical. This might perhaps be inferred a priori, as I once ventured to suggest (Milroy Lectures,
1906), but the main point is that in many of the London outbreaks, the rate of decline is in fact, more
rapid than that of rise. In support, however, of his original view that the curve is and must be symmetrical
and that variation in the germ is all important, two sets of facts are now cited by Dr. Brownlee.
(i.) It is pointed out (loc. cit., p. 245) that after a measles outbreak there still remain children
susceptible to the disease. This is admittedly true ; for it is not of course affirmed on the exhaustion
hypothesis that the number of susceptible children is reduced to zero at the end of a wave of measles.
Every one who has studied measles prevalences in a community of any considerable size will agree as to
this. Dr. Brownlee, however, in pursuing his line of argument goes on to refer to the fact that smallpox
in London in 1901-02 came to an end after only 9,000 persons had been attacked. He writes: " It
can hardly be said that, out of the whole population of London in the year 1901-02, only 9,000 were
susceptible to that disease." The misunderstanding here appears to arise from not taking into account
the influence of panic vaccination and re-vaccination. In point of fact, thanks to the preventive
measures taken, only a small percentage of the population was exposed to risk at the time when the
epidemic was declining; for it may be noted that the number of vaccinations and re-vaccinations carried
out in London, within the few months when the epidemic was rapidly spreading, amounted to not far short
of a million.
(ii.) An attempt is made to show that, at one phase of an outbreak as compared with another,
the organisms do in fact vary in infective power. In support of this conclusion particulars are given
concerning cross infections by measles in scarlet fever wards in Glasgow, and it is found that more such
cross infections occurred on the rising wave and on the top of the wave of measles than later during
the decline.
This phenomenon, however, it may be replied might result from depletion in the numbers of
susceptible persons in the hospital population at the later as contrasted with the earlier period, and the
facts might thus be explicable apart from any change in infecting power on the part of the organism
itself.
The discussion as to the bearing of the two sets of considerations just referred to, however, loses
a good deal of its interest in this particular connection, for the reason that Dr. Brownlee has agreed that
his main contention needs to be reconsidered. Sir Ronald Ross (British Medical Journal, 27th March,
1915, p. 547) had pointed out in his " Prevention of Malaria " (1911), that he had been able, a priori,
to obtain equations, on the assumption that the want of persons liable to infection is the cause of the
delay of the epidemic, from which he had been successful in obtaining a fit to the very nearly symmetrical
curves of certain epidemics. Dr. Brownlee, in replying in the issue of 8th May (loc. cit., p. 799), says :