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of the house-fly were dissected into its constituent parts (by separation of the flies into their three
principal species—Musca, Homalomyia and Stomoxys), one or other of the three contributing curves,
would be found to correspond even more closely with the diarrhoea curve than does the curve for
"total flies.'' It may be noted, however, that the points to which attention was directed in last
year's report, I as suggesting that the correspondence in the fly curve and diarrhoea curve is not quite
of a kind which might be expected, supposing flies to stand in causal relationship to diarrhoea, remain
practically unaffected by a substitution in the comparisons made of a curve for "Musca domestica"
for that for "total flies." (See the right-hand Inset in Diagram I.)
In the second place, Musca domestica is especially common in the poorer parts of towns, and
it is, of course, among the poorer urban populations that diarrhœal mortality is high. More precise information
is desirable under this head, and the question is clearly one which deserves closer attention.
It was pointed out, in the report of last year, that deaths from diarrhoea do not appear to occur, so
far as the matter has been investigated, with greater frequency in populations living close to collections
of stable manure, than in similar populations elsewhere. The experience of Dr. Harris, of Islington, was
referred to last year, and I am now told by Dr. Porter, of Finsbury, that only four deaths occurred, in the
summer of 1908, in a population of some 5,000 persons living in an area surrounding certain large stables
(Centre No. G, of this Report). In an area in Southwark, on the other hand (surrounding a cowshed,
Centre No. 7), Dr. Millson, of Southwark, informs me, there were five deaths, in 1908, in a population
of some 1,500 persons. . The question of diarrhoea mortality, in dwellings near stables, has been
discussed by Dr. Waldo in his Milroy lectures of 1900, by Dr. Niven in his annual reports, and by
Dr. Sykes ("Public Health and Housing").
On the other hand, there are certain further arguments based upon study of the house fly and
diarrhoea curves which seem to tell against belief in the causal relationship of flies to diarrhoea. For
example, it is interesting to compare the two curves of 1908 with the corresponding curves of 1907.
In the last-named year the culminating points of both curves were reached exceptionally late, and in
one other respect the year was quite unique, inasmuch as the amount of diarrhoea was particularly
small. The year 1908 has from both points of view been a more normal year. While, however, the
deaths from diarrhoea were far more numerous in 1908 than in 1907 (during the eight weeks of maximum
diarrhoeal mortality more than twice as many diarrhoea deaths occurred in the latter as in the former
year), the number of flies was not correspondingly excessive; for in 1907 about 1,000,000 flies were
collected at 163 stations, while in 1908 about 1,200,000 flies were collected at 141 stations. The
figures, though very far from being conclusive, suggest that it would be well to ascertain whether the
number of flies undergoes as marked fluctuation year by year as does the amount of fatal diarrhoea.
This consideration, even if fully borne out by more complete enquiry, by no means, however, finally
disposes of a fly-borne diarrhœa hypothesis, for (accepting for the purposes of argument this lastnamed
hypothesis), the fluctuations of diarrhoea may be associated with variations in the amount of
infective material, as well as in the numbers of the supposed carrier, the fly.
Comparison of the curves for 1908 and 1907 brings to light, moreover, a striking fact regarding the
behaviour of the fly and diarrhoea curves in the two years. From the middle of August to the end of
the first week of October, 1907, the number of flies was practically stationary. Again, from September
4th to October 5th, 1908, the number of flies was also practically stationary. Further, judging by the
total number of flies caught, in relation to the number of places of observation in 1907 and 1908,
practically the same extent of prevalence of flies (in the parts of London concerned) was maintained
during these two stationary periods of the two years in question. Yet during the stationary period
of 1907 diarrhoea was steadily increasing, while during the stationary period of 1908 diarrhoea was
steadily declining. In order to explain this phenomenon on the fly hypothesis, it becomes, therefore,
as has been already surmised, necessary to assume that in determining variations in amount of diarrhoea,
importance attaches not so much to the number of carriers of diarrhoea organisms, as to the extent of
distribution of the organism itself. If this be admitted, however, it is (on the assumption that flies cause
diarrhoea) clear that precise correspondence of the carrier curve, i.e. the fly curve, and the diarrhoea
curve cannot be expected to occur.
It may further be pointed out that, just as the various flowers, vegetables and fruits have their
special times of appearing as the year goes by, so the flies and the diarrhoea may be expected to have
a particular seasonal distribution. Thus, to take the case of fruits—strawberries, cherries, currants,
raspberries, greengages, plums, pears, damsons, and blackberries, each in turn come into season throughout
the summer months ; the strawberries beginning in June and the blackberries, damsons and pears
not having quite disappeared in October. If the figures of the Market Returns," or those of the
Board of Trade Journal" giving the number of hundredweights of imported fruits, be used to plot out
curves, it will be found that those appertaining, say, to currants and cherries, or those of plums and
pears (see left-hand Inset in Diagram I.), show as close correspondence as the curves for diarrhoea and
flies. Of course no one would infer that the cherries were caused by the currants, or the pears by the
plums. The closeness of the correspondence in these instances therefore suggests caution as regards
deducing causal relationship of flies to diarrhoea.
In last years report reference was made to the belief, formerly widely entertained, that prevalence
of diarrhoea occurred during the period in which the earth temperature, at a depth of four feet below
the surface, exceeded a particular height. It has been suggested, however, that this correspondence
is, after all, a mere coincidence. Moreover, in 1907, the diarrhoea, judging by the four feet earth temperature
curve, was several weeks behind time, and examination of the London figures for other years
shows that the relation between diarrhoea and the temperature of the earth, four feet below the surface,
is by no means so precise as it has been generally held to be. It is possible that the doubt, now widely
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