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entertained in this instance, may later be extended to the case of flies in relation to diarrhoea. At
any rate, in the existing state of knowledge, too great insistence must not be laid upon the close correspondence
of the two curves as being a proof that the' one phenomenon is due to the other. In the
absence of other evidence confirmatory of causal relationship, the fact of similarity of seasonal distribution
in itself suggests reference to some one common cause, rather than appeal to an hypothesis that
the one seasonal distribution is dependent upon the other. In a paper recently read before the
Epidemiological Section of the Royal Society of Medicine, Dr. Owen H. Peters advanced the hypothesis
that the seasonal rise of diseases such as diarrhoea and enteric fever stands in relation "to the cumu-
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lative effect of certain degrees of temperature extending over a certain time," that, in fact, a maturing
or ripening process similar to that which occurs in the case of the harvest, and indeed in the vegetable
world generally, is necessary. In the light of this theory, the periodicity manifested by the diarrhoea
organism (whatever its nature may be) becomes all important as regards determining the season of
diarrhoea prevalence. Dr. Peters notes, however, that, "since the relative conditions determine that the
diarrhoea curve always falls within 'the period of fly prevalence, it will be impossible by charts, etc. . .
to show that epidemic diarrhoea is propagated without the help of flies."
On the other hand, it may be argued" that, assuming the crop of diarrhoea organisms to be produced
by a gradual ripening process, extending over weeks or months, it is still conceivable, inasmuch
as the fly and diarrhœa seasons begin almost coincidently, that the fly may act as a carrier and thus
play some part in favouring transmission of infection. Thus, to take a case which it might be suggested
is a parallel one—the fertilisation of clover by humble bees. Suppose it to be surmised, in default of
direct evidence, that bees fertilise the clover, because just at the time when the clover is ready to be
fertilised the bees are busy in the fields. It might then be pointed out by some objector that the fertilisation
of the clover must necessarily be preceded by certain processes of growth, having a definite
seasonal periodicity and so on, and that the reason why the precise moment of arrival of the
time, when the clover is ready to be fertilised, coincides with the time when bees are
busily engaged in visiting flowers is that both seasonal periodicities are due to a common process of
maturation—in fact, that no causal relationship between bees and the fertilisation of the clover has
been established. So far as the hive bee is concerned, it appears that the more or less simultaneous
occurrence of bee activity and clover fertilisation does not imply cause and effect relationship ; for,
according to Darwin, the hive bee, which has a short proboscis, cannot lay the clover under contribution
for honey, and it is only the humble bee, with its longer proboscis, which visits the clover and plays a
part in bringing about the process of fertilisation.* Anyone who objected, therefore, to the argument
of simultaneity, so far as clover fertilisation and humble bees are concerned, would be silenced by direct
evidence that the humble bees fertilise the clover, and, similarly, if it should chance that evidence is
ultimately adduced showing that flies transmit diarrhoea, those who are not now satisfied as to the
claims of the fly-borne diarrhoea hypothesis will be compelled to admit its truth.
W. H. Hamer,
Medical Officer (General Purposes).
* Darwin writes, "On the Origin of Species," chapter IV., "The tubes of the corollas of the common red and
incarnate clovers (Trifolium pratense and incarnatum) do not, on a hasty glance, appear to differ in length; yet tho
hive bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited
by humble bees alone; so that whole fields of tho red clover offer in vain an abundant supply of precious nectar to
the hive bee. Thus it might be a great advantage to the hive bee to have a slightly longer or differently constructed
proboscis. On the other hand, I have found by experiment that the fertility of clover depends on be33 visiting and
moving parts of the corollas, so as to push tho pollen on to the stigmatic surface. Hence, again, if humble bees
were to become rare in any country, it might be a great advantage to the red clover to have a shorter or more
deeply divided tube to its corolla, so that the hive bee could visit its flowers. Thus, I can understand how a flower
and a bee might slowly become .... modified and adapted in tho most perfect manner to each other."