PERIODICITY IN ORGANIC LIFE.
53
numbers; when it would be an illustration of the phenomenon of
periodicity, to which I am now calling attention.
Periodicity in abundance is not confined to any class in organic
life ; it occurs in the highest as well as in the lowest, and I propose
to illustrate my meaning by taking examples first from the lowest
forms of life and afterwards from the higher.
Epidemic diseases are, in all classes of organic life, due to a
germ; this theory is, I think, now universally acknowledged. The
disease germ, then, shall be our first example, and as there is one
disease which will enable me to make my ideas on this subject of
periodicity more plain and clear perhaps than any other, I will take
that first.
For many years, ages almost, there existed in the neighbourhood
of Boulogne a disease which was known to the French physicians by
the name of "Diphtherite," and to many of our countrymen who
visited that district as "Boulogne Sore-throat." Possibly there
may have been cases in this country also, and perhaps elsewhere in
Europe, but if so, the cases were so few in number that they attracted
little or no attention. Here, then, we have this species of germ at
its rare or almost extinct period; it existed, but it was not prolific.
In 1852 or thereabout cases began, though rarely, to occur in various
parts of Britain, but they did not spread, and never became epidemic.
Gradually, however, more and more was heard of them—the germ
was approaching its period of activity — and at last, in 1858,
Diphtheria, for this was the disease, overran not only Europe but
the whole world.
This was the period of the abundance of the Diphtheria germ,
but, happily for mankind, its period of rarity seems approaching.
It is not so virulent (although even now bad enough), nor is it so
spreading as in its period of prolifieness and abundance.
This disease also enables me to illustrate the fact that man's
proceedings neither caused the sudden increase in the number of
attacks, nor did he in any way diminish them, excepting that by
care, and the adoption of the teachings of sanitary science, he, to a
certain extent, protected some portions of the population, and in
individual cases placed the affected in a better position to resist the
ravages of the disease. But he certainly did not bring about the
diminished virulence which was so apparent after the disease had
existed for some time, any more than he can be said to have made
the disease epidemic in the first instance. It will no doubt be said