54 PERIODICITY IN ORGANIC LIFE.
that bad sanitary surroundings encouraged and made this disease
epidemic. If so, why did it not spread over the world ages before ?
There was the same neglectfulness of sanitary precautions, and all
unsanitary dwellings and surroundings had not disappeared when the
extreme virulence of the disorder had abated. The same may be
said of the disease I propose next to mention, and, in fact, of any
epidemic.
The phenomena attending the appearance and disappearance of
other epidemics might easily be given, and the horrible disease,
Cholera, now, I fear, approaching our shores, affords another
excellent example of periodicity in abundance and decline. For
ages Cholera existed in India, but it did not put on the virulent
epidemic form which overran the world until about 1830. Then,
after a great increase for several years in India, an approaching
prosperity as I call it, started on its progress of destruction through-
out the world, killing all, or the greater part, of those it attacked,
and after a time following the usual course, diminishing in virulence,
until finally it disappeared from Europe. But it still went on in its
weaker form in India, until its period of abundance gradually
approached, and it then again spread over the world.
These two diseases are as good examples of what I have called
"Periodicity in Organic Life" as could possibly be wished for.
In plant life generally, the same periodical abundance of species
may be noticed. I do not wish any one to infer that there is any
regularity in the intervals between the periods ; there is nothing of
the kind. The times of prosperity in the species are most uncertain,
and cannot always be explained, as I have said before, by either hot
seasons, wet years, or any apparent surrounding causes.
The first illustration I will take shall be from the lower plants, the
Fungi. Some of us remember, and all must have heard of, the first
appearance of the Potato Disease. In 1848, or thereabout, a disease
appeared in the potatoes in many parts of the kingdom, arousing some
interest from its effects on, and the destruction of, the crops which it
attacked. The next year, over large areas, the whole potato crop was
destroyed. In Ireland, where the people depended almost entirely
on the potato for their food, a famine was caused; and throughout
Europe much distress, disease, and suffering. This was the time
I have spoken of as the "period of prosperity" in the species. In
the following years much damage also resulted, but the destruction
has been gradually diminishing since that time, and, let us hope, the