64
PERIODICITY IN ORGANIC LIFE.
follow their usual habits, gradually and persistently moving onwards
in one direction, never returning to the place from whence they
started. What does all this point to ? Simply this—that the species
has arrived at its period of greatest abundance, and that Nature is
about to put a check on its increase, this being one of the ways in
which the period of scarcity is produced in this species, just as the
same migratory habits produce a similar effect in the locust. It
would appear that it is not the want of food which brings the migra-
tion of the Lemming about. The only thing we know is that they
appear in greater numbers than usual, and then are noticed to be
moving in one direction, the horde being increased by the numbers
which join it from every district through which the band of
Lemmings passes. If the gathering is large, of course, like the
swarms of locusts, the animals must do much damage by destroying
all the food of the district in their line of march.
I think from the illustrations I have given, that you all will be
able to understand what I mean by the term "periodicity." I think
that the examples given prove that when abnormal numbers of any
animal or plant occur, we may expect to see this abundance followed
by a period of scarcity, and that this periodic range of maximum and
minimum numbers is the result of a natural law controlling every
organized being, and is an effect not necessarily brought about by
man's agency, nor by climatic changes, nor variation in the supplies
of food.
Applying the same law to diseases, we must not suppose because
the Black-death and other plagues of the Middle Ages have not
appeared for many years that they are necessarily extinct. They
probably exist, but at their minimum period; and we may find at any
moment that one of these dreadful scourges has started on its career
of destruction. It may be that the increase of these diseases will be
the mode by which the enormous increase of the human race will
be checked, an increase which, at a not very distant period, threatens
to so overcrowd the world as to make it a serious problem how all
will be fed. If there is any truth in the idea that there is a law of
periodicity we may make ourselves very easy, feeling sure that in the
armoury of Nature there is some beneficent law which will prevent
the dire results dreaded by many.