58 PERIODICITY IN ORGANIC LIFE.
further examples from the insect world, as both the above-
mentioned species are such excellent illustrations of periodicity, in
the sense I have used the term. Insects are, however, one of the
best classes of animals for working up this subject, as there are so
many species which, during their period of abundance, are so very
numerous in individuals that the changes may be the more readily
observed than if the specimens were less frequent and more difficult
to obtain.
I have no doubt that many of our entomological friends will be
able to call to mind further examples of this periodicity in abundance,
for all must have heard of a "Clouded Yellow year," a "Camberwell-
Beauty year," a "Convolvuli year," and so on,
All of these years of abundance have not been due, in the case of
many insects, to the parents being blown over from the Continent,
although it is just possible that this cause may explain the occasional
apparition of great numbers of "Clouded Yellow" and "Camberwell
Beauty" butterflies.
Occasionally it has happened, and perhaps more frequently than
hitherto noticed, that the period of abundance of a species in this
country coincides with the abundance of the species throughout the
continent of Europe. This was notably so, a few years since, with
regard to the "Gamma-moth (Plusia gamma). Now in this instance,
it could neither be the suitability of the season nor the abundance
of food, as these causes could not have produced the same effect in
the hotter regions of the South of Europe, and the cooler climate of
Scandinavia. For this mysterious something, then, I use the term
"Periodicity," not at all as an explanation, but simply as a term to
note the fact.
It is easy to say in explanation of the immense hordes of locusts
which occur in many parts of the world, that they are migrating for
the purpose of finding food. But is this an explanation of periodicity
in these insects ? How is it they are in such abundance that they
must migrate ? They are always in the districts from whence they
start on their migrations, and to which they never return. How is it
they do not find it necessary to migrate every year ? It cannot be
in consequence of meteorological causes. Nor is it a question of
food, because frequently years elapse between one period of
abundance and another, and at other times for several years in
succession swarms follow swarms. When I come to mention the
Lemming, we shall find the same difficulty in explaining the