PERIODICITY IN ORGANIC LIFE.
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migrations and the abundance of that animal. I might cite almost
any number of instances of periodicity in insects, but I will only
refer to a few more cases, taking the well-known ones.
The Black-veined White Butterfly (Aporia crataegi) used to be
common in many parts of England. But where is it now ? It is
getting rarer every year in the few localities where it is still found, and
looking as if it would soon become extinct. Can any explanation be
given of its diminution ? I know of none. The same may be said
of the Great Blue Butterfly (Lycaena arion) and many others. Again,
numerous species which the earlier Aurelians considered rare are
now common. Why is this ? I may be told we know better how
to find them, but I question if this is the true explanation. I should
rather say the species is undergoing the condition of periodic abun-
dance, quite independent of any proceedings of man. In many
species we, who have given years to the study of insects, know it is
not improved knowledge or ability that enables us to find more of
them, and I would use the same reasoning in dealing with the views
of the older Aurelians, and say that it is not greater skill, but
rather that the insect is more abundant.
There is one other insect I should like to refer to in illustration
of periodicity. In this (the Colchester) district, in the third and
fourth decades of this century, the Blister-fly (Lytta vesicatoria) was
very abundant on every ash tree ; at the present time it is rare.
Every variety of weather occurred during these years, and the
variations from hot to cold and from wet to dry, have been as
frequent since ; and therefore is it not reasonable to suppose that
meteorological conditions have had no effect ? Man's proceedings in
this case certainly had none : for the number of beetle collectors
were so few, and none of these insects were captured for commercial
purposes ; therefore some other cause must have been at work, and
what was it? It could not be natural enemies as birds, for the birds
leave them alone, neither could it be want of food, for as far as we
can see all their various articles of diet were equally common
throughout both periods.
No attempt has ever been made to explain that scarcity or
abundance of Fish, which is so marked in the gregarious species,
such as cod, herring, sprat, mackerel, etc., excepting that it is said
"It is a bad season," or that they have left the part of the coast
where they previously abounded. This change of habitat, no doubt,
explains the scarcity in certain localities, but something more than