PERIODICITY IN ORGANIC LIFE.
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until at last the insects become abundant, the range of abundance
varying with the species; some even in their maximum abundance never
being very common. How then can we explain these variations ?
Meteorological conditions are not sufficient, for the scarcity may be
found in periods of apparently favourable weather, or the abundance
may be apparent in a series of cold and otherwise unsuitable
seasons. Again, the period of scarcity or the reverse continues for
several seasons, all of which cannot present conditions favourable
or unfavourable for the welfare of the species. I will illustrate this
by referring to a few well-known examples. In 1859, and for several
years afterwards, the common Lackey Moth (Bombyx neustria)
was very rare in Essex. I do not think I dislodged with the beating-
stick a dozen larvae in any season for certainly four or five years.
If this scarcity had occurred only in this northern part of the county,
I should not have remarked it so much ; but as I had opportunities,
which I fully embraced, for beating in South Essex as well, it struck
me as something inexplicable. After an interval of this scarcity,
one began again to see the larvae more frequently, and at last
the nests were as apparent on every hedge as in the years of
abundance previously to 1859. Sometimes it is said that an
insect is scarce in consequence of the destruction of its food-
plant in previous seasons by the great abundance of the larvae.
This may be, and is no doubt true of some species; but to a
general feeder like the Lackey Moth, the explanation cannot apply,
as no one in this country has ever seen the whole of the common
trees and shrubs on which this insect feeds defoliated at one
time. There must be some other cause for this condition of
periodicity.
Another insect, the "Small Oak-Egger" (Eriogaster lanestris), at
the time mentioned (1859) was very abundant on every white-thorn
hedge, and in this district a nest might be found every few rods.
This period of abundance having lasted for some years, a decline
gradually set in, until now very few nests are seen, but not having
time to collect, I cannot say whether the insect is absolutely rare.
Defoliation and the consequent starvation of the larvae cannot be
the cause of the scarcity, for in this district, the white-thorn hedges
have never in late years been defoliated, except in some limited
localities, and then not by this insect; it was never in sufficient
numbers.
It may seem scarcely worth while to take up your time with