80 THE ESSEX NATURALIST.
when Biology explains that insects are not the only humble
exploiters of mankind, and points out that practically every
invertebrate group sends representatives to swell the ranks of
Man's sworn and implacable enemies; adding that some of the
most deadly of these enemies belong to a biological borderland
regarding whose inhabitants it is not always easy to decide
whether they can be accounted either animals or plants. Biology
has to depress Human Society further, by explaining that among
the lower forms of plant life Man has almost as many foes as
among the lower forms of animal life: if the pests against which
Man has to contend be usually attributable to the agency of
members of the animal kingdom, the blights which cause Man so
much anxiety are usually due to the activity of members of
the vegetable kingdom. This knowledge brings Man little
comfort when his flocks and herds suffer from murrain and his
crops are the victims of blight. Nor is man relieved, when his
dwelling shows decay, by learning that the damage to the roof-
tree above his head is due to the activities of a boring beetle;
or that the mischief going on in the joists under his floor is caused
by a dry-rot fungus.
But Human Society regains courage when Biology explains
how important is the old observation of Man that "greater
"fleas have lesser fleas upon their backs to bite them, and these
"small fleas have other fleas." The truth embodied in this
statement forms the basis of a most effective method of com-
batting insect pests, which consists of ascertaining the identity
of the organisms that depend for their existence on the ex-
ploitation of the particular pests that exploit Man, and his
animal and plant symbiotic partners; to study the life-histories
of these pests and take steps to parasitize them. Man, fortu-
nately, is himself sufficiently parasitic to welcome this advice
and adopt it with enthusiasm.
Human Society is also greatly relieved to learn from Biology
that Man's empirical methods of selection and breeding, when
applied with modern knowledge, afford one of the surest means
of avoiding the incidence of blights: that it is possible to
include "immunity against particular blights" among the
various qualities for which new strains of staple crops may be
selected and bred. It is, however, fair to admit that while these
lower members of the vegetable kingdom that show an active