d Mr.
H. Newman, who has long attended to the habits of humble-bees, believes
that "more than two thirds of them are thus destroyed all over England."
Now the number of mice is largely dependent, as every one knows, on the
number of cats; and Mr. Newman says, "Near villages and small towns I
have found the nests of humble-bees more numerous than elsewhere, which
I attribute to the number of cats that destroy the mice." Hence it is
quite credible that the presence of a feline animal in large numbers in
a district might determine, through the intervention first of mice and
then of bees, the frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come
into play; some one check or some few being generally the most potent,
but all concurring in determining the average number or even
the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what we
call chance. But how false a view is this! Every one has heard that when
an American forest is cut down, a very different vegetation springs
up; but it has been observed that the trees now growing on the ancient
Indian mounds, in the Southern United States, display the same beautiful
diversity and proportion of kinds as in the surrounding virgin forests.
What a struggle between the several kinds of trees must here have gone
on during long centuries, each annually scattering its seeds by the
thousand; what war between insect and insect--between insects, snails,
and other animals with birds and beasts of prey--all striving to
increase, and all feeding on each other or on the trees or their seeds
and seedlings, or on the other plants which first clothed the ground and
thus checked the growth of the trees! Throw up a handful of feathers,
and all must fall to the ground according to definite laws; but how
simple is this problem compared to the action and reaction of the
innumerable plants and animals which have determined, in the course of
centuries, the proportional numbers and kinds of trees now growing on
the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in th