Seedlings, also, are
destroyed in vast numbers by various enemies; for instance, on a piece
of ground three feet long and two wide, dug and cleared, and where
there could be no choking from other plants, I marked all the
seedlings of our native weeds as they came up, and out of the 357 no
less than 295 were destroyed, chiefly by slugs and insects. If turf
which has long been mown, and the case would be the same with turf
closely browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully grown, plants:
thus out of twenty species growing on a little plot of turf (three
feet by four) nine species perished from the other species being
allowed to grow up freely.
The amount of food for each species of course gives the extreme limit
to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to be
little doubt that the stock of partridges, grouse, and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England, and,
at the same time, if no vermin were destroyed, there would, in all
probability, be less game than at present, although hundreds of
thousands of game animals are now annually killed. On the other hand,
in some cases, as with the elephant and rhinoceros, none are destroyed
by beasts of prey: even the tiger in India most rarely dares to attack
a young elephant protected by its dam.
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