In animals, it is the eggs or the very young that suffer most from their
various enemies; in plants, the tender seedlings when they first appear
above the ground. To illustrate this latter point Mr. Darwin cleared and
dug a piece of ground three feet long and two feet wide, and then marked
all the seedlings of weeds and other plants which came up, noting what
became of them. The total number was 357, and out of these no less than
295 were destroyed by slugs and insects. The direct strife of plant with
plant is almost equally fatal when the stronger are allowed to smother
the weaker. When turf is mown or closely browsed by animals, a number of
strong and weak plants live together, because none are allowed to grow
much beyond the rest; but Mr. Darwin found that when the plants which
compose such turf are allowed to grow up freely, the stronger kill the
weaker. In a plot of turf three feet by four, twenty distinct species of
plants were found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up to their full
size.[5]

But besides having to protect themselves against competing plants and
against destructive animals, there is a yet deadlier enemy in the
forces of inorganic nature. Each species can sustain a certain amount of
heat and cold, each requires a certain amount of moisture at the right
season, each wants a proper amount of light or of direct sunshine, each
needs certain elements in the soil; the failure of a due proportion in
these inorganic conditions causes weakness, and thus leads to speedy
death. The struggle for existence in plants is, therefore, threefold in
character and infinite in complexity, and the result is seen in their
curiously irregular distribution over the face of the earth. Not only
has each country its distinct plants, but every valley, every hillside,
almost every hedgerow, has a different set of plants from its adjacent
valley, hillside, or hedgerow--if not always different in the actual