on in every part of the world, only it is
very difficult for us to trace out the complex reactions that are
everywhere occurring. The general impression of the ordinary observer
seems to be that wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and surroundings,
and therefore having no difficulty in maintaining itself. Before showing
that this view is, everywhere and always, demonstrably untrue, we will
consider one other case of the complex relations of distinct organisms
adduced by Mr. Darwin, and often quoted for its striking and almost
eccentric character. It is now well known that many flowers require to
be fertilised by insects in order to produce seed, and this
fertilisation can, in some cases, only be effected by one particular
species of insect to which the flower has become specially adapted. Two
of our common plants, the wild heart's-ease (Viola tricolor) and the red
clover (Trifolium pratense), are thus fertilised by humble-bees almost
exclusively, and if these insects are prevented from visiting the
flowers, they produce either no seed at all or exceedingly few. Now it
is known that field-mice destroy the combs and nests of humble-bees, and
Colonel Newman, who has paid great attention to these insects, believes
that more than two-thirds of all the humble-bees' nests in England are
thus destroyed. But the number of mice depends a good deal on the number
of cats; and the same observer says that near villages and towns he has
found the nests of humble-bees more numerous than elsewhere, which he
attributes to the number of cats that destroy the mice. Hence it
follows, that the abundance of red clover and wild heart's-ease in a
district will depend on a good supply of cats to kill the mice, which
would otherwise destroy and keep down the humble-bees and prevent them
from fertilising the flowers. A chain of connection has thus been found
between such totally distinct organisms as flesh-eating mammalia and
sweet-smelling flowers, the abundance or scarcity of the one closely
corresponding to that of the other!

The following account of the struggle between trees in the forests of
Denmark, from the researches of M. Hansten-Blangsted, strikingly
illustrates our subject.[8] The chief combatants are the beech and the
birch, the former being everywhere successful in its invasions. Forests
composed wholly of birch are now only found in sterile, sandy tracts;
everywh