their
respective genera, we may suppose that, when they first reached South
Africa they were neglected by the insects of the country; but, being
both remotely like orchids in form of flower, those varieties that
approached nearest to the familiar species of the country were visited
by insects and cross-fertilised, and thus a closer resemblance would at
length be brought about. Another case of close general resemblance, is
that of our common white dead-nettle (Lamium album) to the
stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this
is a case of true mimicry, the dead-nettle being benefited by being
mistaken by grazing animals for the stinging-nettle.[138]

_Colours of Fruits._

It is when we come to the essential parts of plants on which their
perpetuation and distribution depends, that we find colour largely
utilised for a distinct purpose in flowers and fruits. In the former we
find attractive colours and guiding marks to secure cross-fertilisation
by insects; in the latter attractive or protective coloration, the first
to attract birds or other animals when the fruits are intended to be
eaten, the second to enable them to escape being eaten when it would be
injurious to the species. The colour phenomena of fruits being much the
most simple will be considered first.

The perpetuation and therefore the very existence of each species of
flowering plant depend upon its seeds being preserved from destruction
and more or less effectually dispersed over a considerable area. The
dispersal is effected either mechanically or by the agency of animals.
Mechanical dispersal is chiefly by means of air-currents, and large
numbers of seeds are specially adapted to be so carried, either by being
clothed with down or pappus, as in the well-known thistle and dandelion
seeds; by having wings or other appendages, as in the sycamore, birch,
and many other trees; by being thrown to a considerable distance by the
splitting of the seed-vessel, and by many other curious devices.[139]
Very large numbers of seeds, however, are so small and light that they
can be carried enormous distances by gales of wind, more especially as
most of this kind are flattened or curved, so as to expose a large
surface in proportion to their weight. Those which are carried by
animals have their surfaces, or that of the seed-vessel, armed with
minute hooks, or some prickly covering which attaches itself to the hair
of mammalia or the feathers