ructure. Several
beautiful little aquatic plants of the genus Utricularia or bladder-wort
also inhabit bromelia leaves; and these send runners out to neighbouring
plants and thus spread themselves with great rapidity.

_The Importance of Isolation._

Isolation is no doubt an important aid to natural selection, as shown by
the fact that islands so often present a number of peculiar species; and
the same thing is seen on the two sides of a great mountain range or on
opposite coasts of a continent. The importance of isolation is twofold.
In the first place, it leads to a body of individuals of each species
being limited in their range and thus subjected to uniform conditions
for long spaces of time. Both the direct action of the environment and
the natural selection of such varieties only as are suited to the
conditions, will, therefore, be able to produce their full effect. In
the second place, the process of change will not be interfered with by
intercrossing with other individuals which are becoming adapted to
somewhat different conditions in an adjacent area. But this question of
the swamping effects of intercrossing will be considered in another
chapter.

Mr. Darwin was of opinion that, on the whole, the largeness of the area
occupied by a species was of more importance than isolation, as a factor
in the production of new species, and in this I quite agree with him. It
must, too, be remembered, that isolation will often be produced in a
continuous area whenever a species becomes modified in accordance with
varied conditions or diverging habits. For example, a wide-ranging
species may in the northern and colder part of its area become modified
in one direction, and in the southern part in another direction; and
though for a long time an intermediate form may continue to exist in the
intervening area, this will be likely soon to die out, both because its
numbers will be small, and it will be more or less pressed upon in
varying seasons by the modified varieties, each better able to endure
extremes of climate. So, when one portion of a terrestrial species takes
to a more arboreal or to a more aquatic mode of life, the change of
habit itself leads to the isolation of each portion. Again, as will be
more fully explained in a future chapter, any difference of habits or of
haunts usually leads to some modification of colour or marking, as a
means of concealment from enemies; and there is reason to believe that
this diff