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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.


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