t uniform; so that natural selection will tend to modify all
the varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts,
will also be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service
rendered by isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is likewise great in preventing,
after any physical change in the conditions, such as of climate,
elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district
will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be
improved at a slow rate; and this may sometimes be of much importance.
If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical
conditions, the total number of the inhabitants will be small; and this
will retard the production of new species through natural selection, by
decreasing the chances of favourable variations arising.

The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives
a better chance of beneficial variations arising and of their being
selected, accumulated, and fixed. It likewise tends to increase the
direct action of the physical conditions of life, in relation to the
constitution of each organism.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of
the species inhabiting it is small, as we shall see in our chapter on
Geographical Distribution; yet of these species a very large proportion
are endemic,--that is, have been produced there and nowhere else in the
world. Hence an oceanic island at first