hich never intercross, uniformity of character can be retained
amongst them, as long as their conditions of life remain the same, only
through the principle of inheritance, and through natural selection
destroying any which depart from the proper type; but if their conditions
of life change and they undergo modification, uniformity of character can
be given to their modified offspring, solely by natural selection
preserving the same favourable variations.

Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic
and inorganic conditions of life will generally be in a great degree
uniform; so that natural selection will tend to modify all the individuals
of a varying species throughout the area in the same manner in relation to
the same conditions. Intercrosses, also, with the individuals of the same
species, which otherwise would have inhabited the surrounding and
differently circumstanced districts, will be prevented. But isolation
probably acts more efficiently in checking the immigration of better
adapted organisms, after any physical change, such as of climate or
elevation of the land, &c.; and thus new places in the natural economy of
the country are left open for the old inhabitants to struggle for, and
become adapted to, through {105} modifications in their structure and
constitution. Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly improved; and
this may sometimes be of importance in the production of new species. 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 individuals supported on it will necessarily be very small;
and fewness of individuals will greatly retard the production of new
species through natural selection, by decreasing the chance of the
appearance of favourable variations.

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 total number
of the species inhabiting it, will be found to be 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. Hence an oceanic island at first sight seems to have been highly
favourable for the produ