ve that occasional
intercrosses take place with all animals and with all plants. Even if
these take place only at long intervals, I am convinced that the
young thus produced will gain so much in vigour and fertility over the
offspring from long-continued self-fertilisation, that they will have a
better chance of surviving and propagating their kind; and thus, in the
long run, the influence of intercrosses, even at rare intervals, will be
great. If there exist organic beings which 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, etc.; 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 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 vari