individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals
which unite for each birth; but I have already attempted to show that
we have reason to believe 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
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