Intercrossing plays a very important part in nature by keeping the
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, as already stated, we have reason
to believe that occasional intercrosses take place with all animals and
plants. Even if these take place only at long intervals of time, 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 crosses, even at rare intervals, will be great. With respect
to organic beings extremely low in the scale, which do not propagate
sexually, nor conjugate, and which cannot possibly intercross, uniformity
of character can be retained by them under the same conditions of life,
only through the principle of inheritance, and through natural selection
which will destroy any individuals departing from the proper type. If the
conditions of life change and the form undergoes modification, uniformity
of character can be given to the modified offspring, solely by natural
selection preserving similar favourable variations.
Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner.
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