cross only
occasionally, and likewise with animals which unite for each birth, but
which wander little and can increase at a rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body and afterward spread, so that the individuals
of the new variety would chiefly cross together. On this principle
nurserymen always prefer saving seed from a large body of plants, as the
chance of intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always
eliminate the effects of natural selection; for I can bring forward
a considerable body of facts showing that within the same area two
varieties of the same animal may long remain distinct, from haunting
different stations, from breeding at slightly different seasons, or from
the individuals of each variety preferring to pair together.

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
almos