The process must
have been wholly comparable with that of acclimatization. Some species
must have been more adapted to northern climates, others to the soils of
western or eastern regions and so on. These qualities must have decided
the general lines of the distribution, and the species must have been
segregated according to their respective climatic qualities, and their
adaptability to soil and weather. A struggle for life and a natural
selection must have accompanied and guided the distribution, but there
is no reason to assume that the various forms were changed by this
process, and that we see them now endowed with other qualities than they
had at the outset.
Natural selection must have played, in this and in a large number of
other cases, quite the same part as the artificial method of variety
testing.
[120] Indeed it may be surmised that this has been its chief and
prominent function. Taking up again our metaphor of the sieve we can
assert that in such cases climate and soil exercise sifting action and
in this way the application of the metaphor becomes more definite. Of
course, next to the climate and soil in importance, come ecological
conditions, the vegetable and animal enemies of the plants and other
influences of the same nature.
In conclusion it is to be pointed out that this side of the problem of
natural selection and the struggle for life appears to offer the best
prospects for experimental, or for continued statistical inquiry.
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