The paramount importance of this latter circumstance is proved
by the multitude of species which flourish and multiply in a garden, but
cannot exist unless they are protected from other plants. When thus
saved from competition they are able to get whatever they require from
the soil, probably often in excess; and they are thus subjected to a
great change of conditions. It is probably in chief part owing to this
cause that all plants with rare exceptions vary after being cultivated
for some generations. The individuals which have already begun to vary
will intercross one with another by the aid of insects; and this
accounts for the extreme diversity of character which many of our long
cultivated plants exhibit. But it should be observed that the result
will be largely determined by the degree of their variability and by the
frequency of the intercrosses; for if a plant varies very little, like
most species in a state of nature, frequent intercrosses tend to give
uniformity of character to it.
I have attempted to show that with plants growing naturally in the same
district, except in the unusual case of each individual being surrounded
by exactly the same proportional numbers of other species having certain
powers of absorption, each will be subjected to slightly different
conditions. This does not apply to the individuals of the same species
when cultivated in cleared ground in the same garden.
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