)
From the foregoing several considerations we may, I think, conclude that
in the above case of the Digitalis, and even in that of plants which
have grown for thousands of generations in the same district, as must
often have occurred with species having a much restricted range, we are
apt to over-estimate the degree to which the individuals have been
subjected to absolutely the same conditions. There is at least no
difficulty in believing that such plants have been subjected to
sufficiently distinct conditions to differentiate their sexual elements;
for we know that a plant propagated for some generations in another
garden in the same district serves as a fresh stock and has high
fertilising powers. The curious cases of plants which can fertilise and
be fertilised by any other individual of the same species, but are
altogether sterile with their own pollen, become intelligible, if the
view here propounded is correct, namely, that the individuals of the
same species growing in a state of nature near together, have not really
been subjected during several previous generations to quite the same
conditions.
Some naturalists assume that there is an innate tendency in all beings
to vary and to advance in organisation, independently of external
agencies; and they would, I presume, thus explain the slight differences
which distinguish all the individuals of the same species both in
external characters and in constitution, as well as the greater
differences in both respects between nearly allied varieties.
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