Before we consider in detail the several cases given in Table 7/C, some
preliminary remarks must be made. There is the clearest evidence, as we
shall presently see, that the advantage of a cross depends wholly on the
plants differing somewhat in constitution; and that the disadvantages of
self-fertilisation depend on the two parents, which are combined in the
same hermaphrodite flower, having a closely similar constitution. A
certain amount of differentiation in the sexual elements seems
indispensable for the full fertility of the parents, and for the full
vigour of the offspring. All the individuals of the same species, even
those produced in a state of nature, differ somewhat, though often very
slightly, from one another in external characters and probably in
constitution. This obviously holds good between the varieties of the
same species, as far as external characters are concerned; and much
evidence could be advanced with respect to their generally differing
somewhat in constitution. There can hardly be a doubt that the
differences of all kinds between the individuals and varieties of the
same species depend largely, and as I believe exclusively, on their
progenitors having been subjected to different conditions; though the
conditions to which the individuals of the same species are exposed in a
state of nature often falsely appear to us the same. For instance, the
individuals growing together are necessarily exposed to the same
climate, and they seem to us at first sight to be subjected to
identically the same conditions; but this can hardly be the case, except
under the unusual contingency of each individual being surrounded by
other kinds of plants in exactly the same proportional numbers.
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