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Causes
1859 1860 1861 |
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Origin and Causes
1866 1869 1872 |
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| ↑ 4 blocks not present in 1859 1860 1861; present in 1866 1869 1872 |
| At one time it appeared to me probable, as it has to others, that this
sterility might
have been acquired through
natural selection slowly acting on a
slightly lessened degree
of fertility, which
at first spontaneously appeared,
like any other variation, in
certain individuals of one variety when crossed with another
variety.
For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate.
In the first place, it may be remarked that distinct
regions are often inhabited by groups of species and by single species which when brought together and crossed are found to be more or less sterile;
now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued
with truth,
that, if a species were
rendered sterile with some one compatriot, sterility with other species would probably
follow as a necessary contingency.
In the second place, it is as
much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one a
second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first form. |
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of crossing distinct species. And as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first
are incidental on unknown
in their reproductive systems. These
in both cases, follow to a certain extent, as might have been expected, systematic affinity, by which
kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem to
indicate that the greater or lesser difficulty of either grafting or crossing
various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare. |
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Causes
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| We may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. ↑
These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. Even in first crosses, the greater or lesser difficulty in effecting
union apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant having a pistil too long for the pollen-tubes to reach the ovarium. It has also been observed that when pollen of one species is placed on the stigma of a distantly allied species, though the pollen-tubes protrude, they do not penetrate the stigmatic surface. Again, the
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