| → In 1861 1869 1872 |
| It can, in 1859 1860 1866 |
|
| → it may be observed that the amount of external difference between two species is no sure guide to their degree 1869 1872 |
| be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree 1859 1860 1866 |
| we must remember how ignorant we are regarding the precise cause 1861 |
|
| → mutual sterility, so that similar differences in the case of varieties would be no sure guide. 1869 1872 |
| sterility when crossed; and we may apply the same rule to domestic varieties. 1859 1860 1866 |
| sterility, both when species are crossed and when species are removed from their natural conditions. 1861 |
|
| ↑ 4 blocks not present in 1859 1860 1866 1869 1872; present in 1861 |
| On this latter head I have not had space to adduce the many remarkable facts which could have been given; with respect to sterility from crossing, reflect on the difference in the result of reciprocal crosses,— reflect on the singular cases in which a plant can be more easily fertilised by foreign pollen than by its own.
When we think over such cases, and on that of the differently coloured varieties of Verbascum presently to be given, we must feel how ignorant we are, and how little likely it is that we should understand why certain forms are fertile and other forms are sterile when crossed.
It can, in the second place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties.
In the third place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same domestic conditions of life.
|
|
| → circumstances should both induce and eliminate the same 1869 |
| conditions long-continued should likewise induce this 1872 |
|
|
other most closely, are utterly sterile when intercrossed. Several considerations, however, render
fertility of domestic varieties less
→In
the first
→it may be observed that the amount of external difference between two species is no sure guide to their degree
of
→mutual sterility, so that similar differences in the case of varieties would be no sure guide.
↑
It is
certain that with species the cause lies exclusively in differences in their sexual constitution. Now the
to which domesticated animals and cultivated plants have been subjected, have had so little tendency towards modifying the reproductive system in a manner leading to mutual sterility, that we have good grounds for admitting the directly opposite doctrine of Pallas, namely, that such conditions generally eliminate this tendency; so that the domesticated descendants of species, which in their natural state
have been in some degree sterile when crossed, become perfectly fertile together. With plants, so far is cultivation from giving a tendency towards sterility between distinct species, that in several well-authenticated cases already alluded to, certain plants have been affected in an opposite manner, for they have become self-impotent, whilst still retaining the capacity of
and being fertilised by, other species. If the Pallasian doctrine of the elimination of sterility through long-continued domestication be admitted, and it can hardly be rejected, it becomes in the highest degree improbable that similar
→circumstances should both induce and eliminate the same
tendency; though in certain cases, with species having a peculiar constitution, sterility might occasionally be thus
Thus, as I believe, we can understand why with domesticated animals varieties have not been produced which are mutually sterile; and why with plants only
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