a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several considerations, however, render the
fertility of domestic varieties less remarkable
than at first appears.
It can, in
the first 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.
↑| 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.
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| 5 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | It is almost
certain that with species the cause lies exclusively in differences in their sexual constitution.
Now the conditions
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 would
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 fertilising
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 induced.
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 a few such cases, immediately to be given, have been observed.
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| 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 1859 |
| 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 1860 |
| we must remember how ignorant we are on the precise causes of 1866 |
| 4 blocks not present in 1859 1860 1861 1869 1872; present in 1866 | | On this latter head I have not had space to give the many remarkable facts which could have been adduced.
With respect to sterility from crossing, it is good to reflect on the difference in the result of reciprocal crosses, and on those singular cases in which a plant can be more easily fertilised by pollen from a distinct species 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.
In the third place, there is good evidence for believing that a long course of domestication tends to eliminate sterility; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life.
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| 1 blocks not present in 1859 1860 1869 1872; present in 1861 1866 | | Domestic productions are less closely adapted to climate and to the other physical conditions of the countries which they inhabit than are those in a state of nature, for they can generally be removed to other and differently constituted countries with entire
impunity.
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| 5 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | The real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occurred with natural varieties
as soon as they have been modified
in a sufficient and permanent
degree to take rank as species.
We are far from precisely knowing the cause; nor is this surprising, seeing how profoundly ignorant we are in regard to the normal and abnormal action of the reproductive system.
But we can see that species, owing to their struggle for existence with numerous competitors, must
have been exposed to more uniform conditions during long periods of time,
than have been
domestic varieties; and this may well make a wide difference in the result.
For we know how commonly wild animals and plants, when taken from their natural conditions and subjected to captivity, are rendered sterile; and the reproductive functions of organic beings which have always lived and been slowly modified
under natural conditions would probably in like manner be eminently sensitive to the influence of an unnatural cross.
Domesticated productions, on the other hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in their conditions of life, and which can now generally resist with undiminished fertility repeated changes of conditions, might be expected to produce varieties, which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner.
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