| → The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their 1859 1860 1861 |
| With hybrids, in which the reproductive system is in an imperfect condition, and in which this system as well as the 1866 |
| In the case of hybrids, it perhaps depends on their 1869 |
|
| → disturbed 1859 1860 1861 |
| has been disturbed 1866 |
| having been disturbed 1869 |
|
| → species, seems 1859 1860 1861 |
| forms, the sterility apparently is 1866 |
| forms; the sterility being 1869 |
|
| → the crossing of forms only slightly different is favourable to 1859 1860 1861 |
| the crossing of forms only slightly differentiated favours 1866 |
| first, the crossing of forms only slightly differentiated favours 1869 |
| firstly, slight changes in the conditions of life add to 1872 |
|
| → their offspring; and that slight changes in the 1859 1860 1861 1866 |
| their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the 1869 |
| all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different 1872 |
|
| → are apparently favourable to the 1859 1860 1861 |
| apparently add to the 1866 1869 |
| or which have varied, favours the size, 1872 |
|
| → all organic beings. 1859 1860 1861 1866 |
| all organic beings, whilst greater changes are often injurious. 1869 |
| their offspring. 1872 |
|
| ↑ 3 blocks not present in 1859 1860 1861; present in 1866 1869 1872 |
| But the
facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, render
it probable that there is
some unknown bond connecting in all cases
the degree of fertility of first unions with that of their offspring.
The consideration of these facts on dimorphism, as well as the
results of reciprocal crosses, drive us to conclude that in all cases
the primary cause of sterility, both in the parents and in the offspring, is confined to differences in their reproductive systems.
But why
in numerous species, descended from a common parent-form, the reproductive system should in all
have become more or less modified, leading to their mutual infertility, we do not know
in the least; nor whether this has been effected directly, or in correlation with other structural and functional modifications. |
|
| → generally correspond, though 1859 1860 1861 |
| in most cases correspond, even if 1872 |
|
| → of some kind 1859 1860 1861 |
| OMIT 1872 |
|
| → and 1859 1872 |
| from it, and 1860 1861 |
|
| → attempts to express 1859 1860 1861 |
| includes resemblances of 1872 |
|
| ↑ 2 blocks not present in 1859 1860 1861 1872; present in 1866 1869 |
| It is not surprising that the degree of
difficulty in uniting
two species, and the degree
of sterility of
their hybrid-offspring, should generally
correspond, even if due to distinct causes; for both depend on the amount of difference of some kind
between the species which are crossed.
Nor is it surprising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together—though this latter capacity evidently depends on widely different circumstances—should all run, to a certain extent, parallel with the systematic affinity of the forms which are
subjected to experiment; for systematic affinity attempts to express
all kinds
of resemblance between all species.
|
|
| → quite universally, 1859 |
| quite uni- versally, 1860 |
| as is so often falsely stated, universally 1861 |
| as is so often stated, universally 1866 |
| as is so often stated, invariably 1869 1872 |
|
