| → in their present state 1869 |
| OMIT 1872 |
|
| → and fertility have 1869 1872 |
| have 1866 |
|
| → not belonging to a social community, if 1869 1872 |
| if 1866 |
|
| → or indirectly give any advantage to the 1869 1872 |
| to its nearest relatives or to any 1866 |
|
| ↑ 3 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| But it would be superfluous to discuss this question in detail; for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle, quite independent of natural selection.
Both Gärtner and Kölreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of certain other species, for the germen swells.
It is here manifestly impossible to select the more sterile individuals, which have already ceased to yield seeds; so that this acme of sterility, when the germen alone is affected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all cases.
|
|
| → OMIT 1869 |
| same or of other varieties to the 1866 |
|
| → a variety, when deposited on the stigma of the same 1869 |
| any one 1866 |
|
|
through natural
the case of
species
→in their present state
when crossed, produce few and sterile offspring; now, what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility, and which thus approached by one small step towards absolute sterility? Yet an advance of this kind, if the theory of natural selection be brought to bear, must have incessantly occurred with many species, for a multitude are mutually quite barren. With sterile neuter insects we have reason to believe that modifications in their structure
→and fertility have
been slowly accumulated by natural selection, from an advantage having been thus indirectly given to the community to which they belonged over other communities of the same species; but an individual
→not belonging to a social community, if
rendered slightly sterile when crossed with some other variety, would not thus
any advantage
→or indirectly give any advantage to the
other individuals of the same variety, thus leading to their preservation. From these considerations I infer, as far as animals are concerned, that the various degrees of lessened fertility which occur with species when crossed cannot have been slowly accumulated by means of natural selection. ↑ |
|
| With plants, it is possible that the case may be
With
many kinds, insects constantly
pollen from neighbouring plants
the
→OMIT
of each flower; and with some
is effected by the wind.
if the pollen of
→a variety, when deposited on the stigma of the same
should become by spontaneous variation in ever so slight
|
