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See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1866
1869

→ OMIT 1872
in their present state 1869
→ 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
↑ 1 blocks not present in 1859 1860 1861 1872; present in 1866 1869
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.
→ OMIT 1872
between pure species, 1866 1869
sterility could be increased through natural selection to that high degree which is common with so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. After mature reflection it seems to me that this could not have been effected through natural
selection;
selection.
for
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it
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could
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not
OMIT
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have
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been
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of
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any
no
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direct
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advantage
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to
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an
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individual
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animal
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to
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breed
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poorly
badly
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with
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another
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individual
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of
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a
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different
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variety,
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and
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thus
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to
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leave
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few
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offspring;
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consequently
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such
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individuals
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could
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not
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have
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been
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preserved
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or
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selected.
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Or
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take
Take
the case of
two
any two
species
which
which,
→OMIT 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
animal,
animal
→not belonging to a social community, if rendered slightly sterile when crossed with some other variety, would not thus
indirectly give
itself gain
any advantage →or indirectly give any advantage to the other individuals of the same variety, thus leading to their preservation. ↑
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.
We will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids. In the case of first
crosses
crosses,
→OMIT the greater or less difficulty in effecting an union and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a