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See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1861
1866
1869

→ Origin and Causes 1866 1869 1872
Causes 1859 1860 1861
↑ 1 blocks not present in 1866 1869 1872; present in 1859 1860 1861
We may now look a little closer at the probable causes of the sterility of first crosses and of hybrids.
→ of first crosses and of hybrids might 1869 1872
might 1866
→ slowly acquired through the 1869 1872
acquired through 1866
→ of 1869 1872
slowly acting on a 1866
→ OMIT 1872
at first spontaneously appeared, 1866
spontaneously appeared, 1869
→ spontaneously appeared in 1872
in 1866 1869
→ those of another 1872
another 1866 1869
→ species inhabiting distinct 1872
distinct 1866 1869
→ sterile when crossed; 1872
inhabited by groups of species and by single species which when brought together and crossed are found to be more or less sterile; 1866
inhabited by groups of species and by single spe- cies which when brought together and crossed are found to be more or less sterile; 1869
→ form should have been rendered utterly impotent on a 1872
a 1866
form should be rendered utterly impotent on a 1869
→ form; for this peculiar state of the reproductive system could hardly have been advantageous to either species. 1872
form. 1866
form; for this peculiar state of the reproductive system could not possibly be advantageous to either species. 1869
→ in rendering species mutually sterile, the greatest 1872
one great 1866
in rendering species mutually sterile, one great 1869
→ OMIT 1872
on the principle above explained, 1866 1869
→ parent form or 1872
parent-form 1866 1869
→ new species in process of formation. 1869 1872
newly-forming variety. 1866
systematic affinity, by which
every
term every
kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem to
me to
....
indicate that the greater or lesser difficulty of either grafting or crossing
together
....
various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare.
→ Origin and Causes
of
of
the
the
Sterility
Sterility
of
of
first
first
Crosses
Crosses
and
and
of
of
Hybrids .—
Hybrids .—
Hybrids .
Hybrids .
↑ At one time it appeared to me probable, as it has to others, that
this
the
sterility →of first crosses and of hybrids might have been →slowly acquired through the natural selection →of slightly lessened
degree
degrees
of fertility,
which
which,
→OMIT like any other variation, →spontaneously appeared in certain individuals of one variety when crossed with →those of another variety. For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place, it may be remarked that →species inhabiting distinct regions are often →sterile when crossed; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be
argued
argued,
with truth,
....
that, if a species
were
was
rendered sterile with some one compatriot, sterility with other species would
probably
....
follow as a necessary contingency. In the second place, it is
as
almost as
much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one →form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first →form; for this peculiar state of the reproductive system could hardly have been advantageous to either species.
But in
In
considering the probability of natural selection having come into action, →in rendering species mutually sterile, the greatest difficulty will be found to lie in the existence of many graduated steps from
very
....
slightly lessened fertility to
utter and
....
absolute sterility. It may be
admitted,
admitted
→OMIT that it would profit an incipient
species
species,
if it were rendered in some slight degree sterile when crossed with its →parent form or
or
....
with some other variety; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the →new species in process of formation. But he who will take the trouble to reflect on the steps by which this first degree of