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See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1861
1866
1872

→ 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
→ spontaneously appeared, 1869
at first spontaneously appeared, 1866
OMIT 1872
→ in 1866 1869
spontaneously appeared in 1872
→ another 1866 1869
those of another 1872
→ distinct 1866 1869
species inhabiting distinct 1872
→ 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
inhabited by groups of species and by single species which when brought together and crossed are found to be more or less sterile; 1866
sterile when crossed; 1872
act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. And as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first
crosses,
crosses
are incidental on unknown
differences,
differences
chiefly
....
in their reproductive systems. These
differences
differences,
in both cases, follow to a certain extent, as might have been expected, systematic affinity, by which
term every
every
kind of resemblance and dissimilarity between organic beings is attempted to be expressed. The facts by no means seem to
me 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
→spontaneously appeared, like any other variation, →in certain individuals of one variety when crossed with →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 →distinct regions are often →inhabited by groups of species and by single spe- cies which when brought together and crossed are found to be more or less sterile;