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1859
1860
1861
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1869
1872

Compare with:
1860
1861
1866
1869
1872

the 1859 1860 1861 1866 1869
some of the 1872

but that the 1859 1860 1861 1866 1869
The 1872

decreased. 1859 1860 1861
decreased greatly and suddenly. 1866 1869
decreases greatly and suddenly. 1872

1 blocks not present in 1859 1860 1861 1872; present in 1866 1869
With respect to this decrease, it may first be noticed that, when any deviation in structure or constitution is common to both parents, this is often transmitted in an augmented degree to the offspring; and both sexual elements in hybrid plants are already affected in some degree.

I do not doubt that 1859 1860 1861
With respect to this decrease, it may first be noticed that when any deviation in structure or constitution is common to both parents, 1872

usually the case, and that the fertility often suddenly decreases in the first few generations. 1859 1860 1861
often transmitted in an augmented degree to the offspring; and both sexual elements in hybrid plants are already affected in some degree. 1872

in all these experiments the 1859 1860 1861
in nearly all these cases, that the 1866 1869
their 1872

by 1859 1860 1861 1866 1869
in nearly all these cases by 1872

large a body of 1859 1860 1861 1866 1869
many experiments and collected so many 1872

It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so
in- sensibly,
insensibly,
and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this can be required than that the two most experienced observers who have ever lived,
namely
namely,
Kölreuter and Gärtner,
should have
should have
arrived at diametrically opposite conclusions in regard to the very same
forms.
species.
It is also most instructive to
compare—but
compare—
but
but
I have not space here to enter on
details—the
details—
the
the
evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same
observer
author,
from experiments made during different years. It can thus be shown that neither sterility nor fertility affords any
certain
clear
distinction between species and
varieties.
varieties;
but that the evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though Gärtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never
increases,
increased,
but generally
greatly
greatly
decreased. I do not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few generations.
But
Nevertheless
I believe
that
that
in all these experiments the fertility has been diminished by an independent cause, namely,
by too
from
close interbreeding. I have
made
collected
so large a body of facts, showing