Origin of Species Variorum

→ 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

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.