Origin of Species Variorum

→ their own pollen. 1859 1860 1861 1866

pollen from the same plant. 1869 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.

→ might 1866

of first crosses and of hybrids might 1869 1872

→ acquired through 1866

slowly acquired through the 1869 1872

→ slowly acting on a 1866

of 1869 1872

→ at first spontaneously appeared, 1866

spontaneously appeared, 1869

OMIT 1872

→ in 1866 1869

spontaneously appeared in 1872

→ another 1866 1869

those of another 1872

the extraordinary

cases

case

of Hippeastrum,

Lobelia,

Passiflora,

&c., which

seeded

seed

much more freely when fertilised with the pollen of

a distinct

distinct

species, than when

self-fertilised

fertilised

with →their own pollen.

We thus see,

that

that,

although there is a clear and

great

fundamental

difference between the mere adhesion of grafted stocks, and the union of the male and female elements in the 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~~

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

the

Sterility

first

Crosses

and

Hybrids .—

Hybrids .

↑

At one time it appeared to me probable, as it has to others, that

the

this

sterility →might have been →acquired through natural selection →slowly acting on a slightly lessened

degrees

degree

of fertility,

which,

which

→at first spontaneously appeared, like any other variation, →in certain individuals of one variety when crossed with →another variety. For