Origin of Species Variorum

→ the case of species, the sexual elements should so generally 1869

numerous species, descended from a common parent-form, the reproductive system should in all 1866

the case of distinct species, the sexual elements should so generally 1872

→ OMIT 1869

in the least; nor whether this has been effected directly, or in correlation with other structural and functional modifications. 1866

but it seems to stand in some close relation to species having been exposed for long periods of time to nearly uniform conditions of life. 1872

↑ 1 blocks not present in 1866 1869; present in 1872 1859 1860 1861

It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed.

→ in most cases 1869

generally 1866

→ OMIT 1869

of some kind 1866

→ includes resemblances of 1869

attempts to express 1866

→ as is so often stated, invariably 1869 1872

quite universally, 1859

quite uni- versally, 1860

as is so often falsely stated, universally 1861

as is so often stated, universally 1866

→ that they have not been long exposed to uniform conditions of life. 1869 1872

not of differences in the reproductive system. 1859 1860 1861 1866

→ especially kept in mind, 1869 1872

forgotten 1866

But

why

why,

in →the case of species, the sexual elements should so generally have become more or less modified, leading to their mutual infertility, we do not

know

know;

know.

→OMIT ↑

It is not surprising that the

degree of

....

difficulty in

uniting

crossing any

two species, and the

degree

sterility

sterility of

....

their hybrid-offspring, should →in most cases correspond, even if due to distinct causes; for both depend on the amount of difference →OMIT between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together—though this latter capacity evidently depends on widely different circumstances—should all run, to a certain extent, parallel with the systematic affinity of the forms

which are

....

subjected to experiment; for systematic affinity →includes resemblances of all

kinds

resemblance

between

all

species.

First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but

not

not,

→as is so often stated, invariably fertile. Nor is this

nearly general

almost universal

and perfect fertility surprising, when

it is

remembered

remember

how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under

domesti- cation

domestication

by the selection of mere external differences, and →that they have not been long exposed to uniform conditions of life.

Nor

should

also

be →especially kept in mind, that long-continued domestication

apparently

....

tends to eliminate sterility, and is therefore little likely to induce this same quality. Independently of the question of fertility, in all other respects there is the closest general resemblance between hybrids and

mongrels,—

mongrels,

in their variability, in their power of absorbing each other by repeated crosses, and in their inheritance