Origin of Species Variorum

→ sterility, both in the parents and in the offspring, is confined to differences in their reproductive systems. 1866

the sterility is confined to dif- ferences in the sexual elements. 1869

the sterility of crossed species is confined to differences in their sexual elements. 1872

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

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

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

→ 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

OMIT 1869

↑ 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.

→ generally 1866

in most cases 1869

→ of some kind 1866

OMIT 1869

→ attempts to express 1866

includes resemblances of 1869

→ as is so often stated, universally 1866

quite universally, 1859

quite uni- versally, 1860

as is so often falsely stated, universally 1861

as is so often stated, invariably 1869 1872

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

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

→ forgotten 1866

especially kept in mind, 1869 1872

of →sterility, both in the parents and in the offspring, is confined to differences in their reproductive systems. But

why,

why

in →numerous species, descended from a common parent-form, the reproductive system should in all have become more or less modified, leading to their mutual infertility, we do not

know.

know;

know

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

It is not surprising that the

~~degree of~~

degree of

difficulty in

crossing any

uniting

two species, and the

sterility

degree

~~sterility of~~

sterility of

their hybrid-offspring, should →generally correspond, even if due to distinct causes; for both depend on the amount of difference →of some kind 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~~

which are

subjected to experiment; for systematic affinity →attempts to express 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, universally fertile. Nor is this

almost universal

nearly general

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 →not of differences in the reproductive system.

Nor

should

also

be →forgotten that long-continued domestication

~~apparently~~

apparently

tends to eliminate sterility, and is therefore little likely to induce this