Origin of Species Variorum

→ their inarching 1869 1872

them becoming inarched 1859 1860 1861 1866

→ and of their hybrid progeny has not, as far as we can judge, been acquired through natural selection. 1869

between pure species, which have their reproductive systems perfect, seems to depend on several circumstances; in some cases largely on the early death of the embryo. 1859 1860 1861

and of their hybrid progeny has not, as far as we can judge, been increased through natural selection so as to attain that high degree which is universal with species when rendered widely distinct. 1866

and of their hybrid progeny has not been acquired through natural selection. In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo. 1872

→ In the case of 1869

With 1866

→ it 1869

between pure species, in which the reproductive system is in a perfect condition, the sterility 1866

→ instances in chief part 1869

cases largely 1866

↑ 2 blocks not present in 1859 1860 1861 1866 1869; present in 1872

In the case of hybrids, it apparently depends on their whole organisation having been disturbed by being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions of life. He who will explain these latter cases will be able to explain the sterility of hybrids.

→ In the case of hybrids, it perhaps depends on their 1869

The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their 1859 1860 1861

With hybrids, in which the reproductive system is in an imperfect condition, and in which this system as well as the 1866

→ having been disturbed 1869

disturbed 1859 1860 1861

has been disturbed 1866

→ forms; the sterility being 1869

species, seems 1859 1860 1861

forms, the sterility apparently is 1866

→ first, the crossing of forms only slightly differentiated favours 1869

the crossing of forms only slightly different is favourable to 1859 1860 1861

the crossing of forms only slightly differentiated favours 1866

firstly, slight changes in the conditions of life add to 1872

→ their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the 1869

their offspring; and that slight changes in the 1859 1860 1861 1866

all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different 1872

→ apparently add to the 1866 1869

are apparently favourable to the 1859 1860 1861

or which have varied, favours the size, 1872

→ all organic beings, whilst greater changes are often injurious. 1869

all organic beings. 1859 1860 1861 1866

their offspring. 1872

→ in all cases connects 1869 1872

connecting in all cases 1866

→ clearly leads to the conclusion that 1869 1872

drive us to conclude that in all cases 1866

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

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

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

of one species to unite with

another,

another

is incidental on unknown differences in their reproductive systems. There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent

them

their

crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent →their inarching in our forests.

The sterility of first crosses →and of their hybrid progeny has not, as far as we can judge, been acquired through natural selection. →In the case of first crosses →it seems to depend on several circumstances; in some →instances in chief part on the early death of the

embryo,

but

this

apparently

depends

some

imperfection

the

original

act

impregnation.

↑ →In the case of hybrids, it perhaps depends on their whole organisation →having been disturbed by being compounded

from

two distinct →forms; the sterility being closely allied to that

sterility

....

which so frequently affects pure species, when

their

exposed to

natural

unnatural

conditions of

life

have

been

disturbed.

This view is

strongly supported

supported

by a parallelism of another

kind;—

kind:—

kind:

namely,

that

that,

→first, the crossing of forms only slightly differentiated favours the vigour and fertility of →their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the conditions of life →apparently add to the

vigour,

vigour

and fertility of →all organic beings, whilst greater changes are often injurious.

The

But the

facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny,

perhaps render

render

it probable that

there is

....

some unknown bond →in all cases connects the degree of fertility of first unions with that of their offspring. The consideration of these facts on dimorphism, as well as

of the

the

results of reciprocal crosses, →clearly leads to the conclusion that the primary cause of →the sterility is confined to dif- ferences in the sexual elements.