Origin of Species Variorum

→ though they rarely 1866 1869 1872

generally 1859 1860 1861

→ external characters, yet generally differ in fertility 1866 1869 1872

fertility 1859 1860 1861

→ the fertility of a hybrid may be 1872

fertility in the hybrid is 1859 1860 1861 1866 1869

→ cross; that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent; and lastly, 1872

cross. 1859 1860 1861 1866 1869

→ making a first cross between any two species 1872

hybrids 1859 1860 1861 1866 1869

→ always governed by their systematic affinity or degree of resemblance to each other. 1872

related to the degree in which they resemble in external appearance either parent. 1859 1860 1861 1866 1869

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

And lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other.

→ the difference in the result of reciprocal 1861 1866 1869 1872

reciprocal 1859 1860

facility in making reciprocal crosses is extremely common in a lesser degree. He has observed it even between

forms so

....

closely related

(as

forms (as

Matthiola annua and glabra)

that

which

many botanists rank

them

....

only as varieties. It is also a remarkable fact, that hybrids raised from reciprocal crosses, though of course compounded of the very same two species, the one species having first been used as the father and then as the mother, →though they rarely differ in →external characters, yet generally differ in fertility in a small, and occasionally in a high degree.

Several other singular rules could be given from Gärtner: for instance, some species have a remarkable power of crossing with other species; other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring; but these two powers do not at all necessarily go together. There are certain hybrids

which

which,

instead of having, as is usual, an intermediate character between their two parents, always closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are with rare exceptions extremely sterile. So again amongst hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility. These facts show how completely →the fertility of a hybrid may be independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero to perfect fertility, or even to fertility under certain conditions in

excess.

excess;

That

that

their fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately

variable.

variable;

That

that

it is by no means always the same in degree in the first cross and in the hybrids produced from this →cross; that the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent; and lastly,

That

that

the

fertility

facility

of →making a first cross between any two species is not →always governed by their systematic affinity or degree of resemblance to each other. ↑ This latter statement is clearly proved by →the difference in the result of reciprocal crosses between the same two species,

for

for,

according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an