Origin of Species Variorum

→ Causes 1859 1860 1861

Origin and Causes 1866 1869 1872

↑ 4 blocks not present in 1859 1860 1861; present in 1866 1869 1872

At one time it appeared to me probable, as it has to others, that this sterility might have been acquired through natural selection slowly acting on a slightly lessened degree of fertility, which at first spontaneously appeared, like any other variation, in certain individuals of one variety when crossed with another variety. For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place, it may be remarked that distinct regions are often inhabited by groups of species and by single species which when brought together and crossed are found to be more or less sterile; now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been effected through natural selection; but it may perhaps be argued with truth, that, if a species were rendered sterile with some one compatriot, sterility with other species would probably follow as a necessary contingency. In the second place, it is as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one a second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first form.

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~~

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.

→ Causes

the

Sterility

first

Crosses

and

Hybrids .—

Hybrids .

Hybrids .—

We may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. ↑ These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. Even in first crosses, the greater or lesser difficulty in effecting

union apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule, as would be the case with a plant