{ 243} <Note in the original.> If domestic animals are descended from several species and _become_ fertile _inter se_, then one can see they gain fertility by becoming adapted to new conditions and certainly domestic animals can withstand changes of climate without loss of fertility in an astonishing manner.
{ 244} See Suchetet, _L'Hybridité dans la Nature_, Bruxelles, 1888, p. 67. In _Var. under Dom._, Ed. ii. vol. II. hybrids between the fowl and the pheasant are mentioned. I can give no information on the other cases.
{ 245} _Origin_, Ed. i. p. 250, vi. p. 370.
{ 246} This was the position of Gärtner and of Kölreuter: see _Origin_, Ed. i. pp. 246-7, vi. pp. 367-8.
The difference in nature between species which causes the greater or lesser degree of sterility in their offspring appears, according to Herbert and Kölreuter, to be connected much less with external form, size, or structure, than with constitutional peculiarities; by which is meant their adaptation to different climates, food and situation, &c.: these peculiarities of constitution probably affect the entire frame, and no one part in particular{ 247}.
{ 247} <Note in the original.> Yet this seems introductory to the case of the heaths and crocuses above mentioned. <Herbert observed that crocus does not set seed if transplanted before pollination, but that such treatment after pollination has no sterilising effect. (_Var. under Dom._, Ed. ii. vol. II. p. 148.) On the same page is a mention of the Ericaceæ being subject to contabescence of the anthers. For _Crinum_ see _Origin_, Ed. i. p. 250: for _Rhododenron_ and _Calceolaria_ see p. 251.>
From the foregoing facts I think we must admit that there exists a perfect gradation in fertility between species which when crossed are quite fertile (as in Rhododendron, Calceolaria, &c.), and indeed in an extraordinary degree fertile (as in Crinum), and those species which never produce offspring, but which by certain effects (as the exsertion of the pollen-tube) evince their alliance. Hence, I conceive, we must give up sterility, although undoubtedly in a lesser or greater degree of very frequent occurrence, as an unfailing mark by which _species_ can be distinguished from _races_, _i.e._ from those forms which have descended from a common stock.