All other attempts which have been made to arrive at a definite and logical determination of the idea of organic “species” have, like the last, been utterly futile, and led to no results. Considering the nature of the case, it cannot be otherwise. The idea of species is just as truly a relative one and not absolute, as is the idea of variety, genus, family, order, class, etc. I have proved this in detail in the criticism of the idea of species in my “General Morphology” (Gen. Morph. ii. 323-364). I will waste no more time on this unsatisfactory discussion, and now only add a few words about the _relation of species to hybridism_. Formerly it was regarded as a dogma, that two good species could never produce hybrids which could reproduce themselves as such. Those who thus dogmatized almost always appealed to the hybrids of a horse and donkey, the mule and the hinny, which, truly enough, are seldom able to reproduce themselves. But the truth is that such unfruitful hybrids are rare examples, and in the majority of cases hybrids of two totally different species are fruitful and able to reproduce themselves. They can almost always fruitfully mix with one or other of the parent species, and sometimes also among themselves; and in this way completely new forms can originate according to the laws of “mixed transmission by inheritance.” Thus, in fact, _hybridism is a source of the origin of new species_, distinct from the source we have hitherto considered—natural selection. I have already spoken occasionally of these _hybrid species_ (species hybridæ), especially of the hare-rabbit (Lepus Darwinii), which has arisen from the crossing of a male hare and a female rabbit; the goat-sheep (Capra ovina), which has arisen from the pairing of a he-goat and ewe; also the different species of thistles (Cirsium), brambles (Rubus), etc. It is possible that many wild species have originated in this way, as even Linnæus assumed. At all events, these hybrid species, which can maintain and propagate themselves as well as pure species, prove that hybridism cannot serve in any way to give an absolute definition to the idea of species.