HYBEIDISM PRODUCES SPECIES. 275 



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 repro- 

 duce 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 orimnate accordino^ to the laws of " mixed trans- 

 mission by inheritance." 



Thus, in fact, hyhvidisTYi is a source of the origin of oiew 

 sjpecies, distinct from the source we have hitherto considered 

 — natural selection. I have already spoken occasionally of 

 these hybrid species (species hybridse), 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 (Eubus), etc. It is possible that 

 many wild species have originated in this way, as even 

 Linnaeus assumed. At all events, these hybrid species, 

 which can maintain and propagate themselves as weU as 

 pure species, prove that hybridism cannot serve in any way 

 to give an absolute definition to the idea of species. 



I have already mentioned (p. 47) that the many vain 

 attempts to define the idea of species theoretically have 

 nothing whatever to do with the practical distinction of 

 species. The extensive practical application of the idea of 

 species, as it is carried out in systematic zoology and botany, 

 is very instructive as furnishing an example of human folly. 

 Hitherto, by far the majority of zoologists and botanists, in 

 distinguishing and describing the different forms of animals 

 and plants, have endeavoured, above all things, to dis- 



