HYBRIDISM 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 originate according to the laws of “mixed trans- 
mission by inheritance.” 
Thus, in fact, hybridism is a source of the origun of new 
species, distinct from the source we have hitherto considered 
—natural selection. I have already spoken occasionally of 
these hybrid species (species hybridze), 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 
Linnzeus 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. 
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- 
