MORE ABOUT SPECIES 101 



animals to breed under actual observation that the in- 

 dividuals of a species pair with one another, and not 

 with individuals of other species, and further, that the 

 young which they produce are like the parents — show 

 themselves, in fact, to be of the same "species." The 

 species continually year after year reproduces itself with 

 little variation, though some variation does occur. The 

 faculty of pairing only within the group, of never 

 naturally breeding with members of other groups, has 

 accordingly been adopted as a test of species. Hybrids 

 between two species do not occur, except in very rare 

 cases, in the state of nature. It is not always the case 

 that the members of two species cannot possibly pair 

 together, but it is the fact that they do not do so. Man 

 sometimes brings about such crossing or hybridization, 

 and it is a curious fact that the hybrids are often infertile 

 or give rise only to weakly offspring, which could not 

 survive in the natural struggle for existence. Sometimes, 

 however, when the two hybridized species happen to 

 come from regions of the world remote from one another, 

 the resulting hybrids establish a vigorous race. There 

 are real obstacles (of which I will say more below) in 

 natural conditions to hybrid-breeding between any two 

 species which occur naturally in the same territory. Thus 

 the idea of a species is expanded so as to be not merely 

 " a group of individuals of constant likeness in form and 

 characteristics," but we add to that definition a living or 

 constitutional quality expressed by the words, " which 

 produce fertile offspring by pairing with one another, but 

 do not pair with the members of other species." 



This enables us to distinguish the conception of a 

 " species " from that of a " variety " or a " race." We 

 find occasionally peculiarly-marked examples of a species 

 of plant or animal, or even local races of peculiar 

 form ; but we do not regard them as " distinct species " 



