THE STRUGGLE FOR EXISTENCE 400 



even if opportunity offers, successfully mate or interbreed 

 with mainland individuals the island type^constitutes a 

 new species. That is, our distinction between species 

 rests not only on structural differences, but on the impossi- 

 bility of interbreeding (at least for the production of fertile 

 young). Such a combination of the action of natural 

 selection and the condition of isolation (as illustrated by 

 the case of island animals), is probably the most potent 

 factor in the production of new species of animals (and 

 plants). 



For accounts of the struggle- for existence, variations, 

 adaptations, natural selection and species-forming see 

 Darwin's "Origin of Species," Wallace's *' Island Life," 

 and Romanes' " Darwin and After Darwin," I. 



Artificial selection. When a selection among the 

 individuals of a species, that is, the choosing and preserv- 

 ing of individuals which show a certain trait or traits and 

 the destroying of those individuals not possessing this 

 trait, is done by man, it is called artificial selection. To 

 artificial selection we chiefly owe all the many races or 

 varieties of our domesticated animals and plants. For 

 example, from the ancestral jungle fowl have been devel- 

 oped by artificial selection (and by cross-breeding) all our 

 kinds of domestic fowl, as Brahmas, black Spanish, 

 bantams, game-cocks, etc. ; from the wild rock-dove have 

 been developed our various fancy pigeons, as carriers, 

 pouters, fan tails, etc. 



For an account of artificial selection see Darwin's 

 "Plants and Animals under Domestication," and 

 Romanes' " Darwin and After Darwin," I. 



