xiii NATURAL SELECTION 227 



species to its surrounding conditions by virtue of this 

 " survival of the fittest." 



Let us suppose the conditions to change. Gradual alter- 

 ations in climate and other conditions are known to take 

 place, owing to subsidence or elevation of the land. But 

 conditions might be changed in many other ways : some 

 animal or plant previously used as food might become ex- 

 terminated, or a new enemy might find its way into the 

 district inhabited by the species. Then such individuals 

 as presented variations which enabled them better to cope 

 with the new surroundings, would have the advantage over the 

 others, and would have a much better chance of surviving 

 and leaving progeny. The useful variations thus produced 

 and transmitted to the progeny would tend to increase, 

 generation after generation, until a form sufficiently distinct 

 to be regarded as a new species had become developed from 

 the original one. 



That very different varieties of animals and plants can, 

 and have been produced, in a comparatively short time, by 

 man selecting those forms which tend to vary in a desired 

 direction, is a well-known fact. All the breeds and varieties 

 of our domestic animals have been produced by this process 

 of artificial selection ; and " if maa. can by patience select 

 variations useful to him, why, under changing and com- 

 plex conditions of life, should not variations useful to 

 Nature's living products often arise, and be preserved or 

 selected ? " 



It does not come within the scope of the present work to 

 discuss either the causes of variability or those which deter- 

 mine the elevation of a variety to the rank of a species : 

 both questions are far too complex to be adequately treated 

 except at considerable length, and anything of the nature 

 of a brief abstract would only be misleading. As a pre. 



