UNDER ONE SET OF EXTERNAL CONDITIONS. 



503 



have an opportunity of being preserved. New shades of colour, 

 for example, would not expose the owners to the attacks of 

 enemies. Variations of shape, if not inconsistent with the pur- 

 suit of food, would be no disadvantage. 



3rd. By continual Change in the Character of the Natural 

 Selection. — Still further, we can see that when competition arises 

 from the gradual introduction of animals, either friendly or hurt- 

 ful to the first occupants, the character of the Natural Selection, 

 to which they would thus be subjected would be continually 

 changing ; no one set of characters would have constant advantage 

 through a long series of successive generations. 



In these ways the persistence of form might be impaired, and 

 the variability which we may believe exists in some degree in all 

 organisms might be greatly increased beyond what is usually 

 found. This tendency to comparatively rapid variation having 

 been established, the evolution of species would be correspondingly 

 rapid, and the areas of each proportionately limited. 



Imaginary Case, illustrating Evolution without change in the 

 External Conditions. 



If a bird should carry a leaf bearing two individuals of some 

 species and drop it a mile beyond the limits already reached by 

 others of that species, they might there find the same trees to 

 which they were accustomed, and multiply for some tens of years 

 before the first scattering individuals from the slowly advancing 

 wave of migration would reach them. They might, by this time, 

 have increased to many thousands ; and having been entirely 

 separated from the original stock for a considerable number of 

 generations, with a preexisting tendency to rapid variation, a 

 certain variety of form and colour might have partially established 

 itself amongst them. The arrival of a few individuals representing 

 the old stock would, amongst the multitudes of the new variety, 

 have no influence in bringing back the succeeding generations to 

 the original form. The new characters would become from year 

 to year more distinctly set. Owing to an intervening ridge acting 

 as a partial barrier, the number of individuals of the original 

 stock coming amongst them might be always restricted ; and even 

 if no such barrier existed, the individuals arriving from abroad 

 could never be more than a very small number compared with 

 those produced on the spot and possessing the local characteristics. 



