502 REV. J. T. aULICK ON BIVERSITT OE EYOLUTION 



competition, or, according to the language in which Professor 

 Owen lias stated the doctrine, the more certainly does the " Battle 

 of Life " extinguisli all A'ariations from that one form. "When a 

 species is subjected to severe competition of the same kind for 

 countless generations, we may well believe that it gains a stability 

 of type that is not found in one that has during the same time 

 been, either comparatively free from competition, or under the 

 influence of a succession of different competitors and enemies*. 



Stdbility of Type in Island Fauna may he impaired : — 

 1st. £y Freedom from the Competition that limits Variation. — 

 We can see that when animal life commences upon an island 

 where vegetation has already become abundant, the first species 

 that appears on the arena, unless immediately followed by other 

 creatures capable of being either friends or foes, will enjoy for a 

 time complete freedom from competition. If the vegetation is 

 suited, it will also have an abundance of food. Under these cir- 

 cumstances every variation that occurs, unless decidedly mal- 

 formed, will have a chance of living and exerting an influence 

 upon the final, result. 



2ind. By Competition accelerating Variation. — If the intro- 

 duction of competitive animals is long delayed, the first struggle 

 for life will occur between the members of the one stock. Eut 

 competition of this kind does not tend to prevent variation, but 

 rather to accelerate it, by driving portions of the race into new 

 spheres. Supposing the animals first inhabiting the island to be 

 a species of arboreal mollusks, there would soon be an excess of 

 occupants on the trees best suited to them in the region where 

 they first appeared. The portion of the population that would 

 survive this exigency would, in the first place, be those that found 

 sustenance on trees of other kinds. Some of these would either 

 themselves, or through their descendants, reach localities where 

 the trees arc again Ibund on which the stock commenced its 

 career. Those that, in this way, returned to the original trees, 

 would have accjuired some new tendencies to variation through tlic 

 ordeal through which they had passed ; and those that remained 

 upon the other kinds of trees would rapidly develop new cha- 

 racters : in either case, there would be no outside competition 

 limiting them to one definite form. New forms of variation would 



* The only terrestrial mollusks with which the Achatinellince have to compete 

 are a few Helices much inferior in size, and not arboreal in their habits. 



