UNDER ONE SET OF EXTERNAL CONDITIONS. 



501 



species of the ground-genus Amastra is about twenty miles. 

 Again, the difference of type is quite as great between the species 

 of Acliatinella found in the mountains near the eastern end of 

 Oahu and those found forty miles distant, on the other end of the 

 same range of mountains, as the difference between the species of 

 Achatina found in Sierra Leone and those in the region of Port 

 Natal, nearly four thousand miles distant. 



The birds that prey upon these snails are probably few ; but 

 the forests are populous with fruit- and nectar-feeding birds, that 

 might be supposed to give as effectual means of transportation as 

 could be given by any. The number of species represented by 

 these birds is no doubt less than would in most cases be found in 

 an equal extent of continental forest ; but the number of indivi- 

 duals is probably greater than the average number inhabiting 

 equal areas in other parts of the world. 



If we find no reason for attributing the small areas occupied 

 by these species to deficient means of transportation, may we not 

 believe that rapidity of variation has had influence in determining 

 the result ? 



Stability of Type as affected by Cultivation. 

 It is known that there is a great difference in the stability of 

 type in different species of plants and animals that have been sub- 

 jected to cultivation. One produces striking varieties in a single 

 generation ; another requires careful selection of certain characters 

 for many generations before well-marked varieties can be secured. 

 "We also know that continued cultivation will, in many instances, 

 break down the stability of type in a species that, in the first 

 place, adhered with great persistency to one form. It often 

 happens that when the stability has once been disturbed, a wide 

 range of variation may afterwards be obtained with comparative 

 rapidity. 



Is it not possible that similar changes may sometimes take 

 place in species in their wild state ? Two important elements of 

 the cultivation which tends to develop varieties are the removal 

 of competitors and enemies, and the abundant supply of nourish- 

 ment ; but both these conditions may sometimes be furnished by 

 nature without the intervention of man. 



The Natural Selection that prevents Variation. 

 The more severe the competition the more rigidly does Natural 

 Selection adhere to the one form that is best suited to meet that 



