DARWIN 



variation has a wide field, but environment and cir- 

 cumstance do not make a prison wherein the organ- 

 ism must live or else die. The margin of existence is 

 not so narrow. New forms can adapt themselves to 

 new conditions, but, while variation may proceed 

 along directed lines to a great degree, after a time the 

 active and creative energies of growth pass the bounds 

 of physical and physiological equilibrium. Then 

 weakness has set in and the species may not find itself 

 fit to survive either changed or harsh conditions. We 

 are entitled to use the customary metaphor^ and to 

 see in natural selection an inexorable force, whose 

 function is not to create, but to destroy, individuals. 

 Even after we have so narrowed the scope and sphere 

 of natural selection, it is hard to understand; the 

 causes of extinction are often well-nigh as hard to 

 comprehend as are those of the origin of species. If 

 we consider any exaggerated form which has become 

 extinct, there are kindred forms which survive; and 

 in other cases extinction occurs where we can discover 

 no observable disadvantage. 



I am even willing to grant that the struggle for ex- 

 istence and natural selection may be the causes for 

 the extinction of certain species, — and for the very 

 obvious reason that if a species passes out of exist- 

 ence, there must have been either some change in en- 

 vironment very unfavourable to the species, or some 

 powerful and destructive organic enemy must have 

 attacked it in overwhelming numbers, or the species 



C 239 3 



