288 THE ORIGIN OF SPECIES 



certain individuals, at any age or during any season, over 

 those with which they come into competition, or better 

 adaptation in however slight a degree to the surround- 

 ing physical conditions, will, in the long run, turn the 

 balance. 



With animals having separated sexes, there will be in 

 most cases a struggle between the males for the posses- 

 sion of the females. The most vigorous males, or those 

 which have most successively struggled with their con- 

 ditions of life, will generally leave most progeny. But 

 success will often depend on the males having special 

 weapons, or means of defence, or charms; and a slight 

 advantage will lead to victory. 



As geology plainly proclaims that each land has 

 undergone great physical changes, we might have ex- 

 pected to find that organic beings have varied under 

 nature, in the same way as they have varied under 

 domestication. And if there has been any variability 

 under nature, it would be an unaccountable fact if 

 natural selection had not come into play. It has often 

 been asserted, but the assertion is incapable of proof, 

 that the amount of variation under nature is a strictly 

 limited quantity. Man, though acting on external char- 

 acters alone and often capriciously, can produce within a 

 short period a great result by adding up mere individual 

 difEerences in his domestic productions; and every one 

 admits that species present individual differences. But, 

 besides such differences, all naturalists admit that natural 

 varieties exist which are considered sufficiently distinct to 

 be worthy of record in systematic works. No one has 

 drawn any clear distinction between individual differences 

 and slight varieties; or between more plainly marked 



