CiiAr. XIV. RECAPITULATION. 419 



of many animals and plants during a succession of peculiar 

 seasons, and when naturalized in a new country. More indi- 

 viduals are born than can possibly survive. A grain in the 

 balance may determine which individuals shall live and which 

 shall die — which variety or species shall increase in number, 

 and which shall decrease, or linally become extinct. As the 

 individuals of the same species come in all respects into the 

 closest competition with each other, the struggle will gener- 

 ally be most severe between them ; it will be almost equally 

 severe between the varieties of the same species, and next in 

 severity between the species of the same genus. On the other 

 hand, the struggle will often be very severe between beings re- 

 mote in the scale of nature. The slightest advantage in cer- 

 tain 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 surrounding physical condi- 

 tions, will turn tlic balance. 



With animals having separated sexes there will be in most 

 cases a struggle between the males for the possession of the 

 females. The most vigorous males, or those which have most 

 successfully struggled with then' conditions of life, will gener- 

 ally leave most progeny. But success will often depend on 

 the males having special weapons, or means of defence, or 

 charms ; and a slight advantage Avill lead to victory. 



A3 geology plainly proclaims that each land has under- 

 gone great physical changes, we might have expected to find 

 that organic beings have varied under Nature, in the same way 

 as they have varied under domestication. And if there be any 

 variability under Nature, it would be an unaccountable fiict if 

 natural selection did 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 fjuantity. 

 i\Ian, though acting on external characters alone and often 

 capriciously, can produce w'ithin a short peritxl a great result 

 by adding up mere individual differences in his domestic pro- 

 ductions; and every one admits that species present individual 

 difFerences. 15ut, besides such dill'erences, all naturalists admit 

 that varieties exist, which are considered sulliciently distinct 

 to be worthy of record in systematic works. No one has 

 drawn any clear distinction between individual dilVerences and 

 slight varieties ; or between more plainly-marked varieties 

 and sub-species, and species. f)n separate contincMits, and on 

 dilFercnt parts of the same continent when divided hy l)arriers 



