278 RECAPITULATION. [Chap. XV 



acted so efficiently under domestication should not have 

 acted under nature. In the survival of favoured indi- 

 viduals and races, during the constantly-recurrent 

 Struggle for Existence, we see a powerful and ever- 

 acting form of Selection. The struggle for existence 

 inevitably follows from the high geometrical ratio of 

 increase which is common to all organic beings. This 

 high rate of increase is proved by calculation, — by the 

 rapid increase of many animals and plants during a 

 succession of peculiar seasons, and when naturalised in 

 new countries. More individuals 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 finally become extinct. As the indi- 

 viduals of the same species come in all respects into the 

 closest competition with each other, the struggle will 

 generally be most severe between them; it will be al- 

 most 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 severe between beings remote in the- scale of nature. 

 The slightest advantage in 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 surrounding 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 pos- 

 session of the females. The most vigorous males, or 

 those which have most successfully struggled with their 

 conditions of life, will generally leave most progeny. 

 But success will often depend on the males having spe- 



