76 NATURAL SBLEGTION. 



of the occurrence of profitable variations. IjTiless such 

 occur, natural selection can do nothing. Under the 

 term of " variations/' it must never be forgotten that 

 mere individual differences are included. As man 

 can produce a great result with his domestic animals and 

 plants by adding up in any given direction individual dif- 

 ferences, so could natural selection, but far more easily 

 from having incomparably longer time for action. 'Nov 

 do I believe that any great physical change, as of climate, 

 or any unusual degree of isolation, to check immigration, is 

 necessary in order that new and unoccupied places should 

 be left for natural selection to fill up by improving some 

 of the varying inhabitants. For as all the inhabitants of 

 each country are struggling together with nicely balanced 

 forces, extremely slight modifications in the structure or 

 habits of one species would often give it an advantage over 

 others; and still further modifications of the same kind 

 would often still further increase the advantage, as long as 

 the species continued under the same conditions of life 

 and profited by similar means of subsistence and defence. 

 No country can be named in which all the native inhab- 

 itants are novy so perfectly adapted to each other and to 

 the physical conditions under which they live, that none 

 of them could be still better adapted or improved; for in 

 all countries the natives have been so far conquered by 

 naturalized productions that they have allowed some for- 

 eigners to take firm possession of the land. And as for- 

 eigners have thus in every country beaten some of the 

 natives, we may safely conclude that the natives might 

 have been modified with advantage, so as to have better 

 resisted the intruders. 



As man can produce, and certainly has produced, a great 

 result by his methodical and unconscious means of selec- 

 tion, what may not natural selection effect? Man can act 

 only on external and visible characters; Nature, if I may 

 be allowed to personify the natuial preservation or survival 

 of the fittest, cares nothing for appearances, except in so 

 far as they are useful to any being. She can act on every 

 internal organ, on every shade of constitutional difference 

 on the whole machinery of life. Man selects only for his 

 own good; Nature only for tliat of the being which she 

 tends. Every selected character is fully exercised by her, 



