Chap. IV.J NATURAL SELECTION. 101 



man can produce a great result with his domestic animals 

 and plants by adding up in any given direction individ- 

 ual differences, so could natural selection, but far more 

 easily from having incomparably longer time for action. 

 Nor do I believe that any great physical change, as of 

 climate, or any unusual degree of isolation to check im- 

 migration, 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 modifica- 

 tions 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 inhabitants are now so| 

 perfectly adapted to each other and to the physical con- 

 ditions 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 naturalised 

 productions, that they have allowed some foreigners to 

 take firm possession of the land. And as foreigners 

 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 

 selection, what may not natural selection effect? JS^a 

 can act only on external and visible characters: Nature^ 



