226 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



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 con- 

 tinued under the same conditions of life and profited by similar means 

 of subsistence and defense. No country can be named in which all the 

 native inhabitants are now 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 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 efi'ect? Man can act only on external and visible 

 characters: Nature, if I may be allowed to personify the natural pres- 

 ervation 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 that of the being which she tends. Every selected character 

 is fully exercised by her, as is implied by the fact of their selection. 

 Man keeps the natives of many climates in the same country; he 

 seldom exercises each selected character in some peculiar and fitting 

 manner; he feeds a long- and a short-beaked pigeon on the same food; 

 he does not exercise a long-backed or long-legged quadruped in any 

 peculiar manner; he exposes sheep with long and short wool to the 

 same climate. He does not allow the most vigorous males to struggle 

 for the females. He does not rigidly destroy all inferior animals, but 

 protects during each varying season, as far as lies in his power, all 

 his productions. He often begins his selection by some half-monstrous 

 form; or at least by some modification prominent enough to catch the 

 eye or to be plainly useful to him. Under nature, the slightest differ- 

 ences of structure or constitution may well turn the nicely balanced 

 scale in the struggle for life, and so be preserved. How fleeting are the 



