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 effect? 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 
