Races{238} produced by these natural means of selection{239} we may expect would differ in some respects from those produced by man. Man selects chiefly by the eye, and is not able to perceive the course of every vessel and nerve, or the form of the bones, or whether the internal structure corresponds to the outside shape. He{240} is unable to select shades of constitutional differences, and by the protection he affords and his endeavours to keep his property alive, in whatever country he lives, he checks, as much as lies in his power, the selecting action of nature, which will, however, go on to a lesser degree with all living things, even if their length of life is not determined by their own powers of endurance. He has bad judgment, is capricious, he does not, or his successors do not, wish to select for the same exact end for hundreds of generations. He cannot always suit the selected form to the properest conditions; nor does he keep those conditions uniform: he selects that which is useful to him, not that best adapted to those conditions in which each variety is placed by him: he selects a small dog, but feeds it highly; he selects a long-backed dog, but does not exercise it in any peculiar manner, at least not during every generation. He seldom allows the most vigorous males to struggle for themselves and propagate, but picks out such as he possesses, or such as he prefers, and not necessarily those best adapted to the existing conditions. Every agriculturalist and breeder knows how difficult it is to prevent an occasional cross with another breed. He often grudges to destroy an individual which departs considerably from the required type. He often begins his selection by a form or sport considerably departing from the parent form. Very differently does the natural law of selection act; the varieties selected differ only slightly from the parent forms{241}; the conditions are constant for long periods and change slowly; rarely can there be a cross; the selection is rigid and unfailing, and continued through many generations; a selection can _never be made_ without the form be _better_ adapted to the conditions than the parent form; the selecting power goes on without caprice, and steadily for thousands of years adapting the form to these conditions. The selecting power is not deceived by external appearances, it tries the being during its whole life; and if less well <?> adapted than its _congeners_, without fail it is destroyed; every part of its structure is thus scrutinised and proved good towards the place in nature which it occupies. {238} In the _Origin_ the author would here have used the word _variety_. {239} The whole of p. 94 and 15 lines of p. 95 are, in the MS., marked through in pencil with vertical lines, beginning at "Races produced, &c." and ending with "to these conditions." {240} See _Origin_, Ed. i. p. 83, vi. p. 102.