It will be as well to quote Darwin himself on this point.  He says; “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, while Nature, if I may be allowed to personify the natural 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 the good 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-beaked 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 hair and short wool in 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, so 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 differences of structure or constitution may- well turn the nicely balanced scale in the struggle for life, and so be preserved.  How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will be his results, compared with those accumulated by nature during whole geological periods!  Can we wonder, then, that Nature’s productions should be far ‘truer’ in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?”

Darwin’s theory of survival of the fittest is begun by the statement of the fact that the number of organisms that survive are very small compared with the number that are born.  To quote his own words, “There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair.  Even slow-breeding man has doubled in twenty-five years, and at this rate in less than a thousand years there would literally not be standing room for the progeny.”  It has been computed that if the offspring of the elephant, which is believed to be the slowest breeding animal known, were to survive, there would be about 20,000,000 elephants on the earth in 750 years.  The roe of a single cod contains eight or nine millions of eggs, and if each egg were to hatch, and the fish survive, the sea would shortly become a solid mass