Professor Huxley showed the probable descent of the horse with its hoofed foot from the extinct three-toed Hipparion of Europe, and that from the four-toed Orohippus of the Eocene formation. He declared it probable that a five-toed horse would be found, and Professor Marsh, in the West, has found the Eohippus, corresponding very nearly to Professor Huxley's description. The question among naturalists was, "How can plants and animals have become thus changed?" Darwin showed how it was possible to effect most of these changes by "natural selection," or the choosing of the best to survive in the struggle for existence. As man by grafting secures the finest fruit, and by care in animal life the swiftest horses for speed as well as the strongest for labor, so nature selects her best for the higher development of the race. Darwin says, "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 his progeny.... The elephant is reckoned the slowest breeder of all known animals; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years, there would be nearly nineteen million elephants alive, descended from the first pair." In various ways the weakest are destroyed. Darwin, on a piece of ground three feet long and two wide, says, "I marked all the seedlings of our native weeds as they came up, and, out of 357, no less than 295 were destroyed, chiefly by slugs and insects." He gives this interesting instance of the struggle for existence. "I find from experiments that humble-bees are almost indispensable to the fertilization of the heart's-ease, for other bees do not visit this flower.... Humble-bees alone visit red clover, as other bees cannot reach the nectar.... Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heart's-ease and red clover would become very rare, or wholly disappear. The number of humble-bees in any district depends in a great measure upon the number of field-mice, which destroy their combs and nests; the number of mice is largely dependent, as every one knows, on the number of cats." Hence, as Mr. Darwin shows, the frequency of certain flowers in a district may depend upon the number of cats!