Seite - 89 - in The Origin of Species

STRUGGLE FOR EXISTENCE 89 to the ground according to definite laws; but how simple is tlie problem where each shall fall compared to that of the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the pro- portional numbers and kinds of trees now growing on the old Indian ruins ! The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. This is likewise sometimes the case with those which may be strictly said to struggle with each other for existence, as in the case of locusts and grass- feeding quadrupeds. But the struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers. In the case of varie- ties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon de- cided : for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varie- ties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the other varieties. To keep up a mixed stock of even such extremely close varieties as the variously-coloured sweet peas, they must be each year harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily decrease in number and disappear. So again with the varieties of sheep; it has been asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. The same result has followed from keeping together different varieties of the medicinal leech. It may even be doubted whether the varie- ties of any of our domestic plants or animals have so ex- actly the same strength, habits, and constitution, that the original proportions of a mixed stock (crossing being pre- vented) could be kept up for half-a-dozen generations, if they were allowed to struggle together, in the same manner as beings in a state of nature, and if the seed or young were not annually preserved in due proportion.