The reason why this kind of struggle goes on is apparent if we consider that the allied species fill nearly the same place in the economy of nature. They require nearly the same kind of food, are exposed to the same enemies and the same dangers. Hence, if one has ever so slight an advantage over the other in procuring food or in avoiding danger, in its rapidity of multiplication or its tenacity of life, it will increase more rapidly, and by that very fact will cause the other to decrease and often become altogether extinct. In some cases, no doubt, there is actual war between the two, the stronger killing the weaker; but this is by no means necessary, and there may be cases in which the weaker species, physically, may prevail, by its power of more rapid multiplication, its better withstanding vicissitudes of climates, or its greater cunning in escaping the attacks of the common enemies. The same principle is seen at work in the fact that certain mountain varieties of sheep will starve out other mountain varieties, so that the two cannot be kept together. In plants the same thing occurs. If several distinct varieties of wheat are sown together, and the mixed seed resown, some of the varieties which best suit the soil and 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. As an effect of this principle, we seldom find closely allied species of animals or plants living together, but often in distinct though adjacent districts where the conditions of life are somewhat different. Thus we may find cowslips (Primula veris) growing in a meadow, and primroses (P. vulgaris) in an adjoining wood, each in abundance, but not often intermingled. And for the same reason the old turf of a pasture or heath consists of a great variety of plants matted together, so much so that in a patch little more than a yard square Mr. Darwin found twenty distinct species, belonging to eighteen distinct genera and to eight natural orders, thus showing their extreme diversity of organisation. For the same reason a number of distinct grasses and clovers are sown in order to make a good lawn instead of any one species; and the quantity of hay produced has been found to be greater from a variety of very distinct grasses than from any one species of grass.