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.
It may be thought that forests are an exception to this rule, since in the north-temperate and arctic regions we find extensive forests of pines or of oaks. But these are, after all, exceptional, and characterise those regions only where the climate is little favourable to forest vegetation. In the tropical and all the warm temperate parts of the earth, where there is a sufficient supply of moisture, the forests present the same variety of species as does the turf of our old pastures; and in the equatorial virgin forests there is so great a variety of forms, and they are so thoroughly intermingled, that the traveller often finds it difficult to discover a second specimen of any particular species which he has noticed. Even the forests of the temperate zones, in all favourable situations, exhibit a considerable variety of trees of distinct genera and families, and it is only when we approach the outskirts of forest vegetation, where either drought or winds or the severity of the winter is adverse to the existence of most trees, that we find extensive tracts monopolised by one or two species. Even Canada has more than sixty different forest trees and the Eastern United States a hundred and fifty; Europe is rather poor, containing about eighty trees only; while the forests of Eastern Asia, Japan, and Manchuria are exceedingly rich, about a hundred and seventy species being already known. And in all these countries the trees grow inter-
