II 
THE STRUGGLE FOR EXISTENCE 
35 
As ati 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 arc 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- 
