II THE STRUGGLE FOR EXISTENCE 35 



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 Ave 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 oi-ders, thus showing their extreme 

 diversity of organisation. For the same reason a number of 

 distinct gi'asses 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, Avhere 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 diff"erent forest trees, and 

 the Eastern United States a hundred and fifty ; Europe is 

 rather poor, containing about eighty trees only ; Avhile 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- 



