adow, 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
intermingled, so that in every extensive forest we have a considerable
variety, as may be seen in the few remnants of our primitive woods in
some parts of Epping Forest and the New Forest.

Among animals the same law prevails, though, owing to their constant
movements and power of concealment, it is not so