In a certain sense, all animated nature is but a single vast co-operative society. I am no foolish optimist: I will admit, indeed, that the members of the society so composed often display to one another the most unfriendly and unfraternal spirit. The hawks, for instance, show a distinct want of true brotherly love towards the larks or the tom-tits: and the mice and lizards find the owls and the cats by no means clubbable. The co-operative society is hardly what one could call a happy family. Still, in spite of the fact insisted upon by the poet that "Nature is one with rapine--a harm no preacher can heal," it is none the less true that a certain rough balance, an accommodation or adjustment of part to part, occurs in every department of animal and vegetable life. When we come to think, it could hardly be otherwise. Things can only exist if they contain in themselves the conditions necessary to existence. An unadapted animal or plant perishes instantly. Spiders could not live in an island which contained no flies; kingfishers necessarily presuppose fish; and silkworms imply the presence of mulberry leaves. You cannot have vultures wild in a country where there are no dead animals lying about loose: nor can you keep bees except where there are honey-bearing flowers. Dutch clover depends for its very existence upon a few insects which fertilize it and set its seeds. The draining of the fens killed out a dozen species of English plants and animals; the inclosure of the prairies deprived the buffaloes of their chance of pasture. In this sense, all nature hangs together as it were; each species fills some place in the great mosaic which cannot be altered without considerable disturbance of adjacent pieces. Destroy the rabbits in a given area, and you have nothing left for the weasels to feed upon. Sometimes, too, apparently unimportant or unnoticed creatures perform in the aggregate some valuable work for the rest of the plant and animal community, which little suspects its real indebtedness to them. Darwin showed long ago that the humble and despised earthworm was really answerable for the greater part of that rich layer of vegetable mould or soil which covers the bare rocks; it deposits the material in which all our plants root and from which they derive a large element of their sustenance. Kill out the earthworms over the whole of our earth, and you would reduce a vast proportion of it to the condition of a desert. For the worms pull down green leaves into their neat little burrows; and the refuse of these leaves, continually renewed from season to season by the industrious small workmen, forms by far the greater share of that dark layer of vegetable mould which is the chief source of the fertility in plains and lowlands. Sandy upland spots, where worms are few, form little or no soil, and will only support a poor moorland growth of gorse and heather. You must have plenty of worms if you want to grow corn or turnips.