say--deprives it of certain special mineral or animal constituents
needful for that particular crop, and makes the growth of the plant,
therefore, feeble or even impossible. To avoid this misfortune, he lets
the land lie fallow, or varies his crops from year to year according to
a regular and deliberate cycle. Well, natural selection forced the same
discovery upon the plants themselves long before the farmer had dreamed
of its existence. For plants, being, in the strictest sense, 'rooted to
the spot,' absolutely require that all their needs should be supplied
quite locally. Hence, from the very beginning, those plants which
scattered their seeds widest throve the best; while those which merely
dropped them on the ground under their own shadow, and on soil
exhausted by their own previous demands upon it, fared ill in the
struggle for life against their more discursive competitors. The result
has been that in the long run few species have survived, except those
which in one way or another arranged beforehand for the dispersal of
their seeds and fruits over fresh and unoccupied areas of plain or
hillside.

I don't, of course, by any means intend to assert that seeds always do
it by the simple device of wings or feathery projections. Every variety
of plan or dodge or expedient has been adopted in turn to secure the
self-same end; and provided only it succeeds in securing it, any
variety of them all is equally satisfactory. One might parallel it with
the case of hatching birds' eggs. Most birds sit upon their eggs
themselves, and supply the necessary warmth from their own bodies. But
any alternative plan that attains the same end does just as well. The
felonious cuckoo drops her foundlings unawares in another bird's nest:
the ostrich trusts her unhatched offspring to the heat of the burning
desert sand: and the Australian brush-turkeys, with vicarious maternal
instinct, collect great mounds of decaying and fermenting leaves and