iciently
explains the apparent absence of such useful productions in Australia
and the Cape of Good Hope, notwithstanding that they both possess an
exceedingly rich and varied flora. These countries having been, until a
comparatively recent period, inhabited only by uncivilised men, neither
cultivation nor selection has been carried on for a sufficiently long
time. In North America, however, where there was evidently a very
ancient if low form of civilisation, as indicated by the remarkable
mounds, earthworks, and other prehistoric remains, maize was cultivated,
though it was probably derived from Peru; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.

_Conditions favourable to the production of Variations._

In order that plants and animals may be improved and modified to any
considerable extent, it is of course essential that suitable variations
should occur with tolerable frequency. There seem to be three conditions
which are especially favourable to the production of variations: (1)
That the particular species or variety should be kept in very large
numbers; (2) that it should be spread over a wide area and thus
subjected to a considerable diversity of physical conditions; and (3)
that it should be occasionally crossed with some distinct but closely
allied race. The first of these conditions is perhaps the most
important, the chance of variations of any particular kind being
increased in proportion to the quantity of the original stock and of its
annual offspring. It has been remarked that only those breeders who keep
large flocks can effect much improvement; and it is for the same reason
that pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great a number of
persons, have produced such strange and numerous varieties. In like
manner, nurserymen who grow fruit and flowers in large quantities have a
great advantage over private amateurs in the production of new
varieties.

Although I believe, for reasons which will be given further on, that
some amount of variability is a constant and necessary property of all
organisms, yet there appears to be good evidence to show that changed
conditions of life tend to increase it, both by a direct action on the
organisation and by indirectly affecting the reproductive system. Hence
the extension of civilisation, by favouring dom