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.