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 domestication under altered
conditions, facilitates the process of modification. Yet this change
does not seem to be an essential condition, for nowhere has the
production of extreme varieties of plants and flowers been carried
farther than in Japan, where careful selection continued for many
generations must have been the chief factor. The effect of occasional
crosses often results in a great amount of variation, but it also leads
to instability of character, and is therefore very little employed in
the production of fixed and well-marked races.
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