tic productions would ever become so completely habituated
to the conditions under which they now live, as to cease varying, we have
no sufficient means for judging. But, in fact, our domestic productions are
never exposed for a great length of time to uniform conditions, and it is
certain that our most anciently cultivated plants, as well as animals,
still go on varying, for all have recently undergone marked improvement. In
some few cases, however, plants have become habituated to new conditions.
Thus Metzger, who cultivated in Germany during many years numerous
varieties of wheat, brought from different countries,[625] states that some
kinds were at first extremely variable, but gradually, in one instance
after an interval of twenty-five years, became constant; and it does not
appear that this resulted from the selection of the more constant forms.

* * * * *

_On the Accumulative Action of changed Conditions of Life._--We have good
grounds for believing that the influence of changed conditions accumulates,
so that no effect is produced on a species until it has been exposed during
several generations to continued cultivation or domestication. Universal
experience shows us that when new flowers are first introduced into our
gardens they do not vary; but ultimately all, with the rarest exceptions,
vary to a greater or less extent. In a few cases the requisite number of
generations, as well as the successive steps in the progress of variation,
have been recorded, as in the often-quoted instance of the Dahlia.[626]
After several years' culture the Zinnia has only lately (1860) begun to
vary in any great degree. "In the first seven or eight years of high
cultivation the Swan River daisy (_Brachycome iberidifolia_) kept to its
original colour; it then varied into lilac and purple and other minor
shades."[627] Analogous facts have been recorded with the Scotch rose. In
discussing the variability of plants several experienced horticulturists
have spoken to the {262} same general effect. Mr. Salter[628] remarks,
"Every one knows that the chief difficulty is in breaking through the
original form and colour of the species, and every one will be on the
look-out for any natural sport, either from seed or branch; that being once
obtained, however trifling the change may be, the result depends upon
himself." M. de Jonghe, who has had so much success in raising new
varieties of pears and strawberries,[