ORIGIN OF VARIETIES. 
9^3 
is proved most conclusively by the fact that seeds from the same fruit produce 
different varieties, either exclusively or together with the hereditary parent-form. 
Although the production of varieties and the form they assume are not the 
direct results of external influences, yet the continuance of the existence of a variety 
may be determined by these influences. When a variety is produced, the question 
arises whether it will thrive best in damp or in dry ground, in sunny or shady places, 
and so forth ; whether it can reproduce itself under these circumstances, or whether 
it will die out. The conclusion follows that hereditary varieties arise independently 
of direct external influences, but that the continuance of their existence depends on 
external causes. A variety which occurs only in a particular locality is not produced 
by the conditions of this particular locality ; but it alone furnishes the peculiar con- 
ditions of life which this particular variety requires, while other varieties which have 
arisen at the same place disappear. 
It has already been shown in Sect. 34 that hybrids show in general a tendency 
to the production of varieties. Two diff'erent sets of hereditary characters are com- 
bined in a hybrid, and there is hence a strong tendency towards the formation of new 
characters which may be more or less hereditary. Hybridisation is therefore one of 
the most important means at the command of the horticulturist for disturbing the 
constancy of inherited characters and producing a number of varieties from two dis- 
tinct ancestral forms ^ But even the ordinary sexual union of two individuals of a 
species, as in dioecious, dichogamous, or dimorphic plants, may be considered as a 
kind of hybridisation ; in these cases also the individuals which unite must cer- 
tainly be diff'erent, since otherwise their cross-fertilisation would be no more pro- 
ductive than self-fertflisation, In these cases therefore two sets of characters which 
diff"er, though it may be but slightly, also unite in the descendants ; and if a hybrid 
from two diff'erent species exhibits a strong tendency to variation, the cross-fertil- 
isation of two diff'erent individuals of one and the same species may at least give 
rise to a slight tendency in the same direction. It is therefore probable that in the 
cross-fertilisation of different individuals — towards which there is always a tendency 
in nature even in hermaphrodite flowers — we have a perpetual cause of variation in 
plants. But this is by no means the only cause of variation, as is shown by the 
existence of bud-variation, and by the reflection that the difference between indi- 
viduals which produce a variable progeny is itself due to slight variation. 
A great number of facts point to the conclusion that almost every plant has a tendency 
to vary continually and in different directions, while every new character which is not 
produced directly by external agencies tends at the same time to become hereditary. 
If notwithstanding this many wild plants and some cultivated ones are very constant and 
produce no varieties which can be distinguished externally, this is mainly the result of the 
fact that the newly produced varieties are unable to exist in the conditions by which they 
are surrounded, or at least soon disappear, a point to which I shall recur more in detail. 
The hereditary transmissibility of acquired characters exhibits itself in a most peculiar 
way when it does not affect the whole of the parent-plant, but only a particular branch. 
A still more remarkable case was observed by Bridgman. He noticed that the spores 
from the lower inner part of the lamina of the leaves of the varieties Scolopendrium 
^ See also Naudin, Coiript. rend. 1864, vol. LIX. p. 837. [Journ. Roy. Hort. Soc. new series, 
vol, I. p. I.] 
