ORIGIN OF VARIETIES. 825 



reason — because varieties are to so great an extent independent of external influences 

 — that they are hereditary. A change produced in a plant by moisture, shade, or any 

 similar cause, is for the same reason not hereditary, because its descendants, when 

 placed under other vital conditions, acquire again other non-permanent characters. 

 That hereditary characters, or those which may become so, are not produced by ex- 

 ternal influences, is proved most conclusively by the fact that seeds from the same 

 fruit produce diff'erent varieties, either entirely so or together with the inherited 

 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 has arisen, 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 perish. 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 vital 

 conditions which this particular variety requires, while other varieties which have 

 arisen at the same place disappear. 



It has already been shown in Sect. 32 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 tjie 

 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-fertilisation. 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 difterent 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 there must always be a slight 

 diff"erence between individuals which produce a variable progeny. 



A great number of facts point to the conclusion that almost every plant has a tend- 

 ency 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. 



' See also Naudin, Compt. rend. 1S64, vol. LTX, p. S37. [Jouni. Roy. Ho.i. Soc. new series, 

 vol. 1, p. I.] 



