XIV 



former should belong to the same family as the herbaceous 

 ones of our own woods and glades, that the Lily of the 

 Valley and other humble plants should be representatives 

 here of a tribe which is illustrated in Africa by the Dracce- 

 nas, some of the largest and oldest of tropical productions ; . 

 and that the Mallows, which constitute trees of gigantic 

 size near the Equator, should appear only in the form of 

 herbaceous plants within the confines of Europe. Not 

 however that we have any evidence of heat being capable 

 of converting an herbaceous into an arborescent plant by 

 any continuation of its action, for herbaceous species trans- 

 ported to the Tropics do not become arborescent, nor do 

 arborescent ones dwindle into herbaceous ones in colder 

 climes ; but only that the stimulus of great heat was indis- 

 pensable to the existence of those species whose habit it is 

 to become caulescent, and unpropitious to that of others 

 which continue herbaceous and therefore diminutive. 



We may even go further, and assume, that, in temperate 

 regions, the unequal length of the day at different periods 

 of the year, as contrasted with the uniformity in that respect 

 which prevails within the Tropics, would be likely to intro- 

 duce corresponding deviations in the character of the vege- 

 tation in these two cases. A shorter duration of the sun's 



