378 GEOGRAPHICAL DISTRIBUTION, Chap. XL 



by a dry climate ; for Dr. Falconer informs me that it 

 is the damp with the heat of the tropics which is so 

 destrnctive to perennial plants from a temperate cli- 

 mate. On the other hand, the most humid and hottest 

 districts will have afforded an asylum to the tropical 

 natives. The mountain-ranges north-west of the Hima- 

 laya, and the long line of the Cordillera, seem to have 

 afforded two great lines of invasion : and it is a striking 

 fact, lately communicated to me by Dr. Hooker, that all 

 the flowering plants, about forty-six in number, common 

 to Tierra del Fuego and to Europe still exist in North 

 America, which must have lain on the line of march. 

 But I do not doubt that some temperate productions 

 entered and crossed even the lowlands of the tropics at 

 the period when the cold was most intense, — when 

 arctic forms had migrated some twenty-five degrees 

 of latitude from their native country and covered the 

 land at the foot of the Pyrenees. At this period of ex- 

 treme cold, I believe that the climate under the equator 

 at the level of the sea was about the same with that now 

 felt there at the height of six or seven thousand feet. 

 During this the coldest period, I suppose that large 

 spaces of the tropical lowlands were clothed with a 

 mingled tropical and temperate vegetation, like that 

 now growing with strange luxuriance at the base of the 

 Himalaya, as graphically described by Hooker. 



Thus, as I believe, a considerable number of plants, a 

 few terrestrial animals, and some marine productions, 

 migrated during the Glacial period from the northern 

 and southern temperate zones into the intertropical re- 

 gions, and some even crossed the equator. As the warmth 

 returned, these temperate forms would naturally ascend 

 the higher mountains, being exterminated on the low- 

 lands ; those which had not reached the equator, would 

 re-migrate northward or southward towards their former 



