378 
GEOGRAPHICAL DISTRIBUTION, 
Chap. XI. 
even crossed tlie equator. The invasion would, of course, 
have been greatly favoured by high land, and perhaps 
by a dry climate; for Dr. Falconer informs me that it 
is the damp with the heat of the tropics which is so 
destructive 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 Df 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- 
