And it is certain that many temperate plants, if
protected from the inroads of competitors, can withstand a much warmer
climate than their own. Hence, it seems to me possible, bearing in
mind that the tropical productions were in a suffering state and could
not have presented a firm front against intruders, that a certain
number of the more vigorous and dominant temperate forms might have
penetrated the native ranks and have reached or even crossed the
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 climate. 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
Himalaya, 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.
Pages:
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453