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bear in mind is, that all tropical productions will have 

 suffered to a certain extent. On the other hand, the 

 temperate productions, after migrating nearer to the 

 equator, though they will have been placed under some- 

 what new conditions, will have suffered less. 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 th&t 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 Cordil- 

 lera, 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 extreme 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 tropi- 

 cal and temperate vegetation, like that now growing 



