as we see at the
present day crowded together at the Cape of Good Hope, and in parts of
temperate Australia. As we know that many tropical plants and animals
can withstand a considerable amount of cold, many might have escaped
extermination during a moderate fall of temperature, more especially by
escaping into the warmest spots. But the great fact to 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 somewhat 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 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. 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 tropical and temperate vegetation, like that now