and would be still exposed
to nearly the same enemies, as insects and other animals; whereas the
invading temperate productions, though finding a favouring temperature,
would have some of their conditions of life new, and would be exposed to
many new enemies. But I fully admit the difficulty to be very great.
I cannot see the full force of your difficulty of no known cause of
a mundane change of temperature. We know no cause of continental
elevations and depressions, yet we admit them. Can you believe, looking
to Europe alone, that the intense cold, which must have prevailed when
such gigantic glaciers extended on the plains of N. Italy, was due
merely to changed positions of land within so recent a period? I cannot.
It would be far too long a story, but it could, I think, be clearly
shown that all our continents existed approximately in their present
positions long before the Glacial period; which seems opposed to such
gigantic geographical changes necessary to cause such a vast fall of
temperature. The Glacial period endured in Europe and North America
whilst the level of the land oscillated in height fully 3,000 feet,
and this does not look as if changed level was the cause of the Glacial
period. But I have written an unreasonably long discussion. Do not
answer me at length, but send me a few words some time on the subject.

I have had this copied, that it might not bore you too much to read it.

A few words more. When equatorial productions were dreadfully distressed
by fall of temperature, and probably by changed humidity, and changed
proportional numbers of other plants and enemies (though they might
favour some of the species), I must admit that they all would be
exterminated if productions exactly fitted, not only for the climate,
but for all the conditions of the equatorial regions during the Glacial
period existed and could everywhere have immigrated. But the productions
of the temperate regions would have probably found, under the equator,
in their new homes and soils, considerably different conditions of
humidity and periodicity, and they would have encountered a new set of
enemies (a most important consideration); for there seems good reason
to believe that animals were not able to migrate nearly to the extent to
which plants did during the Glacial period. Hence I can persuade
myself that the temperate productions would not entirely replace and
exterminate the productions of the cooled tropics, but would b