d moderated, tropical plants immigrated from the other
non-chilled parts of the world. But this is impossible unless you bridge
over the tropical parts of the Atlantic--a doctrine which you know I
cannot admit, though in some respects wishing I could. Oswald Heer would
make nothing of such a bridge. When the Glacial period affected the
Old World, would it not be rather rash to suppose that the meridian
of India, the Malay Archipelago, and Australia were refrigerated, and
Africa not refrigerated? But let us grant that this was so; let us
bridge over the Red Sea (though rather opposed to the former almost
certain communication between the Red Sea and the Mediterranean); let
us grant that Arabia and Persia were damp and fit for the passage of
tropical plants: nevertheless, just look at the globe and fancy the cold
slowly coming on, and the plants under the tropics travelling towards
the equator, and it seems to me highly improbable that they could escape
from India to the still hot regions of Africa, for they would have to go
westward with a little northing round the northern shores of the Indian
Ocean. So if Africa were refrigerated first, there would be considerable
difficulty in the tropical productions of Africa escaping into the
still hot regions of India. Here again you would have to bridge over the
Indian Ocean within so very recent a period, and not in the line of
the Laccadive Archipelago. If you suppose the cold to travel from the
southern pole northwards, it will not help us, unless we suppose that
the countries immediately north of the northern tropic were at the same
time warmer, so as to allow free passage from India to Africa, which
seems to me too complex and unsupported an hypothesis to admit.
Therefore I cannot see that the supposition of different longitudinal
belts of the world being cooled at different periods helps us much.
The supposition of the whole world being cooled contemporaneously (but
perhaps not quite equally, South America being less cooled than the
Old World) seems to me the simplest hypothesis, and does not add to
the great difficulty of all the tropical productions not having been
exterminated. I still think that a few species of each still existing
tropical genus must have survived in the hottest or most favourable
spots, either dry or damp. The tropical productions, though much
distressed by the fall of temperature, would still be under the same
conditions of the length of the day, etc.,