My dear Hooker

I have been recalling my thoughts on the question whether the Glacial period affected the whole world contemporaneously, or only one longitudinal belt after another.¹ To my sorrow my old reasons for rejecting the latter alternative seem to me sufficient;² and I should very much like to know what you think. Let us suppose that the cold affected the two Americas either before or after the Old World; let it advance first either from north or south, till the Tropics became slightly cooled, and a few temperate forms reached the Silla of Caraccas and the mountains of Brazil. You would say I suppose that nearly all the tropical productions would be killed; and that subsequently, after the cold had 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.³ Oswald Heer would make nothing of such a bridge.⁴

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 former almost certain communication between Red Sea and 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 region of India.— Here again you would have to bridge over the Indian Ocean within so very a recent 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; S. 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 length of day, &c; and would be still exposed to nearly 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 N. America, whilst the level of the land oscillated in height fully 3000 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 sometime on subject.

My dear Hooker | Ever yours | Ch. Darwin

I have had this copied that it might not bore you too much to read it.—⁵

A few words more: when equatorial productions were dreadfully distressed by fall of temperature & probably by changed humidity & changed proportional numbers of other plants & enemies (though this 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 & could everywhere have immigrated. But the productions of the temperate regions would have probably found under the Equator in their new homes & soils considerably different conditions of humidity & periodicity; & 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 which plants did during the G. period. Hence I can persuade myself that the temperate productions would not entirely replace & exterminate the productions of the cooled Tropics; but would become partially mingled with them. I am far from satisfied with what I have scribbled. I conclude that there must have been a mundane glacial period; & that the difficulties are much the same whether we suppose it contemporaneous over world, or that longitudinal belts were affected one after the other.—⁶ For Heaven’s sake forgive me!