Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/465

1869.] MURPHY—GLACIAL CLIMATE. 355 was not one of great heat, but moderate and moist. If so, the glacial and the carboniferous climates may have coexisted, being only separated by a few degrees of latitude. We see an approach to such a state of things at present; in the straits of Magellan the climate is so far glacial that glaciers reach the sea and give origin to icebergs, while in the Falklands it is so far carboniferous that all the vegetation, and that not mosses but flowering plants, is converted when dead into peat.

If I am right that both hemispheres were never glaciated at once, it follows that the equatorial regions were never glaciated at all; and this accounts for what Darwin remarks with surprise, that the vegetable species of the tropics have undergone much less extinction during the glacial period than might have been expected ('Origin of Species,' 4th edition, p. 454). But he regards it as proved that at a comparatively recent period, which in all probability was the glacial, there has been a good deal of intermigration of species between the two temperate zones, which must have crossed the equator when its temperature was cooler than it is now. Both of these facts may be explained by supposing, what is in itself very probable, that during the glacial period the equatorial climate was much what it is now, except in some places which were cooled by ice-bearing currents. The floating ice would also be a most efficient agent in transporting seeds. Agassiz and Mr. Wallace have found traces of glacial action in the valley of the Amazon—action of icebergs probably, not glaciers; for no one supposes that the valley of the Amazon, from the Andes to the Atlantic, was ever filled with a glacier (Alfred R. Wallace on Ice-marks in North Wales, 'Quarterly Journal of Science,' Jan. 1867).

M. Martins (in his article on "Les Glaciers actuels et la Periode Glaciaire," Revue des deux Mondes, March 1, 1867) objects to Mr. Croll's theory, that it would require the glacial periods of the two hemispheres to have occurred at different times, while geological evidence shows that they occurred at the same time. If this objection is valid against Mr. Croll's theory, it is equally valid against mine; for mine is, in fact, only Mr. Croll's inverted: his theory and mine place the glacial epochs of the opposite hemispheres in opposite periods of the same cycle.

I reply to this that geological evidence does not and cannot show whether a glacial period in the northern hemisphere and in the southern—in Scotland, for instance, and in Patagonia—were actually contemporary or separated by an interval of several thousand years. The period during which the excentricity of the earth's orbit is near its maximum is very long, several times 25,000 years. The precession of the equinoxes completes its cycle in 25,000 years, at the end of which time will recur the same position of the solstices with respect to the earth's perihelion and aphelion. According to my theory a glacial period occurs during the period of the greatest excentricity of the earth's orbit, at the time when the earth's aphelion is near the summer solstice; consequently it would occur in the same hemisphere after an interval of 25,000 years, and in the oppo-