1869.] MUBPHT GLACIAL CLIMATE. 355 



was not one of great heat, but moderate and moist. K so, the gla- 

 cial 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 ice- 

 bergs, 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 be- 

 tween the two temperate zones, which must have crossed the equa- 

 tor 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 pro- 

 bably, 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 Sci- 

 ence,' 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 ob- 

 jection is valid against Mr. CroU's theory, it is equally valid against 

 mine ; for mine is, in fact, only Mr. CroU'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 pre- 

 cession 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 ex- 

 centricity 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- 



