284 Correspondence—MUr. Alfred R. Wallace. 
CORRS Oia eae: 
DR. CROLL’S EXCENTRICITY THEORY. 
Str,—In your last number Mr. Searles V. Wood advances what he 
considers to be “the conclusive objection” to Dr. Croll’s theory of 
excentricity as a cause of the glacial epoch, viz. that North 
America was glaciated further south than Europe, in proportion to 
its present difference of winter climate, while Dr. Croll admits his 
theory ‘‘to be baseless unless there was a complete diversion of the 
warm ocean currents from the hemisphere glaciated.” 
I do not myself remember that Dr. Croll ever made such an 
admission, and it is certainly not necessary for the application of his 
theory. But whether there was a partial or a complete diversion of 
the Gulf-stream from the coasts of Europe, the result anticipated by 
Mr. 8. V. Wood—a complete similarity in the extension of ice over 
the two continents—was not be expected, because they are subject to 
very different conditions, independently of the action of ocean 
currents. 
EKurope is interpenetrated by seas having a southward opening, 
while the mass of land in Western Europe is trifling compared to 
that of North America. Transfer the Mediterranean to America and 
you have a sea entering south of Cape Hatteras, and extending quite 
across the continent to the Sierra Nevada of California, with north- 
ward branches reaching to Lake Huron! The influence of sucha 
sea receiving the waters of one of the largest tropical rivers (the 
Nile), together with the broken form of the western coast of Europe 
and the narrowness of the land, must be alone sufficient to give 
Western Europe an insular climate as compared with Hastern 
America. But at the same time we have on the American side 
conditions tending in the very reverse direction. The enormous 
ice-bearing masses of Greenland and Grinnell’s Land immediately 
to the north and north-east, and the Highlands of Labrador in the 
latitude of the Germanic plain, combined with the great cul-de-sac 
of Hudson’s Bay, to receive icebergs from the north, and pile them 
up in its southern inlet, almost in the latitude of London, must have 
tended to lower the climate of North America during the Glacial 
epoch as much as the Mediterranean and the Bay of Biscay must 
have ameliorated that of Europe. 
These causes of difference of climate depend on broad geo- 
graphical facts, which we have every reason to believe existed 
during the Glacial epoch as they do now, and they appear to me 
amply sufficient to account for the 10° or 12° further southward 
extension of the ice in America than in Europe, even if the Gulf- 
stream were “completely diverted.” But I do not believe it was 
completely, but only partially diverted and also diminished in 
intensity, and it therefore still exerted some differential action on 
the climates of the opposite coasts of the Atlantic. I would also 
point out that the difference between the latitudes of points with 
the same winter isothermals in West Europe and East America 
averages about 20°, which is much greater than the difference of the 
