160 ISLAND LIFE part i 



of them, which we may suppose to have occurred when the 

 excentricity was considerably diminished, that exhibit any 

 indications of a climate at all warmer than that which now 

 prevails.^ 



Prohdble Date of the Glacial Epoch, — The state of extreme 

 glaciation in the northern hemisphere, of which we gave 

 a general description at the commencement of the pre- 

 ceding chapter, is a fact of which there can be no doubt 

 whatever, and it occurred at a period so recent geologically 

 that all the mollusca were the same as species still living. 

 There is clear geological proof, however, that considerable 

 changes of sea and land, and a large amount of valley 

 denudation, took place during and since the glacial epoch, 



^ In the Geological Magazine, April, 1880, Mr. Searles Y. Wood 

 adduces what he considers to be the " conclusive objection" to Dr. CroU's 

 excentricity theory, which is, that during the last glacial epoch Europe 

 and North America were glaciated very much in proportion to their 

 respective climates now, which are generally admitted to be due to the 

 distribution of oceanic currents. But Dr. Croll admits his theory **to 

 be baseless unless there was a complete diversion of the warm ocean 

 currents from the hemisphere glaciated," in which case there ought to be 

 no difference in the extent of glaciation in Europe and North America. 

 Whether or not this is a correct statement of Dr. CrolFs theory, the above 

 objection certainly does not apply to the views here advocated ; but as I 

 also hold the "excentricity theory" in a modified form, it may be as well 

 to show why it does not apply. In the first place I do not believe that the 

 Gulf Stream was "completely diverted" during the glacial epoch, but 

 that it was diminished in force, and (as described at p. 144) ^ar^Z2/ diverted 

 southward. A portion of its influence would, however, still remain to 

 cause a difference between the climates of the two sides of the Atlantic ; 

 and to this must be added two other causes — the far greater penetration 

 of warm sea- water into the European than into the North American conti- 

 nent, and the proximity to America of the enormous ice-producing mass 

 of Greenland. We have thus three distinct causes, all combining to 

 produce a more severe winter climate on the west than on the east of the 

 Atlantic during the glacial epoch, and though the first of these — the Gulf 

 Stream — was not nearly so powerful as it is now, neither is the difference 

 indicated by the ice-extension in the two countries so great as the present 

 difference of winter-temperature, which is the essential point to be con- 

 sidered. The ice-sheet of the United States is usually supposed to have 

 extended about ten, or, at most, twelve, degrees further south than it did 

 in Western Europe, whereas we must go twenty degrees further south in 

 the former country to obtain the same mean winter-temperature we find 

 in the latter, as may be seen by examining any map of winter isothermals. 

 This difference very fairly corresponds to the difference of conditions 

 existing during the glacial epoch and the present time, so far as we are 

 able to estimate them, and it certainly affords no grounds of objection to 

 the theory by which the glaciation is here explained. 



