592 CENOZOIC ERA— AGE OF MAMMALS. 



epochs many times in the history of the earth. Another, according 

 to Croll, is the alternation of colder and warmer periods many times 

 during every period of greatest eccentricity, and a similar alternation 

 of each between the two poles, so that the cold period at one pole cor- 

 responds with the warm period at the other. Of alternations of colder 

 and warmer periods during the Glacial epoch there are many evidences 

 both in Europe and America, but there is no evidence that these were 

 so numerous (seven or eight) as the theory requires. Of the recurrence 

 of many Glacial epochs in the history of the earth there is as yet no 

 reliable evidence, but much evidence to the contrary. It is true that 

 what seem to be Glacial drifts with scored bowlders, etc., have been 

 found on several geological horizons, but these are usually in the vicin- 

 ity of lofty mountains, and are probably, therefore, evidences of local 

 glaciation, not of a Glacial epoch. On the other hand, all the evidence 

 derived from fossils plainly indicates warm climates even in polar re- 

 gions during all geological periods until the Quaternary. The evi- 

 dence at present, therefore, is overwhelmingly in favor of the unique- 

 ness of the Glacial epoch. This fact is the great objection to Croll's 

 theory. 



Mr. Wallace has attempted to remove this objection by modifying 

 Croll's theory. He substantially accepts Croll's view, but thinks that 

 astronomical changes alone will not produce a Glacial epoch, but must 

 be coincident with geographical changes favoring the same result. He 

 maintains that, until the Quaternary, geographical conditions favored 

 warm, uniform climates, especially by several open current- ways from 

 tropical to polar seas, notably one from the Indian Ocean through 

 Western Asia; and that at the beginning of the Quaternary these 

 warm currents were cut off by northern elevation, which we know oc- 

 curred at this time, while the elevation itself would tend still further 

 to increase the cold. The Glacial epoch was therefore the result of 

 several causes, astronomical and geographical, viz., aphelion winter, 

 maximum eccentricity, and northern elevation. On this view it is easy 

 to see why there should have been but one Glacial epoch. 



Furthermore, Mr. Wallace thinks that, during a period of maximum 

 eccentricity, the great accumulation of ice, once effected, would tend to 

 self -perpetuation, i. e., would conserve the cold and tide over the shorter 

 precessional cycles, so that these would have but little effect, and hence 

 there would not be, and in fact were not, seven or eight alternations of 

 cold and hot periods during the Glacial epoch, as Croll thinks. There 

 was but one interglacial period, and this was determined by changes in 

 eccentricity, as seen in Fig. 964. 



Mr. Wallace has certainly put the astronomic theory in its best 

 form, and his view may be regarded as probable if we admit that 

 among his concurring causes the geographical are the most important. 



