134 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Very little is known of the Pleistocene in the West Indies, 

 though probably to this date should be assigned the notable 

 oscillations of level which are recorded in the raised sea- 

 terraces of Cuba and other islands. The Windward groups 

 were joined, at least in part, to the continent and large extinct 

 rodents reached Antigua, which would not be possible under 

 present conditions. The Isthmus of Panama was 200 feet 

 or more higher than it is now and correspondingly wider, but 

 was depressed to a lower than the present level, and finally 

 raised to the height it now has. Marine beds, of presumably 

 Pleistocene date and certainly not older, extend from the 

 Caribbean shore to Gatun, some seven miles, and are nowhere 

 more than a few feet above sea-level. 



The question of Pleistocene climates is a very vexed one 

 and is far from having received a definitive answer. Limita- 

 tions of space forbid a discussion of the problem here and I 

 shall therefore merely state the conclusions which seem best 

 supported by the evidence so far available. Such immense 

 accumulations of ice might be due either to greatly increased 

 snow-fall, or to a general lowering of the temperature. The 

 balance of testimony is in favour of the latter factor and no 

 great refrigeration is required. Professor Penck has calcu- 

 lated that a reduction of 6° or 7° in the average yearly tem- 

 perature would restore glacial conditions in Europe. Even 

 the tropics were affected by the change, as is shown not only 

 by the glaciation of the Andes, but also by Mt. Kenya, which 

 is almost on the Equator in eastern Africa and still has glaciers. 

 The presumably Pleistocene ice covered the whole mountain 

 like a cap, descending 5400 feet below the present glacier 

 limit. It was pointed out above that the interglacial stages 

 had greatly ameliorated climatic conditions and that, in some 

 of them at least, the climate was warmer than it is to-day in 

 the same localities. The cause of these astonishing fluctuations 

 and of the climatic changes in general, to which Geology 

 bears witness, still remains an altogether insoluble mystery. 



