SUMMARY OF THE LIFE OF THE PLEISTOCENE 375 



the biota was not seriously affected until the Sangamon interval. This author 

 says: " Until toward the close of third Interglacial Times no traces of northern 

 much less arctic forests and animals are discovered anywhere, excepting along 

 the borders of the ice fields. It would appear as if the animal and plant life 

 of Europe and America were, in the main, but slightly affected by the first three 

 glaciations. We cannot entertain for a moment the belief that in glacial times 

 all the warm fauna and flora migrated southward and then returned, because 

 there is not a shred of evidence for this theory. It is far more in accord with 

 the known facts to believe that all southern and eastern forms of life had be- 

 come very hardy, for we know how readily animals now living in warm earth 

 belts are acclimatized to northern conditions. " 



It is probable that Osborn here has in mind the biota living at the margin 

 or somewhat to the south of the ice front, because all the evidence indicates 

 that all life moved southward in the path of the advancing glacier or suffered 

 extinction. There was undoubtedly an unusual commingling of arctic, temper- 

 ate, and warm faunas and floras in the region south of the ice sheet and the 

 southern forms probably did become more or less acclimatized to a climate 

 colder than was normal for them; but when the ice sheet melted and retreated 

 the arctic and temperate animals undoubtedly migrated northward, as is 

 plainly indicated by the remains of these animals and plants in territory to 

 the north of the southern boundary of the till sheets (cf. Aftonian and 

 Yarmouth). 



Salisbury 15 sums up the effect of the Ice Age on the biota and his conclusions 

 seem to well describe the situation as it is indicated by the facts at present 

 known. 



''The great changes in the physical processes which this on-coming of the 

 ice-sheets brought into operation, effected corresponding changes in life and 

 in the processes which depend on life. In the first place, the total amount of 

 land life must have been greatly reduced. If account be taken of mountain 

 glaciation in both hemispheres as well as of the ice-sheets, it is probably within 

 the limits of truth to say that conditions became so far inhospitable as nearly to 

 eliminate land life from about one-seventh of the land of the globe, and to 

 have rendered conditions relatively inhospitable over a still larger area. . . 



"The crowding of land life off 8,000,000 square miles, more or less, must 

 have tended to concentrate it upon the land which still remained hospitable, 

 and to decimate or exterminate those forms which could not migrate readily. 

 ... It would seem, from the series of physical changes sketched, that very 

 profound changes in life should have followed, but it must be confessed that, 



u Outlines of Geologic History with especial Reference to North America, pp. 271-272. 

 References to marine conditions are omitted. 



