376 



LIFE OF THE PLEISTOCENE 



in spite of the conditions which it would seem must have been favorable for 

 great destruction of life, and for imposing great modifications upon that which 

 survived, statistical evidences of the changes which followed are less impressive 

 than would have been expected. The data at hand point to extensive migra- 

 tions, but not to exterminations and profound modifications which might have 

 been expected. It seems impossible to think that the changes of climate which 

 drove musk oxen to Kentucky and Virginia, and Arctic plants and reindeer 

 to the lowlands of central Europe and to the Mediterranean, were without very 

 profound biologic significance, unless the life of the earth had reached a condi- 

 tion of far greater stability than that of earlier times, when lesser physical 

 changes seem to have produced greater biological changes. " 



Table showing Distribution of Life during each Interglacial Interval 



Interglacial Interval 



u 



W 



"Si 



G 



OS 



"3 



o 



< 



.a 



3 



o 



g 



a 



o 

 £ 



a 



c 



o 

 <u 

 Ph 



J3 



"3 



o 

 H 



Plants 



7 



6 

 



104 

 

 

 4 

 2 



87 



23 



4 

 1 



14 

 

 1 



4 



2 



79 



12 



50 

 

 

 

 



1 







24 



14 



91 

 

 

 1 

 

 

 



18 



68 



132 

 2 



85 

 

 

 1 

 



25 







66 

 

 

 

 

 

 



24 



66 



244 

 2 

 9 



2 

 

 



1 

 21 



145 



Animals 



Mollusca 



298 



Crustacea 



4 



Insecta 



108 



Pisces 



3 



Amphibia 



1 



Reptilia 



6 



Aves 



3 



Mammalia 



117 







Total Animals 



203 

 210 



105 

 128 . 



75 

 87 



110 

 124 



245 

 313 



90 

 90 



279 

 345 



540 



Total Life 



685 







It is apparently in accord with the ascertained facts, as indicated by the 

 fossil remains found between the till sheets, and listed in this work, to believe 

 that at each interglacial interval there was first a migration northward into 

 territory laid bare by the retreating ice and later a southward migration as the 

 climate changed with the on-coming of the new glaciaton ; that this to and fro 

 migration was repeated four times and is now in its fifth northward migration 

 period; and that the effect of such forced migrations has been to cause the ex- 

 tinction of many species of insects and mammals, while causing little change in 

 plants and mollusks. There were probably other causes contributary to the 

 extinction of the insects and mammals, but we must believe, from the strati- 

 graphical evidence, that the physical changes brought on by the Glacial Period 

 contributed largely to the dying out of so many species of these two highly 



