914 GEOLOGY 



THE LIFE OF THE PLEISTOCENE PERIOD 



Destructive effects of glaciation. Glaciation was the groat 

 physical event of the Pleistocene period, and its effect on the life 

 of the times is the topic of chiefest interest in connection with the 

 life of the period. It is reasonable to believe that the successive 

 ice-sheets, several million square miles in extent, destroyed much 

 life, and caused great change in that which survived. The logic 

 is so cogent that we must believe it to be true; yet so far as the 

 record shows, the difference between the preglacial life and the post- 

 glacial is less than might have been anticipated. Thus more tha:i 

 half the known species of the marine Pliocene invertebrates arc 

 still living, whereas, in the transition between several of the more 

 ancient periods, nearly all species disappeared. Of the Pliocene 

 plant species, a very considerable percentage are still living. The 

 land vertebrates, on the other hand, were very generally replaced 

 by new species, and the same appears to have been true of the 

 insects. 



At the height of glaciation, the sum total of life on the globe 

 must have been greatly reduced. Even the re-expanded life of 

 to-day is probably inferior, quantitatively, to that of the middle 

 Tertiary. Not only this, but existing life is, on the whole, probal >ly 

 poorly adjusted to its surroundings, for it is improbable that, in 

 the millions of square miles where life was destroyed by the ice, 

 there has yet been worked out the best balance between the veget a- 

 tion and the soils and climate on which it depends, between the 

 carnivorous animals and the herbivores on which they prey, not 

 to speak of all the complicated minor relations that are involved 

 in a well-adjusted peopling of the earth. 



To-and-fro migration. An important biological effect of the 

 ice-sheets on life, was enforced migration in latitude. With every 

 advance of the ice, the whole fauna and flora of the affected region 

 had to move on in front of it, or suffer extinction. The arctic 

 species immediately adjacent to the ice border crowded upon the 

 sub-arctic forms next south of them, the sub-arctic forms crowded 

 upon the cold- temperate, and these in turn upon the warm-tem- 

 perate and so on. It is not unlikely that the limits of t he t ropical 



