CHAPTER VIL 



CHANGES OF CLIMATE WHICH HAVE INFLUENCED THE 

 DISPERSAL OF ORGANISMS : THE GLACIAL EPOCH. 



Proofs of the recent occurrence of a Glacial Epoch — Moraines — Travelled 

 Blocks — Glacial deposits of Scotland : the " Till " — Inferences from the 

 glacial phenomena of Scotland — Glacial phenomena of North America 

 — Effects of the Glacial Epoch on animal life — Warm and cold periods 

 — Palasontological evidence of alternate cold and warm periods — 

 Evidence of interglacial warm periods on the Continent and in North 

 America — Migrations and extinctions of Organisms caused by the 

 Glacial Epoch. 



We have now to consider another set of physical revolutions 

 which have profoundly affected the whole organic world. 

 Besides the wonderful geological changes to which, as we have 

 seen, all continents have been exposed, and which must, with 

 extreme slowness, have brought about the greater features of 

 the dispersal of animals and plants throughout the world, there 

 have been also a long succession of climatal changes, which, 

 though very slow and gradual when measured by centuries, may 

 have sometimes been rapid as compared with the slow march of 

 geological mutations. 



These climatal changes may be divided into two classes, which 

 have been thought to be the opposite phases of the same great 

 phenomenon — cold or even glacial epochs in the Temperate zones 

 on the one hand, and mild or even warm periods extending into 

 the Arctic regions on the other. The evidence for both these 

 changes having occurred is conclusive ; and as they m.ust be 

 taken account of whenever we endeavour to explain the past 



