CHAPTER VII 



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 

 — Palaeontological Evidence of Alternate Cold and Warm Periods — 

 Evidence of Intcrglacial 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 revolu- 

 tions 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 has 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 



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