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 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 must be 
taken account of whenever we endeavour to explain the past 
