Chap. XI. 
DUKING THE GI/ACIAL PERIOD. 
377 
body of geological evidence, that the whole world, or a 
large part of it, was during the Glacial period simulta¬ 
neously much colder than at present. The Glacial 
period, as measured by years, must have been very 
long; and when we remember over what vast spaces 
some naturalised plants and animals have spread within 
a few centuries, this period will have been ample for 
any amount of migration. As the cold came slowly on, 
all the tropical plants and other productions will have 
retreated from both sides towards the equator, followed 
in the rear by the temperate productions, and these by 
the arctic; but with the latter we are not now con¬ 
cerned. The tropical plants probably suffered much 
extinction; how much no one can say; perhaps for¬ 
merly the tropics supported as many species as we see 
at the present day crowded together at the Cape of 
Good Hope, and in parts of temperate Australia. As 
we know that many tropical plants and animals can 
withstand a considerable amount of cold, many might 
have escaped extermination during a moderate fall of 
temperature, more especially by escaping into the lowest, 
most protected, and warmest districts. But the great 
fact to bear in mind is, that all tropical productions will 
have suffered to a certain extent. On the other hand, 
the temperate productions, after migrating nearer to the 
equator, though they will have been placed under some¬ 
what new conditions, will have suffered less. And it is 
certain that many temperate plants, if protected from 
the inroads of competitors, can withstand a much warmer 
climate than their own. Hence, it seems to me possible, 
bearing in mind that the tropical productions were in 
a suffering state and could not have presented a firm 
front against intruders, that a certain number of the 
more vigorous and dominant temperate forms might 
have penetrated the native ranks and have reached or 
