Chap. XI. DURING THE GLACIAL PERIOD. 377 



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 

 warmest spots. But the great fact to bear in mind is, 

 that all tropical productions will have suffered to a cer- 

 tain extent. On the other hand, the temperate pro- 

 ductions, after migrating nearer to the equator, though 

 they will have been placed under somewhat new con- 

 ditions, 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 

 even crossed the equator. The invasion would, of course, 

 have been greatly favoured by high land, and perhaps 



