THE DISTRIBUTION OF ANIMALS AND PLANTS 403 



Greenland and Spitzbergen.^ The age in which we hve is 

 thus one of mediocrity, attaining neither to the Arctic rigour of 

 the later Pleistocene, nor to the universal mildness of the 

 preceding Miocene. 



The causes of these changes of climate we have discussed 

 elsewhere. It remains for .us now to consider the actual 

 condition of our present continents, and the bearing of past 

 conditions on the distribution of their living inhabitants. 



In speaking of continents and islands, it may be as well to 

 remark at the outset that all the land existing, or which 

 probably has at any time existed, consists of islands great 

 or small. It is all surrounded by the ocean. Two of the 

 greater masses of land are, however, sufficiently extensive to 

 be regarded as continents, and from their very extent and 

 consequent permanence may be considered as the more special 

 homes of the living beings of the land. Two other portions 

 of land, Australia and the Antarctic polar continent, may be 

 regarded either as smaller continents or large islands, but 

 partake of insular rather than continental characters in their 

 animals and plants. All the other portions of land are pro- 

 perly islands; but while these islands, and more especially 

 those in mid-ocean, cannot be regarded as the original homes 

 of many forms of life, we shall find that they have a special 

 interest as the shelters and refuges of numerous very ancient 

 and now decaying species. 



The two great continents of America and Eurasia have been 

 the most permanent portions of the land throughout geological 

 time, some parts of them having always been above water, 

 probably from the Laurentian age downward, though at various 

 times they have been reduced to little more than groups of 

 islands. On them, and more especially in their more northern 



1 As I have elsewhere shown, a warm climate in an Arctic region seems 

 to have afforded the necessary conditions for the great colonizing floras of 

 all geological periods. 



