ROSS ON EVOLUTION. 4ST 



tion thither of the Plants and Animals of North America, implying, 

 of course, a continuity of the Continents at that time. But while 

 North America has made little progress comparatively in the 

 differentiation of its Plants since the Eocene, it has been far different 

 on the Great Continent, which is consequently now far in advance of 

 North America, and though the differentiation of the higher Animals 

 in North America has been much greater comparatively than of 

 its Plants, yet in this respect also it is in every way inferior and 

 behind the Great Continent. South America may be said to 

 represent in a general way the Eocene of North America, and 

 Australia the Cretaceous of North America and the Cretaceous and 

 the Eocene of the Great Continent, while New Zealand with its 

 gigantic birds as the highest type, represents an earlier Mesozoic 

 Epoch, and the Gallapagos Islands with their gigantic Reptiles, 

 probably represent a still earlier Epoch. In each of these cases the 

 comparative cessation of progress referred to, seems to have been 

 the result of isolation from the then Great Continents — the chief 

 centre of progress and of differentiation, or in other words, of 

 progress upwards and of progress outwards, — outwards, not only 

 in space but in those adaptations which have given to each great 

 group representatives suited for every possible mode of existence. 

 South America has more recently been again united to North 

 America, but climatic causes have prevented a rapid migration of 

 North American types. 



These are a few typical illustrations of a principle illustrated 

 everywhere, since in fact every considerable Island or Archipelago 

 illustrates it, and even on the Continents, great mountain ranges, 

 deserts, &c. serve as barriers to the migrations of land Species, 

 and the Continents themselves to those which inhabit the sea, while 

 to those which inhabit the shallow waters, the ocean depths present 

 a barrier hardly less impassable than to land Species. It must be 

 remembered too that each great group has its own centres, and 

 subordinate groups theirs also, and that these all vary in position 

 with the varying changes of climate, elevation, &c. As a single 

 example of local centres for subordinate groups, the Humming-bird 

 may be given, of which more than a thousand Species inhabit 

 South America, though none are known ever to have existed out 



