A NATURALIST IN BRAZIL 



variety of species, while its great masses of land gave them oppor- 

 tunities of migration. But if this was the most prolific source of life, 

 room had to be found for its creations, and the earlier forms were 

 thrust towards the outer limits of the hemisphere. The older species 

 withdrew before the newer; the oldest were driven furthest; and 

 this explains why to-day we find the most ancient species at the 

 southern extremities of Africa and America, and also in Australia 

 and New Zealand — that is, in the regions of the globe most remote 

 from the focus of their emergence. 



One may ask why the older forms of animal life did not defend 

 the soil which was their due, and drive the newer species from their 

 proper home. But it would seem that youth confers greater energy, 

 and that fresh modes of attack break down the firmest resistance. 

 Just as European man has spread over the whole earth, driving back 

 or expelling the indigenous populations in Africa, America and 

 Australia, so twelve species of European snakes have established 

 themselves in North America, but not a single American snake has 

 made its way to Europe. In Australia the two indigenous beasts of 

 prey, of the order of the marsupials — the Marsupial Wolf and the 

 Tasmanian Devil — have had to give way to the Dingo, the yellow 

 wild dog introduced by man — -just as the indigenous Australian 

 song-birds are giving way before those of their European relatives 

 which have been introduced by the white settlers. In Rio, too, the 

 pretty native Bunting, the Ticotico, is retreating before the imported 

 European sparrow. 



If now we follow the traces of these vast migrations, if we 

 scrutinize the skeletal remains which tell us of their paths, we are 

 obliged to assume that there was once land where to-day the ocean 

 rolls, and that South America, in the early Tertiary period, when 

 the principal mammalian types emerged, was connected with the 

 northern continents of North America, Europe, Africa and Asia. 

 For the mammals, being for the most part genuine land animals, 

 could not swim across wide areas of sea ; if they spread to other 

 continents, it could only be by land. 



Since we cannot search the bottom of the sea for the relics of the 

 Tertiary period, we must fall back on hypotheses if we wish to recon- 

 struct the old connecting-links between the continents. Of such 

 hypotheses there are many. One supposes a great continent filling 

 the southern hemisphere, and including South America, South 

 Africa and Australia. Recently the opinion has emerged that South 

 America may, so to speak, have broken away from Africa, since its 

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