222 



ISLAND LIFE. 



[part I. 



archaic forms, we may mention the mud-fishes and the ganoids, 

 confined to limited fresh-water areas ; the frogs and toads, which 

 still maintain themselves vigorously in competition with higher 

 forms ; and among mammals the Ornithorhynchus and Echidna 

 of Australia ; the whole order of Marsupials — which, out of 

 Australia where they are quite free from competition, only 

 exist abundantly in South America, which was certainly long 

 isolated from the northern continents; the Insectivora, which, 

 though widely scattered, are generally nocturnal or subterranean 

 in their habits ; and the Lemurs, which are most abundant in 

 Madagascar, where they have long been isolated, and almost 

 removed from the competition of higher forms. 



Glimatal Revolutio'ns as an agent in producing Organic 

 Changes. — The geographical and geological changes we have 

 been considering are probably those which have been most 

 effective in bringing about the great features of the distribution 

 of animals, as well as the larger movements in the development 

 of organised beings ; but it is to the alternations of warm and 

 cold, or of uniform and excessive climates — of almost perpetual 

 spring in arctic as well as in temperate lands, with occasional 

 phases of cold culminating at remote intervals in glacial epochs, 

 — that we must impute some of the more remarkable changes 

 both in the specific characters and in the distribution of 

 organisms.^ Although the geological evidence is opposed to 

 the belief in early glacial epochs except at very remote and 

 distant intervals, there is nothing which contradicts the occur- 

 rence of repeated changes of chmate, which, though too small 

 in amount to produce any well-marked physical or organic 

 change, would yet be amply sufficient to keep the organic world 

 in a constant state of movement, and which, by subjecting the 

 whole flora and fauna of a country at comparatively short 

 intervals to decided changes of physical conditions, would 

 supply that stimulus and motive power which, as we have seen, 



1 Agassiz appears to have been the first to suggest that the principal 

 epochs of life extermination were epochs of cold ; and Dana thinks that 

 two at least such epochs may be recognised, at the close of the Palaeozoic 

 and of the Cretaceous periods — to which we may add the last glacial 

 epoch. 



