THE RATE OF ORGANIC CHANGE 229 



of climate, since they afforded the means, at long intervals, 

 of bringing the most diverse forms into competition, and 

 of spreading all the great animal and vegetable types 

 widely over the globe. But the isolation of considerable 

 masses of land for long periods also afforded the means 

 of preservation to many of the lower types, which thus 

 had time to become modified into a variety of distinct 

 forms, some of which became so well adapted to special 

 modes of life that they have continued to exist to the 

 present day, thus affording us examples of the life of early 

 ages which would probably long since have become extinct 

 had they been always subject to the competition of the 

 more highly organised animals. As examples of such 

 excessively 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. 



Climatal Eevohttions 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 culmin- 

 ating 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.^ 

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



