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. 
Climatal Revolutions 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 . 1 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 climate, 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 he recognised, at the close of the Palaeozoic 
and of the Cretaceous periods — to which we may add the la3t glacial 
epoch. 
