392 DARWINISM chap. 



any kind of special creation, or by sudden advances of organisa- 

 tion in the offspring of preceding types, such close relationship 

 would not be found ; and facts of this kind become, therefore, 

 to some extent a test of evolution under natural selection or 

 some other law of gradual change. Of course the relationship 

 will not appear when extensive migration has occurred, by 

 which the inhabitants of one region have been able to take 

 possession of another region, and destroy or drive out its 

 original inhabitants, as has sometimes happened. But such 

 cases are comparatively rare, except where great changes of 

 climate are known to have occurred ; and we usually do find 

 a remarkable continuity between the existing fauna and flora 

 of a country and those of the immediately preceding age. A 

 few of the more remarkable of these cases vnW now be briefly 

 noticed. 



The mammalian fauna of Australia consists, as is well 

 known, wholly of the lowest forms — the Marsupials and Mono- 

 tremata — except only a few species of mice. This is accounted 

 for by the complete isolation of the country from the Asiatic 

 continent during the whole period of the development of the 

 higher animals. At some earlier epoch the ancestral mar- 

 supials, which abounded both in Europe and North America 

 in the middle of the Secondary i:)eriod, entered the country, 

 and have since remained there, free from the competition of 

 higher forms, and have undergone a special development in 

 accordance with the peculiar conditions of a limited area. 

 While in the large continents higher forms of mammalia have 

 been developed, which have almost or wholly exterminated the 

 less perfect marsupials, in Australia these latter have become 

 modified into such varied forms as the leaping kangaroos, the 

 burrowing wombats, the arboreal phalangers, the insectivorous 

 bandicoots, and the carnivorous Dasyurida3 or native cats, 

 culminating in the Thylacinus or " tiger- wolf " of Tasmania — 

 animals as vmlike each other as our sheep, rabbits, squirrels, 

 and dogs, but all retaining the characteristic features of the 

 marsupial type. 



Now in the caves and late Tertiary or Post-Tertiary deposits 

 of Australia the remains of many extinct mammalia have been 

 found, but all are marsupials. There are many kangaroos, 

 some larger than any living species, and others more allied to 



