GEOGEAPHICAL ISOLATION 331 



area is proportional to the length of time for which and the 

 extent to which that area has been isolated from other areas. 

 Thus Australia, which has probably been separated from the 

 Asiatic continent ever since the Cretaceous period, has a most 

 peculiar fauna and flora. We have already referred to the 

 numerous different kinds of marsupials kangaroos, wombats, 

 phalangers, native bears, native cats and so forth which have 

 not as yet been supplanted by the more recently developed 

 groups of mammals found in other parts of the world. Australia 

 is also still the home of those most primitive and reptile-like 

 of all the mammals, the Monotremata (Figs. 91, 92). The 

 Australasian forests, again, are composed principally of eucalypts 

 of many different species, which are found nowhere else in the 

 world. In New Zealand, which is even more isolated than 

 Australia, we find no less peculiar inhabitants, including the 

 wonderful tuatara (Fig. 113), the oldest surviving type of terrestrial 

 vertebrate, together with the kiwi (Fig. Ill) and other remarkable 

 flightless birds. 



The reasons why the degree of peculiarity of the fauna and 

 flora of any region is proportional to the degree of geographical 

 isolation are not difficult to find. On the one hand ancient types, 

 such as the tuatara, the monotrenies and the marsupials, may be 

 preserved from competition with more modern forms long after 

 they have been exterminated elsewhere. On the other hand, indivi- 

 duals accidentally introduced from distant areas at rare intervals 

 will have few opportunities of breeding with others of the same 

 species, and thus whatever variation occurs amongst them will 

 be less liable to be swamped by intercrossing with the parent 

 form. New races and ultimately new species will thus become 

 established more readily in such areas than elsewhere. This 

 principle of geographical isolation as a factor in the production 

 of new species is of great importance and we shall have to refer 

 to it again in a subsequent chapter. 



The zoological or botanical affinities of the inhabitants of any 

 given area, not only with one another but also with those of 

 adjacent areas, are exactly what we should expect in accordance 

 with the views which we are advocating. It is impossible to 

 believe that the existing marsupials were (with the exception of 

 the few American species) all specially created in Australasia 

 when we know perfectly well that marsupials used to exist in 

 Europe in past geological times and can still exist in Europe 



