598 CENOZOIC ERA— AGE OF MAMMALS. 



other factor of change ; and doubtless partly also by the more active 

 operation of other factors of change, which we do not yet understand. 

 This last cause tends to produce not only more rapid general evolution, 

 but also to destroy extreme geographical diversity ; and since it oper- 

 ates on animals rather more than plants, plant species are more apt to 

 be local, and are less certainly carried along with the stream of general 

 evolution, and are, therefore, less reliable in determining geological age 

 than animals. 



4. Ee-isolations in new positions. This would again produce diver- 

 gence of geographical faunas and floras increasing with time, as long 

 as the isolation continued. Thus it is seen that geographical diversity 

 is a product of three factors — viz., difference of environment, isolation, 

 anal time. 



The last of these critical periods was the Quaternary. Therefore in 

 the changes of physical geography and climate of this period we find 

 the main cause of the present distribution of species ; and, conversely, 

 this distribution furnishes the key to the geographical changes and the 

 direction of migrations during the Quaternary. 



The principles enumerated above are so important, that some ex- 

 amples illustrating seem necessary. 



1. Australia. — The fauna and flora of Australia are the most pe- 

 culiar known anywhere. Confining our attention to mammals ; of about 

 one hundred and thirty species known in Australia, all except two or 

 three bats and rats (of all animals the most likely to be accidentally 

 introduced) are non-placentals, i. e., marsupials and monotremes. And, 

 moreover, with the exception of several opossums in America, North 

 and South, non-placentals are not found anywhere except in Aus- 

 tralia and neighboring islands. The explanation is as follows : Of all 

 countries, Australia has been the longest isolated from all other conti- 

 nents. The wide migrations of the Quaternary which mingled the 

 faunas and floras of other continents did not reach this one. It will 

 be remembered that, in Jurassic times, marsupials in great numbers 

 inhabited Europe and America, and doubtless all other great conti- 

 nents, Australia among the number. It will be remembered also that 

 true placental mammals were not introduced until the Tertiary. It is 

 evident, then, that Australia was separated before the Tertiary, and has 

 been isolated ever since. The severe struggle which determined the 

 evolution of placentals elsewhere did not affect that continent. Pla- 

 cental were not evolved there, nor could they get there from abroad. 



2. Africa. — The mammalian fauna of Africa, south of Sahara, as 

 shown by Wallace (Island Life), consist of two very distinct groups — 

 viz., a group of small animals of very generalized type — insectivores and 

 lemurs ; and a group of large, powerful, and highly-specialized animals 

 — carnivores and herbivores. The animals of the former group are 



