I.] DISTRIBUTION OF GROUPS DISSIMILAR. 9 



migrations of the earlier forms must likewise probably have taken 

 place at an early epoch. 



With mammals the case is very different. The earliest known 

 forms, which date from the Triassic and Jurassic rocks, are chiefly 

 marsupials and forms apparently allied to the monotremes, and it 

 is probable that most of the descendants of these, as is more fully 

 indicated in the sequel, migrated southwards during the early part 

 of the Tertiary epoch, to find in Australasia a refuge from the 

 competition of higher forms. Of the higher placental mammals, 

 none of the modern types make their appearance before the 

 Oligocene and Miocene periods, while many do not antedate the 

 Pliocene. Their southern migrations accordingly took place later 

 on in the Tertiary period, one of the earliest movements being the 

 wandering of lemuroids, insectivores, and civet-like carnivores 

 into South Africa and Madagascar. On the other hand, many 

 other higher types, such as the hippopotami, giraffes, and antelopes, 

 which were abundant in Europe and southern Asia during the 

 Pliocene, only left their more northern homes to find a permanent 

 abiding place in Africa at a very late epoch in the earth's history. 



Although the glacial epoch probably had a large share in the 

 southern movements of the later Tertiary mammals, some cause 

 with which we are unacquainted would appear to have been the 

 impelling power at earlier epochs. But be this as it may, it is 

 quite evident that a continuous series of waves of migrations of 

 animal life has taken place throughout a very long portion of the 

 earth's history. Similar migrations are also evident in the case of 

 birds (which are likewise a modern group), many forms, such as 

 secretary-birds and trogons, now exclusively southern in their 

 distribution, being represented in Europe during the middle part of 

 the Tertiary period. 



From this inequality in the ages, and consequently in the date 

 of migration, of different groups of animals, it is 

 manifest that there will be great differences in the 



present distribution of such groups; and hence it different 



. . .. , Geographical 



will be evident that zoological provinces indicated Distribution. 

 by one group will not hold good for others. Notable 

 instances of this are afforded by the very different divisions into 

 which the globe is divided by those who take mammals, reptiles, 



