6io GEOGRAPHICAL DISTRIBUTION 



the Toxodontia. In other words, there is no real distinction 

 between adaptive and genetic characters. 



The second explanation assumes that the two discon- 

 tinuous types were at one time continuous, and that the 

 intermediate members have now died out. Upon the evo- 

 lution of a successful type it naturally spreads in every 

 suitable direction, and later, when the type has had its 

 day and becomes replaced by others, it dies out first in 

 the central areas where competition is fiercest but may 

 linger on in more remote parts. There is no doubt that 

 this is the actual course of events in many cases. Thus we 

 find traces of tapirs in Europe, India and North America. 

 Remains of lemurs are also found in Europe and North 

 ' America. 



The second point is with regard to the course of evolu- 

 tion. There is much evidence for assuming that the northern 

 hemisphere has been the scene of early mammalian evolu- 

 tion, and that a succession of mammalian types have radiated, 

 especially southwards, from this centre. The Prototherian 

 wave reached the southern limit in Australia, where it still 

 lingers. The Metatherian wave appears to have spread down 

 to Australia, Africa and South America. Extinguished in 

 Africa, it still lingers in America and has reached and passed 

 its climax in Australia. A third wave consists of the Eden- 

 tata^ the lowest of Eutheria. These also reached their 

 zenith in South America, where they still linger. Yet a 

 fourth wave, of more recent date, of the lemur type, lingers 

 in the outlying parts of South East Asia (Malay) and reaches 

 a climax in the isolated region of Madagascar. Finally, 

 the most "up-to-date" types of Ungulata^ Carnivora and 

 Rodentia are either at their world-wide zenith or have not 

 yet reached the outlying regions and extend mainly over the 

 Holarctic region. 



Lastly, we may recall the instances we have had of 

 *' oceanic " islands, like New Zealand, with no indigenous 

 mammals. By gradation we are led through types like 

 Madagascar, Ceylon, and others which have a fauna differing 

 in degree from that of the adjoining continent, till finally we 

 reach islands, such as Britain, which have a fauna usually 

 approximating closely to that of the mainland, though often 

 differing in quantity. Geological history usually gives us 



