610 GEOGRAPHICAL DISTRIBUTION 
the Zoxodontia. 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 im 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 
