OF MAMMALIA. 609 
In conclusion, we may touch upon a few special points. 
The first of these is the phenomenon. of discontinuous 
distribution (cf. p. 64). All mammalian species are found in 
continuous or contiguous areas, but the different species of 
a genus may in certain instances occur in widely separated 
areas A good example, usually quoted, is that of the 
tapirs, which are found in Malay and South America. Dis- 
continuous distribution of a family is also fairly common ; 
we may instance the Zvaguid@e or Chevrotains of Africa 
and India. Of a discontinuous order, we may instance the 
Diprotodontia, which has one family (opossum-rats) in 
America and the rest in Australia, and a similar case in the 
Polyprotodontia, with the opossums in South America. An 
instance of much the same kind is the distribution of the 
Primates, the lemurs being found in Madagascar and Africa, 
on the one hand, and in Further India and Malay, on the 
other, and the Axshropoidea occurring in America, Africa 
and India. 
There are two possible explanations of this phenomenon. 
The first is based on the assumption that the discontinuity 
is fundamental and that the genera, families, or orders have 
been separately evolved from the same earlier ancestors, 
their resemblances being due to parallel or convergent 
evolution. As an instance of this we may quote the Anthro- 
potdea. It is quite conceivable that the New World monkeys 
and those of the Old World have been separately evolved 
from primitive types which were not monkeys. The 
same applies to the Diprotodontia, which may have been 
separately evolved from Polyprotodontia. There is very 
strong evidence for supposing that horses and rhinoceroses 
were independently evolved from primitive ungulates in each 
hemisphere. 
Without entering into the question of the polyphyletism 
of the class Mammata—by which we mean the separate 
evolution of mammalian types from pre-existent amphibio- 
reptiles—we may note that this very highly differentiated 
class would lend itself more than any other to the phenom- 
enon of parallel evolution. Rodentia are specially distin- 
guished as an order by their peculiar incisor-dentition, yet 
the same modification is found in the Dasse (Hyracoidea), 
the Aye-Aye (Lemuroidea), the Wombat (Diprotodontia) and 
M. 40 
