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 Tragulidce or Chevro tains 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 

 Folyprotodontia^ 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 Anthropoidea 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- 

 poidea. 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 

 w^ere independently evolved from primitive ungulates in each 

 hemisphere. 



Without entering into the question of the polyphyletism 

 of the class Mammalia — 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. Rode?itia 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 {Lemi^roidea), the Wombat {Diprotodontia) and 

 M. 40 



