THE NEOTROPICAL REGION 55 



so favourable for mammalian development as the more 

 open and varied country of Africa. 



This deficiency in mammalian life is, however, more 

 than counterbalanced by the abundance of other groups 

 of animals, more especially of birds and insects, to the 

 development of which the luxuriant tropical vegetation 

 seems to be especially conducive. 



Again, the mammalian fauna of the Neotropical Region 

 is quite as remarkable for what it does not possess (lipotypes) 

 as for what it has. Everything points to the conclusion 

 that during a long geological age, probably throughout the 

 greater part of the Tertiary epoch, South America was 

 entirely isolated from the rest of the world. Thus the 

 present fauna has arisen from two quite different sources 

 — first, from the original fauna of early Tertiary times; 

 and, secondly, from immigrants from the north, some 

 of these being of rather long standing, and others of 

 later arrival. 



Of the nine Orders of Terrestrial Mammals, repre- 

 sentatives of eight occur in the Neotropical Region, the 

 only Order entirely absent being the Monotremes, which 

 are absolutely confined to Australia. 



The Marsupials are represented in the Neotropical 

 Region by a single family only, 1 out of seven into which 

 this order is usually divided. This is the Didelphyidee, or 

 Opossums, of the twenty-four generally recognised species 

 of which one (Didelphys marsupialis) ranges north with 

 some modification into the Nearctic Region. The Quica 

 Opossum (D. opossum) (Fig. 7, p. 56) is another well-known 



1 Since this was written Mr. Thomas has described his wonderful new 

 South-American genus Ccenolestes, which seems to belong to the Australian 

 Diprotodonts. See P. Z. S., 1895, p. 870. 



