182 ORIGIN OF LIFE IN AMERICA 



newly emerged land which has been populated from the north 

 within recent geological times. The most noteworthy species 

 that might be mentioned is the Florida water vole (Microtus 

 alleni) . It is not alone the only North American water vole, 

 but also the sole member of the sub-genus Neofiber, and thus 

 forms a connecting link between the genera Microtus and 

 Fiber, that is to say, between the meadow voles and the 

 musk rats.* 



I have endeavoured to describe in almost every chapter the 

 range of some typically 'North American mammal. Some have 

 not yet been alluded to, among them the opossum. Two 

 species are now often distinguished in North America,f one of 

 which is supposed to extend its range into South America. As 

 a matter of fact the northern and southern forms are 

 extremely difficult to discriminate from one another, and 

 many authorities are of opinion that they merely represent 

 varieties of one species (Didelphys marsupialis) which thus 

 has an enormously extensive distribution in North and South 

 America. 



The existing marsupials, or pouched animals as we may call 

 them, are generally regarded as modern survivors of one of the 

 ancient groups of mammals, which apparently spread almost 

 all over the globe before the superior beasts of more recent 

 times had made their appearance. It is believed that at the 

 time -of their prime, Australia became separated from the 

 mainland of Asia, so that this southern continent now forms 

 the headquarters of the group, while in other parts of the 

 world they succumbed in the struggle with superior and better 

 fitted animals. Yet in America, where competition with the 

 more highly developed and more aggressive beasts ought to 

 be very keen, opossums, which belong to the marsupials, 

 are by no means on the verge of extinction. On the 

 contrary, they hold their own perfectly against the more 

 modern competitors. The genus Didelphys is much more 

 abundantly represented in South America than on the 

 northern continent, and other genera of marsupials occur 

 there besides opossums. To judge from these modern repre- 



* Bangs, Outram, " Mammals of Florida and Coast Eegion." 

 + Allen, J. A., " North American Opossums." 



