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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, 
hut 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 
he 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 Region.” 
+ Allen, J. A., “ North American Opossums.” 
