
MAMMALS OF MINNESOTA. 153 
hordes of emigrants, which, in spite of the normally nocturnal 
habits of the animal, pressed on in a solid phalanx, harried on 
all sides by hawks and wolves, crossing rivers and facing death 
in a hundred forms, driven by the fiat of necessity, and thus 
demonstrating the Malthusian principle as applied at least to 
mice. The rodents attain the maximum development in South 
America. Thirty-two of the thirty-seven genera are restricted 
to that continent. The haresand squirrels constitute the most 
universally distributed families. Africa, similarly, is rich in 
endemic forms, while even the island of Madagascar has its 
peculiar rodent fauna. The distribution in the circumpolar 
continent is more general and presents fewer exceptional fea- 
tures. Eleven or so of the twenty-four North American genera 
are peculiar to this continent, and most of the restrictions and 
limitations are such as may be accounted for by the physical fea- 
tures of the land. The mice are found in all continents, even 
Australia having representatives. The hares and squirrels are 
found on all other continents, and are rather close families. 
East India is poorest in rodents, and for no obvious reason, so 
that we are forced to seek the explanation of this and other 
anomalies in the historical development of the order. Repre- 
sentatives of the genus Myoxus, and the squirrels have been 
found among Eocene fossils in Europe, and the genera continue 
to the present time. The Eocene of Wyoming affords remains 
of Paramys and Sciwravus, and in the upper Eocene the mar- 
mot-like Plesiarctomys. 
Other species very imperfectly known are referred to unchar- 
acterized genera, as Colonymys, Taxymys, Tillomys, Mysops, 
Heliscomys, etc. Enough, at least is known to indicate a 
numerous line of successors to the early Eocene rodents and to 
convince us that the various families were early differentiated. 
Mice, squirrels and porcupines have existed since the Hocene— 
that period so marvelously productive of new mammalia. 
The Miocene was the period of greatest development of the 
type, and it is claimed that at that time some genera now 
restricted to the Americas roamed over Europe. The numer- 
ous recent discoveries of paleontology leave us quite in doubt 
as to the primitive source of the rodent type, beyond the vague 
suggestion that the earliest rodent was probably a marsupial— 
a convenient way of dissembling sheer ignorance. 
It would be interesting did our limits permit to compare 
the curious extremes of structure and variations in habit exhib- 
SMe, 
