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 hares and 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 Sciuravus, 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 Eocene 

 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- 



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