South America has an astonishingly rich and varied assemblage of rodents, both indigenous and immigrant, but the former are much the more important, varied and abundant. Of the four divisions of the order, all of which are represented, three are immigrants from the north and the fourth is autochthonous, but this far outnumbers the other three combined. The hares and rabbits have but very few species, one of which occurs in Brazil and is separated by a very wide interval from the one in Costa Rica, while the pikas are absent. Of the squirrel division, only the true squirrels are found, and of these there are many species, the ground-squirrels, marmots, prairie-dogs and beavers all being lacking. In the same way the rat and mouse division is represented by a single family. The vesper or white-footed mice (_Sitomys_) have invaded the southern continent and a number of peculiar genera have arisen there, but all of northern ancestry, such as the groove-toothed mice (_Rheithrodon_) and the fish-eating rats (_Ichthyomys_). The voles, or meadow-mice, the muskrats, jumping mice, kangaroo-rats and pocket-gophers of the northern continent are all absent. While the immigrant suborders have thus but one family each in South America, the case is very different with the fourth or porcupine group, of which that continent is to-day, as it has been for ages past, the headquarters. No less than six families and twenty-nine genera are known, all of the genera and four of the families being restricted to the Neotropical region. Contrast this assemblage with the extreme scantiness of this group in North America, where but a single genus, the Short-tailed or Canada Porcupine (_Erethizon_) represents it, and that is a late immigrant from the south. [Illustration: FIG. 104.—Neotropical rodents. _A._ Vizcacha (_Viscaccia_). _B._ Paca (_Agouti paca_). _C._ Rock Cavy (_Cavia rupestris_). _D._ Water-Hog, or Carpincho (_Hydrochærus_). _D_, by permission of the N.Y. Zoölog. Soc. _A_, _B_, _C_, by permission of W. S. Berridge, London.] [Illustration: FIG. 105.—Chinchilla (_Chinchilla laniger_).—By permission of W. S. Berridge, London.]