178 



THE GNAWERS OR RODENTS. 



jerboas, and the pikas are represented in the 

 Old World and in the colder parts of North 

 America ; the warmer zones of the Old World 

 and South America have the octodons and 

 the spiny rats, which are chiefly developed 

 in America. On the other hand, all the Sub- 

 ungulata, as well as the genus Lagidium, are 

 purely of South American origin, while the 

 pouched rats and the tree porcupines are 

 natives of North and South America. The 

 New World thus excels the Old in the num- 

 ber, richness, and diversity of its types, and 

 manifestly South America is the most favoured 

 region of all. 



This remarkable distribution of the rodents 

 cannot be explained by any peculiarities of 

 climate, food, and so forth, such as are usually 

 erroneously regarded as the dominant factors 

 in determining the distribution of faunas. 

 The poverty of the East Indies in rodents 

 has sometimes been attributed to the large 

 number of small beasts of prey there met 

 with, and to the uniformity of the conditions 

 of existence, which is held to have been un- 

 favourable to the development of varieties. 

 It is not easy to understand how such a con- 

 clusion can ever have been arrived at in face 

 of the facts observed in South America, where 

 the rodents are at once most numerous and 

 most varied, notwithstanding the uniformity 

 of the climatic conditions, and notwithstand- 

 ing the abundance of terrestrial and winged 

 enemies which are found there also. 



These differences in distribution, on which 

 we have so often laid special stress, must ac- 

 cordingly be due to another cause, and this 

 can only lie in the origin or descent of the 

 different types. The geographical distribu- 

 tion points us first of all to various centres 

 from which the rodents must have spread; 

 and these centres are unquestionably the two 

 Americas, South Africa, and the Eurasian 

 continent. During the process of distribution 

 over the earth the rodents have naturally 

 undergone greater or less modifications. We 

 must accordingly seek the key to explain the 



phenomena observed at the present day in 

 the facts of palaeontology. 



The Upper Eocene has yielded in Europe 

 the remains of two genera still living, Myoxus 

 and Sciurus, both of which have continued 

 without interruption to the present day. It 

 is a rather notable fact that of these two 

 earliest known genera one has tubercled teeth 

 and the other enamel ridges on the crown, 

 while in both the teeth have roots. It is 

 also worthy of note that the dormice, notwith- 

 standing their occurrence in very remote 

 epochs, have remained confined to the Old 

 World, while the squirrels, which at the pre- 

 sent day are remarkably abundant in the New 

 World, have hitherto yielded no remains in 

 America older than the Quaternary strata. 

 On the other hand, we find in the Eocene of 

 Wyoming two extinct genera, Paramys and 

 Sciuravus, the latter of which, as the name 

 itself indicates, is closely allied to the true 

 squirrels; and we find, moreover, that there is 

 in the Upper Eocene a genus allied to the 

 marmots, Plesiarctomys. The mice are re- 

 presented in the Eocene of America by the 

 extinct genus Myops, and a genus called 

 Megamys, belonging to the family of the 

 Octodontida, is mentioned as belonging to the 

 South American Eocene, together with doubt- 

 ful remains referred to the genus Arvicola 

 (the voles). The European genus Theridomys, 

 found in the Upper EoceneofAuvergne, would 

 be regarded as the ancestor of the spiny rats. 



Mice, dormice, squirrels, octodons, and 

 spiny rats would thus all have their ancestors 

 in the Upper Eocene. 



In the Miocene we find a greater wealth of 

 families and genera; only the mice make an 

 exception in this respect, they remaining 

 almost stationary. The jerboas first appear 

 in Europe, represented by the extinct genus 

 Issyodrimys; the beaver family makes its 

 appearance with the genera Castor (the true 

 beavers), Chalicomys, and Steneofiber in Eu- 

 rope (the two latter now extinct), and with 

 the extinct Palaeocastor in America. In India 



