136 ZOOLOGICAL GEOGRAPHY. 



We here introduce four of the most characteristic mammalia of the great 

 American plains or prairies, three of them being types confined to IS'orth 

 America. The graceful animals on the left are the prong-horned antelopes 

 (Antilocapra americana), whose small horns, though hollow like those of 

 the antelopes, are shed annually like those of the deer. To the right we 

 have the prairie-dogs of the trappers (Cynomys ludovicianus) which, as will 

 be easily seen, are rodents, and allied to the marmots of the European 

 Alps. Their burrows are numerous on the prairies, and the manner in 

 which they perch themselves on little mounds and gaze on intruders, is 

 noticed by all travelers. On the left, in the foreground, is one of the ex- 

 traordinary pouched rats of America (Geomys bursarius). There are bur- 

 rowing animals, feeding on roots; and the mouth is, as it were, double, the 

 outer portion very wide and hairy, behind which is the small inner mouth. 

 Its use may be to keep out the earth from the mouth while the animal is 

 gnawing roots. A mouth so constructed is found in no other animals but 

 in these l!^orth American rats. In the distance is a herd of bisons (Bison 

 americanus) , the typical beast of the prairies. * * * 



The remaining part of this volume will consist of a systematic review 

 of the distribution of each family of animals, and an application of the 

 principles already established to elucidate the chief phenomena they pre- 

 sent. The present chapter must, therefore, be considered as the conclusion 

 of the argumentative and theoretical part of the present work ; but it must 

 be read in connection with the various discussions in Parts II. and III., in 

 which the conclusions to be drawn from the several groups of facts have 

 been successively given ; and especially in connection with the general ob- 

 servations at the end of each of the six chapters on the Zoological Eegiens. 



The hypothetical view, as to the more recent of the great geographical 

 changes of the earth's surface, here set forth, is not the result of any pre- 

 conceived theory, but has grown out of a careful study of the facts accu- 

 mulated, and has led to a considerable modification of the author's previous 

 views. It may be described as an application of the general theory of 

 Evolution, to solve the problem of the distribution of animals; but it also 

 furnishes some independent support to that theory, both by showing what 

 a great variety of curious facts are explained by its means, and by answer- 

 iag some of the objections, which have been founded on supposed difiicul- 

 ties in the distribution of animals in space and time. 



It also illustrates and supports the geological doctrine of the general 

 permanence of our great continents and oceans, by showing how many 

 facts in the distribution of animals can only be explained and understood 

 on such a supposition; and it exhibits, in a striking manner, the enormous 

 influence of the G-lacial epoch, in determining the existing zoological fea- 

 tures of the various continents. 



And, lastly, it furnishes a more consistent and intelligible idea than has 

 yet been reached by any other mode of investigation, of all the more im- 



