THE PAL^E ARCTIC REGION 195 



{Cervidte), which now commence to be very much more 

 prominent. In the true Sivalik fauna of India there are 

 a good many types which have never yet been found in 

 Europe; such, for instance, as the Camels, which are 

 specially characteristic of the American Tertiary strata. 

 Furthermore, there are found, in the American formations 

 of this age, a large number of forms, such as Bos, Equus, 

 Hippopotamus, and Ursus, which do not appear at all in 

 Europe until the later Pliocene times. 



When the Pliocene times arrive, we begin to find a 

 preponderating number of still existing genera present in 

 the fossil beds, although the greater number of them have, 

 at the present epoch, retreated southwards into the 

 Oriental and Ethiopian Regions. This southward migra- 

 tion seems to have gone on throughout the Pliocene 

 period, and was probably occasioned by the increasing cold 

 caused by the gradual advent of the great Ice-age, which 

 now began to make itself felt over the whole of the 

 northern part of the globe. 



Finally, during the Glacial period the fauna assumed 

 nearly its present form, containing large numbers of species 

 that still survive. At this epoch, too, a connection appears 

 to have been formed between the Old and New Worlds in 

 the neighbourhood of Behring Strait, by means of which 

 an interchange of animals took place, and resulted in 

 occasioning the similarity which forms so marked a 

 feature on a comparison of the Nearctic and Paloearctic 

 faunas. 



It is this similarity that has caused certain writers on 

 geographical distribution to unite the Palsearctic and 

 Nearctic Regions into one, whereas, as a matter of fact, 

 palseontological evidence seems to show that, out of all the 



