

THEIR GEOLOGICAL AGE. 207 



same mammalian fauna, and that they belonged to one geo- 

 logical period. It is not meant to be implied that all the 

 species of the fauna ranged equally everywhere throughout 

 the area ; some in all probability predominated in the south, 

 others in the north ; but I can see no grounds for entertaining 

 the belief that they were broken up into groups in successive 

 periods, corresponding with an older Pliocene, Pleistocene, 

 and Post-Pliocene division. 



In speculating about the probable climatal conditions on the 

 land in Europe during the Pliocene period, one of the fossil 

 pachyderms has appeared to me capable of throwing more 

 light than all the others, namely, Hippopotamus major. Two 

 living species of this genus are known, the one Hippopota- 

 mus amphibius, the other the small Hippopotamus Liberensis. 

 They are both found in the tropical or warmer parts of 

 Africa. In their habits they are strictly aquatic, plunging 

 into rivers during the day, and emerging at night to pasture 

 along the banks. They always hug the margins of the 

 rivers or lakes, and are not known to make inland journeys 

 away from them. When they migrate, they either float 

 with the stream, or, if moving against it, they walk along 

 the beds of the river, only leaving it for a short distance, 

 when their course is interrupted by rapids, and replunging 

 into the stream when the obstacles cease. Wherever they 

 are found they enjoy open water all the year round. Their 

 unwieldy heavy form and short limbs are admirably adapted 

 for their aquatic habits, but unfit them for journeying by 

 land. 



There is no reason to believe that the huge European fossil 

 species was in any respect less aquatic in its habits than its 

 living congeners. Wherever its remains have been discovered 

 in the greatest abundance and perfection, it has invariably 

 been along the margins of rivers or great lakes, such as the 

 Val d'Arno, where the bones of hundreds of individuals have 

 been observed. It appears to have been spread over nearly 

 the whole of the Pliocene area of England, since bones and 

 teeth have been described from the Valleys of the Severn, the 

 Avon, and the Thames, Kirkdale Cave, Kent's Hole, and 

 Durdham Down. The general argument, so ably discussed 

 by Dr. Fleming, that we cannot predicate in many cases from 

 the known food and habits of existing species, what the food 

 and habits of extinct species of the same genus may have 

 been, will not apply to the fossil Hippopotamus major, which 

 must have had open water free from ice, if it lived the whole 

 year round in England. That it was capable of migration 

 by land more than the existing species we have no grounds 

 for believing ; and if it is argued that there may have been 



