120 ISLAND LIFE 



reach Yorkshire. Professor Boyd Dawkins supposes that 

 at this time our summers were warm, as in Middle Asia 

 and the United States, while the winters were cold, and 

 that the southern and northern animals migrated to and 

 fro over the great plains which extended from Britain to 

 the Continent, The following extract indicates how such 

 a migration was calculated to bring about the peculiar 

 association of sub-tropical and arctic forms. 



^' It must not, however, be supposed that the southern 

 animals migrated from the Mediterranean area as far 

 north as Yorkshire in the same year, or the northern as 

 far south as the Mediterranean. There were, as we shall 

 see presently, secular changes of climate in Pleistocene 

 Europe, and while the cold was at its maximum the 

 arctic animals arrived at the southern limit, and while 

 it was at its minimum the spotted hyaena and hippo- 

 potamus and other southern animuls roamed to their 

 northern limit. Thus every part of the middle zone has 

 been successively the frontier between the northern and 

 southern groups, and consequently their remains are 

 mingled together in the caverns and river-deposits, under 

 conditions which prove them to have been contemporaries 

 in the same region. In some of the caverns, such as that 

 of Kirkdale, the hysena preyed upon the reindeer at one 

 time of the year and the hippopotamus at another. In 

 this manner the association of northern and southern 

 animals may be explained by their migration according to 

 the seasons ; and their association over so wide an area as 

 the middle zone, by the secular changes of climate by 

 which each part of the zone in turn was traversed by the 

 advancing and retreating animals." ^ 



When we consider that remains of the hippopotamus 

 have been found in the caves of North Wales and Bristol 

 as well as in those of Yorkshire, associated in all with 

 the reindeer and in some with the woolly rhinoceros or 

 the mammoth, and that the animal must have reached 

 these localities by means of slow-flowing rivers or flooded 

 marshes by very circuitous routes, we shall be convinced 

 that these long journeys from the warmer regions of South 



^ Early Man in Britain and his Place in the Tertiary Period, p. 113, 



