xiii THE GEOLOGICAL EVIDEXCES OF EVOLUTION 377 



divisions often present a widely different series of species ; so 

 that although a certain number of species are common to 

 two or more of the great divisions, the totality of the species 

 that have lived upon the earth must be very much more than 

 twelve times — perhaps even thirty or forty times — the 

 number now living. In like manner, although the species of 

 fossil mammals now recognised by more or less fragmentary 

 fossil remains may not be much less numerous than the 

 liWng species, yet the duration of existence of these was 

 comparatively so short that they were almost completely 

 changed, perhaps six or seven times, during the Tertiary 

 period ; and this is certainly only a fragment of the geological 

 time during which mammalia existed on the globe. 



There is also reason to believe that the higher animals 

 were much more abundant in species during past geological 

 epochs than now, owing to the greater equability of the climate 

 which rendered even the arctic regions as habitable as the 

 temperate zones are in our time. 



The same equable climate Avould probably cause a more 

 uniform distribution of moisture, and render what are no'A' 

 desert regions capable of supporting abundance of animal life. 

 This is indicated by the number and variety of the species of 

 large animals that have been found fossil in very limited areas 

 which they evidently inhabited at one period. M. Albeit 

 Gaudry found, in the deposits of a mountain stream at 

 Pikermi in Greece, an abundance of large mammalia such as 

 are nowhere to be found living together at the present time. 

 Among them were two species of Mastodon, two different 

 rhinoceroses, a gigantic wild boar, a camel and a giraffe 

 larger than those now living, several monkeys, camivora 

 ranging from martens and civets to lions and hyaenas of the 

 largest size, numerous antelopes of at least five distinct genera, 

 and besides these many forms altogether extinct. Such were 

 the great herds of Hipparion, an ancestral form of horse ; the 

 Helladotherium, a huge animal bigger than the giraffe ; the 

 Ancylotherium, one of the Edentata ; the huge Dinotherium ; 

 the Aceratherium, allied to the rhinoceros ; and the monstrous 

 Chalicotherium, allied to the swine and ruminants, but as large 

 as a rhinoceros ; and to prey upon these, the gTeat Mac- 

 hairodus or sAbre-toothed tiger. And all these remains were 



