XIII 
THE GEOLOGICAL EVIDENCES 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 
living 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 would probably cause a more 
uniform distribution of moisture, and render what are now 
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. Albert 
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, carnivora 
ranging from martens and civets to lions and hysenas 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 Dinotheriiun ; 
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 great Mac- 
hairodus or sabre-toothed tiger. And all these remains were 
