148 
ORIGIN OF LIFE IN AMERICA 
it would seem as if the family Equidae had originated in 
America and had sent certain offshoots to the Old World 
during such times when America was connected by land 
with either Asia or Europe. Professor Deperet * certainly 
takes the view that both Ancbitherium and Hipparion 
reached Europe by means of a land connection with America, 
and that the two continents were several times joined to one 
another by land during the Tertiary Era. This opinion is 
amply supported by the most weighty zoogeographical evi¬ 
dence, as will be shown in one of the succeeding chapters 
(pp. 226—231). There is, indeed, a very general agreement 
among palaeontologists on this point. The only difference of 
opinion concerns the exact location of the site of these ancient 
land bridges. 
One of the most characteristic animals of the great plains 
is the so-called “ prairie dog ” (Cynomys ludovicianus). The 
name has been applied to it on account of the peculiar barking 
sound it emits when alarmed. Otherwise it has nothing to do 
with the dog family, being more nearly related to the ground 
squirrels and marmots. It loves the sunshine and a dry 
atmosphere, and becomes less and less numerous as we ap¬ 
proach the humid prairies from the west. The prairie dog is 
a social creature living in colonies, and these, according to 
Or. Merriam,f are sometimes from twenty to thirty miles in 
length. The damage done to crops by these animals is enor¬ 
mous, while their increase is greatly favoured by the spread 
of agriculture. The cultivation of the soil enables them to 
support larger families, whereas the cultivator further pro¬ 
tects them by destroying their natural enemies. 
The prairie dog inhabits a vast area between Montana in 
the north and southern Texas in the south. Altogether seven 
species of Cynomys are known, some of which range into 
Arizona and Mexico, whereas none occur in the eastern or 
extreme western States of America. No fossil remains of 
Cynomys from Tertiary deposits have been identified, except 
from the Miocene Republican River deposits of Kansas and 
Nebraska, and even they only doubtfully belong to the genus. 
* Deperet, Ch., “ Transformations of the Animal World,” p. 313. 
+ Merriam, C. H., “Prairie Dog,” pp. 258—263. 
