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 Anchitherium 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 

 Dr. Merriam,t 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. 



* Dep6ret, Ch., " Transformations of tlie Animal World," p. 313. 

 t Merriam, 0. H., "Prairie Dog," pp. 258—263. 



