PLEISTOCENE EQUID^E. 695 



Africa and have taken up their abode in Southern regions 

 which are now the home of Burchell's zebra. Hence, 

 it has been assumed that the ancestors of this zebra 

 took part in this migration, which was apparently caused 

 by the increasing cold of the Glacial Age. These con- 

 siderations seem to indicate that E. a sinus atlanticus was 

 an ancestor of Burchell's zebra, but not of the horse or ass. 



M. Boule has also found this pillar of enamel in the milk 

 teeth of specimens of E. stenonis obtained from deposits 

 in Puy-de-D6me, Haute Loire and Val d'Arno. 



Unfortunately, we have no information as to the time 

 when the mountain zebra and quagga departed from the 

 line of descent of the horse. 



M. Boule points out in his admirable paper that there 

 were two varieties of Equus stenonis, namely, Equus ligeris, 

 and E. robustus (E. plicidcns of Owen). The former, 

 which is the one he considers to be an ancestor of Burchell's 

 zebra, was of comparatively small size, and the enamel 

 of its molar teeth took far less complicated forms than 

 that of the horse. The latter was comparatively large, 

 and its enamel was folded in a manner somewhat 

 similar to that of the domestic horse, which M. Boule 

 regards as a descendant of E. robustus. As the horse 

 thrives best in a temperate or cold cHmate, and as the 

 zebra is a tropical animal, we can readily understand that, 

 although the intense cold of the Great Ice Age banished the 

 E. ligeris tribe from Europe and North Africa, it had not 

 a similar effect on the E. robustus tribe, which can now 

 be found in a wild state in parts of Siberia that are not 

 very far from the Arctic circle. As Darwin suggests, 

 the instinct which horses possess of scraping away, with 

 their fore feet, snow that covers the ground, so as to get 

 at the underlying grass, shows that they probably came 

 from a country in which the winters were severe. 



Although scientific research has been unable, up to the 

 present, to tell us what animal was the latest common 

 ancestor of the Equidse of to-day, the fact that asses and 



