August 13, 1909] 



SCIENCE 



219 



item of the journey. In these later years, 

 when amateur travel in the west is frequent, 

 a detailed record of this kind will be of value 

 to seekers after adventure; even if certain 

 parts of the river are unduly dangerous, there 

 are other long stretches in which a boat trip 

 might well be undertaken in a summer vaca- 

 tion without too great disregard of a safe 

 return home. Whether made by scientist, 

 hunter or artist, the journey would surely be 

 repaying in high degree, as one may be as- 

 sured from the plates, as well as from Dellen- 

 baugh's vivid descriptions. The solitude must 

 be impressive as one floats down the smooth 

 reaches beneath a mighty architecture of bare 

 cliffs. The excitement of running rapids 

 would seem to be sufficient for the most ardent 

 seeker of new impressions. Many of the plates 

 are excellent, although reproduced from 

 photographs taken nearly forty years ago. 

 W. M. D. 



SPECIAL ARTICLES 



THE POSSIBLE ANCESTORS OF THE HORSES LnTNG 



UNDER domestication' 



During the later part of the nineteenth cen- 

 tury, it was generally taken for granted (1) 

 that " the seven or eight species of Equidse now 

 existing are all descended from an ancestor of 

 a dun colour more or less striped " ;° (2) that 

 the common ancestor of the living horses, 

 asses and zebras was connected by a single line 

 of descent with the four-toed " fossil " horses 

 of the Eocene period; (3) that the domestic 

 horses are descended from Pleistocene species 

 characterized by large molars with a long an- 

 terior internal pillar, a large heavy head and 

 coarse limbs; (4) that in various parts of 

 Europe and Asia, domestic races increased in 

 size and were improved in make, speed and 

 disposition, as a result of artificial selection 

 and favorable surroundings. 



On the continent it seems to be still gener- 

 ally assumed that the domestic breeds have de- 

 scended from a single species,' but in Eng- 



' Abstract of a paper presented to the Royal 

 Society, London. 



= Darwin, "Animals and Plants," Vol. II., p. 17. 



•The latest suggestion is that domestic horses 

 are the descendants of Equus fossilis Riitimeyer, 



land and America many naturalists now be- 

 lieve: (1) That domestic horses have sprung 

 from several wild species connected by several 

 lines of descent with the three-hoofed " fossil 

 horses " of the Miocene period, and (2) that 

 while some of the wild ancestors were adapted 

 for living in the vicinity of forests and upland 

 valleys, others were adapted for a steppe, 

 plateau or desert life. 



Of possible ancestors of the domestic breeds, 

 the following may be mentioned: Eqxius siva- 

 lensis, E. stenonis, E. gracilis (Owen's Asinus 

 fossilis), E. namadicus* E. fossilis and E. 

 roiu^tus. 



These species mainly differ in the teeth, 

 size and deflection of the face and in the 

 bones of the limbs. In the first three species 

 the grinding surface of the anterior internal 

 pillar (a fold of enamel on the inner surface 

 of the cheek teeth) of the premolars and first 

 molar, is short — in the last premolar, pm. 4, 

 it may only be one third the length of the 

 crown — in the second three species the ante- 

 rior internal pillar of pm.4 and m. 1, is long 

 — at least half the antero-posterior length of 

 the crown. One of the ancestral types (E. 

 rohustus) was broad-browed and had a short 

 face almost in a line with the cranium; 

 another (E. sivalensis), also broad-browed, 

 had a long tapering, strongly deflected face; 

 a third (E. fossilis) had a long narrow face, 

 not so strongly bent downwards as in E. 

 sivalensis, and a fourth (E. gracilis) had a 

 fine narrow, but only slightly deflected, face. 



In E. gracilis the middle metacarpal (can- 

 non bone) was so slender that the length was 

 seven and a half times the width, while in E. 

 robust us the length of the metacarpal was 

 sometimes only flve and a half times the 'width. 



Of these possible ancestors, the first three 

 occur in Pliocene deposits, the second three 

 have only hitherto been found in Pleistocene 

 deposits. 



a Pleistocene species closely allied to the wild 

 horse of Mongolia — E. przewaJsJiii. 



* E. namadicus seems to be closely allied to E. 

 compUcatus. a species widely distributed in North 

 America during the Pleistocene period. 



