Januarv, 1904.] 



KNOWLEDGE. 



17 



must iinajjiue our ent;ineer to be iutroduced to that of the 

 horse. " Here we have," he would say, " the very 

 ideal aniiuiil you want, aud I can sugjjest absolutely no 

 uieehanioal improvement iu its framework, save that I 

 fail to re;dise the use of the two small splints of bone 

 attached to the sides of the upper part of the eanuon-bone 

 in eaeh limb."' 



Here, indeed, we have in a nutshell the essential differ- 

 ence between the horse (and its near relatives the zebras 

 and asses) and its earlv ancestors, which. althoui;h of 



FiS. 1. — Skeleton of Hyracotherium, of the Lower Eocene Period in 

 North America and Europe. (After Cope.) 



smaller size, were generalized creatures not far removed 

 in their organization from the tapir. In the ancestral 

 tvpe there is abundant room for modification and speciali- 

 zation, whereas in the other the possibilities of improve- 

 ment and advance appear to have Ijeen exhausted, aud the 

 animal is (with the aforesaid exception) practically perfect 

 for its own special mode of life, and is the supreme 

 development of which its line is capable. The mode in 

 which this perfection has been attained during the slow 

 couree of evolution, it is my jiurpose, so far as sjsace 

 permits, to demonstrate in the present article. 



The earliest mammal to which we can at present defi- 

 nitely afiiliate the horse and its relatives is one from the 

 lowest part of the lowest, or Eocene, division of the Tertiary 

 period known as Phenacodus. This was a short-legged 

 creature not larger than a fox, with a relatively small 

 head, long tad, and five toes to each foot. On these five- 

 toed feet the creature probably walked much in the same 

 way as the modern tapir, that is to say that although the 

 wrist and ankle joints were raised well above the ground, 

 all the three bones of each toe were applied to the same, 

 and the sole was provided with cushion-like pads. Very 

 important is the circumstance tliat in each foot the middle 

 toe was symmetrical in itself, and decidedly larger than 

 those on either side. The toes — and more especially those in 

 the fore-foot — were distinctly expanded at the extremity, 

 and during life were encased in horny sheaths which were 

 probably more like hoofs than claws. Not less important 

 is it to notice that in the skull the socket of the eye was 

 not closed behind by a bar of bone, and was thus con- 

 tinuous with the great hollow on the sides of the temples 

 for the reception of the muscles which worked the jaw. 

 As regards the teeth, it must suffice to say, firstly, that 

 they were forty-four in number, as iu so many c)f the early 

 generalized mammals, and that although well-marked 

 tusks, or canines, were present in both jaws, there was no 

 distinct gap at the commencement of the grinding, or 

 cheek-series. Secondly, these cheek-teeth had very short 

 crowns, surmounted by four simple conical elevations, or 

 cusps, between which were a couple of smaller cusps. 

 Into other details of the structure of this primitive 

 creature it would be out of place to enter here. As to its 



coloration in life, no one has, I believe, hitherto ventured 

 to make even a suggestion. 



When, how<'ver, we advance one step further iu the 

 scale, and come to the Hiirdrotlifriuiu of the Lon<lon Clay 

 division of the Koceno, American paheontologists have 

 been bold enough to say that the creaturi! had a. trans- 

 verL^ely striped coat comparalile to that of the modern 

 zebras ; the reason for this 1 icing that all members of the 

 horse trilie display, especially in the case of hybrids, a 

 tendency to throw back, or revert, to a strijx^d typ' of 

 coloration. Whether we are justified in Mieving this 

 ancestral striping to date so far back as the Hyracotherium, 

 it is not for me to say. 



As regards its organization, Hi/racotheriuni differed 

 markedly in many respects from the earlier Phenacodnn, 

 this Ix'ing most clearly displayed in the skeleton of the feet 

 (Fit;. "2). Tilt' fore-foot had, for instance, become unsyni- 

 metrical, owing to the l(»ss of the first, or "great," toe; 

 the outermost of the four remaining digits lieing quite 

 small, and having no fellow ; the foot being thus com- 

 parable to the fore-foot of a tapir. The hind-foot, on the 

 contrary, although more reduced, still retained the 

 symmetrical form of the ancestral type, having lost both 

 the first and the third digit, aud thus being three-toed, 

 like the corresjionding foot of a tapir. Hi/rai-ollieriiim, 

 which was no larger than a fox, still resembled its ancestor 

 in having two bones to the second segment of each limb, 

 that is to say a nidlus and ulna in the fore, and a tibia and 

 fibula in the hind limb. Here it should be mentioned that 

 between Hijracotherhim and Phenacodnt! there may have 

 existed an intermediate type with four toes to each foot. 

 As regards its cheek-teeth, the creature presented a distinct 



Fig. 2 — Bones of Left Hind and Fore Feet of Syracotherium. 



advance on Phenacodue. In the latter, as already said, 

 the crowns of these teeth were surmounted by simple 

 tubercles. On the other hand, in the former, three of these 

 cusps in the upper teeth tend to unite to form an oblique 

 anterior transverse ridge, while the thi-ee hinder ones tend 

 to make a second posterior ridge; at the same time the 

 two outer tubercles show indications of uniting so as to 

 form a continuous outer wall to this part of the crown of 



