Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/185

173 ZOOLOGY.] HOUSE 173 principal forms, separated from each other by many distinct points in their organization, among which one of the most externally conspicuous was the structure of their feet. From this character the one form has received the name of Artiodactyla or &quot;even-toed,&quot; the other Perissodactyla or &quot; odd-toed.&quot; It is only of the latter that we shall have to speak in this article. Perhaps the best notion of a perissodactyle ungulate of the Eocene age can be derived from the tapir of the present day, an animal which has changed less from the primitive and generalized type of the group of that time than any other existing member of the order. These early forms had all the complete number of teeth found in so many of the mammals of that pariod of various orders, arranged according to the well-known formula incisors -|, canines, premolars , molar* = |Y on each side, or 44 in all. The molar teeth had very short square crowns, with transverse or oblique ridges on the grinding surface. In the fore limbs the radius and ulna, and in the hind limbs the tibia and fibula, were distinct and well- developed bones. Whatever the number of toes on each foot, the one corresponding to the middle or third digit of the generalized pentadactyle limb was the longest ; its ungual phalanx was symmetrical in itself, and it formed the centre of the foot, on each sideof which the other toes were arranged in complete or partial symmetry according to the stage of development. In the hind foot in all known cases the symmetry was complete, only one toe on each side of the middle digit being present (fig. 3, c) ; but in the fore foot the primitive symmetry, formed by the presence of two toes on each side of the middle toe, had been lost in nearly all, by the disappearance of one of the outer toes (the first), the condition still retained by the tapirs (fig. 3, a); or it had bsen replaced by the second stage of symmetry, in which both outer toes are absent, and only three remain, as in the modern rhinoceros (fig. 3, c). By no animal of this period had the third, or most highly specialized stage of symmetry, that which, as we shall see, characterizes the modern horses (fig. 3, e), been attained. By various and gradually progressing deviations from the common original type, these animals began at a very early period to break up into several groups, some of which (as Macrauchenia), after undergoing a considerable degree of specialization, have become extinct without leaving successors ; but three of these modified types, already dis tinct at the close of the Eocene period, have continued up to the present day, gradually, as time advanced, becoming more and more divergent from each other. These are now represented by the three families of the rhinoceroses, the tapirs, and the horses. Great as may be the differences between these animals as we see them now, we can trace their history step by step, as revealed by the fragments preserved from former ages, further and further back in time, their differences continually becoming less marked, and ultimately blending together, if not into one common ancestor, at all events into forms so closely alike in all essentials that 110 reasonable doubt can be held as to their common origin. Leaving out of further consideration the two collateral branches, it will be our purpose now to follow the history of the special subject of this article. The remains of the earliest known animals to which it is possible to trace back the modern horse by a series of successive modifications are found in the lowest strata of the great lacustrine formations assigned to the Eocene period, spread over considerable portions of the present territories of New Mexico, Wyoming, and Utah in North America. That similar animals may have existed in other parts of the world is extremely probable. Negative evi dence in such cases is of little value, as may be judged by the fact that it is only within a very few years that the existence of these deposits teeming with fossil remains of previously unsuspected forms has been brought to light, and their systematic exploration has scarcely yet commenced. A little animal, not larger than a fox, E&amp;lt;jldi&amp;gt;i&amp;gt;us of Marsh, presented the most generalized form of the perissodactyle type as yet discovered, as besides the four well-developed toes of the fore foot, found in so many others, it had at least a rudiment of a fifth. All analogy leads to the sup position that this must in its turn have been represented at a still earlier period by another form with all five toes complete, but direct evidence of this is at present wanting. Tiie transition from this horse-like animal of the early period to the horses of modern times has been accompanied by a gradual increase in size. The diminutive Eocene Eohippus and Orohippus were succeeded in the Miocene period by other forms to which the names of Anchitherium and Miohippus have been given, of the size of sheep ; these again in Pliocene times by Hipparion and Pliohippus, as large as the modern donkeys ; and it is only in the Pleisto cene period that Equidce appeared which approached in size the existing horse. Important structural modifications have also taken place, with corresponding changes in the mode of life of the animal. The neck has become elongated, the skull altered in form, the teeth greatly modified, and the limbs have undergone remarkable changes. The last two require to be described more in detail. The teeth in the Eocene forms had, as mentioned above, the characteristic number of forty-four. This number has been retained throughout the series, at least theoretically ; but one tooth on each side of each jaw, the anterior pre- molar, which in all the Eocene and Miocene species was a well-developed tooth, persisting through the life time of the animal, is in all modern horses rudimentary, functionless, and generally lost at an early period of life, evidently passing through a stage which must soon lead to its com plete disappearance. The canines have also greatly dimin ished in size, and are rarely present in the female sex, so that practically a very large number of adult horses of the present day have eight teeth less than the number possessed by their predecessors. The diastema or interval between the incisor and premolar teeth, of essential importance in the domesticated horse to his master, as without it there could be no room for inserting the special instrument of subjugation to his commands, the bit, already existed in the earliest known forms, but has gradually increased in length. The incisors have undergone in comparatively recent times that curious change producing the structure which will be more fully described hereafter, and which distinguishes the horse s incisors from those of all other known animals. Lastly, the molars have undergone a remarkable series of modifications, much resembling in principle those that have taken place in several other groups of herbivorous animals. Distinctions in form which existed between the premolars, at least the anterior members of the series, and the true molars have gradually disappeared, the teeth becoming all very uniform in the shape and structure of their grinding surface. The crowns of all these teeth in the early forms were very short(see fig. 2, a) ; there was a dis tinct constriction, the neck, between the crown and roots; and when the tooth was developing, as soon as the neck once rose fairly above the alveolar margin, the tooth remained per manently in this position. The term &quot; brachyodont &quot; ex presses this condition of teeth, the mode of growth of which j may be illustrated by those of man. The free surface had two nearly transverse curved ridges, with valleys between (fig. 2, a); but the valleys were shallow and had no deposit of cementum filling them, the whole exposed surface of the un worn tooth being formed of enamel. When the ridges be came worn down the dentineof the interior was exposed, form ing islands surrounded by enamel. With the progress of time