224 Reviews — Prof. A. Gaudry — The Evolution of Mammalia. 



any definite generic type of Pachyderm from which the Euminants 

 were evolved, though the evidence of such evolution may be re- 

 garded as conclusive. In the digits of the limbs, the author makes 

 a point of reminding us that existing genera show transitions from 

 types with four metapodial bones to those in which there is but one. 

 And by ti-acing the ancestral forms of the more specialized living 

 genera, a similar series of modifications is met with, until, in the 

 earlier genera, the metapodial bones are all separated. The means 

 by which the surviving types have been evolved are (1) the first, 

 second and fifth metatarsals and first cuneiform are carried back- 

 ward ; (2) the posterior part of the third metatarsal is enlarged to 

 sustain the cuneiform ; (3) the first and second cuneiform elements 

 of the tarsus, the second and fifth metatarsals and the trapezius 

 become very small ; (4) the bones are modified by blending of the 

 various elements of the metacarpus and metatarsus. 



The ancestry of the Horse occupies the fifth chapter, and is ex- 

 amined first by tracing the modifications of the teeth in Miocene 

 and Eocene genera, and afterwards by a study of the limbs in 

 those animals. There is but little here added to the researches 

 of Professor Huxley, and no departure is made from the con- 

 clusions of Professor Marsh. In Europe the Horse is traced from 

 Paloploiherium by way of Pachynolyjphus to AncJiitlierium, which 

 is allied by its teeth to Pal^oilierium, though its limbs approx- 

 imate to Paloplotherium and Hipparion, which last is little more 

 than a Horse with two small supplemental digits, one on each 

 side of the hoof which supports the limb. I^his latter transi- 

 tion is the more interesting, since monstrosities of the living horse 

 sometimes appear, in which the digits on one limb are developed 

 exactly as in the Miocene Hipparion. Such a horse as Equiis Stenonis 

 of the Pliocene, in which the teeth closely approximate to Hipparion, 

 is regarded as the immediate ancestor of the old-world Horse. In 

 the extremities of the limbs Aceroiherium of the Middle Miocene, 

 which has three large digits and a small one on the outside, 

 diverges considerably from the type of the living horse ; but 

 PalcBotlieriiim, which has three digits, of which the middle one is 

 slightly the largest, makes a nearer approximation. In Paloplo- 

 therium the lateral digits and metapodial bones are much more 

 reduced in size ; while in Hipparion the reduction in size is carried a 

 stage further, and the middle digits proportionately enlarged. The 

 author quotes with approval the proposal of Professor Marsh to derive 

 the Horse from the Orohippiis through a series of animals which pro- 

 gressively increased in size ; but apparently without considering 

 that, if his own view and that of Prof. Marsh are both accepted, 

 then we should have two distinct lines of descent for the Horse. 



The sixth chapter is an exceedingly able exposition of the grounds 

 of classification of the Ungulata by means of the teeth and limbs 

 with a view to explaining the nature of the osteological modifica- 

 tions which gradually evolved the Artiodactyla and Perissodactyla 

 from a common stock. The author considers that all the modifica- 

 tions which the extremities of the even and odd-hoofed Ungulata 



