362 ANIMAL BIOLOGY 



a connecting link between the Reptiles and Birds as we know them 

 to-day. (Fig. 233.) 



Archaeopteryx was undoubtedly a bird about the size of a 

 Pigeon, but one with jaws supplied with many small teeth; with 

 a long lizard-like tail formed of many vertebrae, each bearing a 

 pair of quill feathers; with a four-fingered reptilian hand; and 

 so on. In brief, just such a creature as the imagination of an evolu- 

 tionist would picture for a primitive Bird has been actually dis- 

 covered in the lithographic stone quarries of Bavaria, representing 

 the later Jurassic period. 



The ancestry of the modern Horse has been the most impressive 

 fossil pedigree, ever since Professor Marsh collected the famous 

 series of fossil skeletons from the western United States and ar- 

 ranged them in the Yale University Museum. They have been 

 referred to as the "first documentary record of the evolution of a 

 race." Huxley studied this collection and regarded it as conclusive 

 evidence of evolution. 



The essential facts of the evolution of the Horse are these. 

 Horse-like animals probably arose from an extinct group, similar 

 to the Condylarthra, which had five toes on each foot and a large 

 part of the sole resting on the ground. However, the first unques- 

 tionable horse-like forms found in North America are little animals 

 about a foot in height, known as Eohippus, from rocks of the 

 Eocene epoch. The fore foot of Eohippus has four complete toes 

 (digits 2, 3, 4, and 5) but no trace of the inner digit, or thumb, 

 while the hind foot has three complete digits (2, 3, and 4) with 

 vestigial remains, or splints, of the first and fifth. Later in the 

 Eocene appears Orohippus showing a somewhat larger central 

 digit in the fore foot and the disappearance of the splints in the 

 hind foot. Passing up to the Oligocene epoch, Mesohippus, an 

 animal about the size of a wolf, occurs with fore and hind feet 

 three-toed, but with the side toes much smaller than the central 

 one, and just a trace of the fifth digit in the fore foot as a splint. 

 Then Merychippus appears in the Miocene epoch, with still 

 shorter side toes (digits 2 and 4) so that they do not reach the 

 ground, and the weight is borne solely on the hoofed tip of the third 

 digit. This same general reduction of the lateral digits and advance 

 in size and functional importance of the middle digit is carried 

 further during the Pliocene epoch in Pliohippus, which is usually 

 regarded as the first type of 'one-toed' Horse, and leads finally 



