INTRODUCTION 3 



leaves at night to seek for food. The transformation, 

 too, of tadpoles into air-breathing frogs and toads, is an 

 example familiar to everybody who frequents the country 

 ditches in the summer-time, and is too common an 

 occurrence to excite remark ; whilst the breathing of 

 atmospheric air by inhabitants of the ocean, which never 

 leave it, is another case in point, exhibited by the whales 

 and porpoises. 



At the bottom of the chalk sea were deposited 

 myriads of fossils, shells, and fish, which are marked 

 features of the formation at the present day ; but there 

 are no traces of mammals. The few that have been 

 discovered during the Secondary Period appear to have 

 been of a small marsupial animal, and also a small 

 insectivorous one. 



Although the actual genesis of the horse is thus 

 veiled in obscurity, like that of other mammals, warm- 

 blooded animals which suckle their young, its evolution is 

 clearly traced in the Old World from the Hyracotherium, 

 a horse-like animal not larger than a fox. It was only 

 about eleven inches high, existing in the Lower Eocene 

 Period, the first, and lowest, of the Tertiary deposits, and 

 possessing four toes on each fore-foot, with rudiments of 

 another, and three on each hind-foot. In the New 

 World the earliest direct ancestor was the Eo-hippus, 

 which had the same characteristics as the Hyracotherium. 

 Another small animal existed about the same period which 

 possessed five toes on each foot, had the same charac- 

 teristics as the Hyracotherium, and there is little doubt 

 was a still more remote ancestor of the horse. It is 

 termed the Phrenacodus, the first specimen being found 

 by Professor Cope, who dug it up from the Eocene marl 

 on Bear River, in Wyoming. All mammals, even 

 elephants, are descended from similar five-toed ancestors 

 (no larger than a fox), and there are no fossil remains 

 (excepting those of which mention has been made 

 above) of any of them earlier than the sands and clays 

 forming the Tertiary deposits. These are divided into 

 Lower, Middle, and Upper, as far as this country is 



