ORIGIN OF DOMESTIC MAMMALS. 571 



forms, combining in a remarkable degree characters more 

 elaborated, and in great detail, in different orders of living 

 mammals, especially the Ungulates. For example, from the 

 Eocene Oorypliodon, a generalized ungulate animal, have 

 probably been derived the ruminants, the tapirs, hog, hip- 

 popotamus-like forms, the rhinoceros, and, finally, the 

 horse. This inference is based on the fact that the bones 

 and teeth of Oorypliodon present characters which are no 

 longer combined in any one species of mammals, but which 

 are found worked out in detail in the members of the differ- 

 ent orders referred to. 



Moreover, the early Tertiary mammals had brains much 

 smaller than in any existing forms, and with only one ex- 

 ception, without convolutions — showing that the develop- 

 ment of the size of the brain and its convolutions, and con- 

 sequently of the intellects, has kept pace with the successive 

 stages in the specialization shown in existing forms, and 

 which agree with the increasing complexity of the Ameri- 

 can Continent and the subdivision of the western part of 

 the continent into distinct basins, with separate mountain 

 systems and river-valleys. The result of all this apparent 

 waste of generalized forms, and the survival of the few 

 favored t3rpes now existing, has been the preservation of 

 animals which have been domesticated by man, such, as the 

 dog, pig, horse, ox, camel, elephant, and of others useful as 

 food or as intelligent servants ministering to his every-day 

 wants. 



The earliest mammals were small insectivorous or gnaw- 

 ing marsupials, none larger than a cat, and first appearing 

 in Jurassic strata. 



The Mammalia are divided into three sub-classes — viz., the 

 Ornithodelpliia (duckbill and EcMdna), the DidelpMa or 

 marsupials, and the Monodelphia, comprising all the higher 

 mammals. 



Suh-class 1. Ornithodelphia. — The duckbill and spiny ant- 

 eater (Pig. 493, Echidna hystrix) are the only representatives 

 of the sub-class, of which there is but a single order, called 

 Monotremes, and are distinguished by the following char- 

 acters. The oviducts, vasa deferentia and ureters, open into 



