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 Coryphodon, 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 Coryphodon 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 intellect, 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 types 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 the Triassic. They may have originated from Theromorph 
reptiles. 
The Mammalia are divided into three sub-classes—viz., the 
Ornithodelphia (duckbill and Hehidna), the Didelphia or 
marsupials, and the Monodelphia, comprising all the higher 
mammals. 
Sub-class 1. Ornithodelphia.—The duckbill and spiny ant- 
eater (Fig. 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 inte 
