FOSSIL VERTEBRA TES— MAMMALIA 829 
extreme interest as indicating the origin of the group which has 
been for long one of the greatest problems of the paleontologist. 
Other than the interest attaching to the origin of the group is 
that of the geological possibility of the forms getting into the 
southern continent in the early Tertiary time. These forms 
undoubtedly originated in the United States. As there is no 
trace of them in the basal Eocene of South America. and they 
appear in great numbers and highly developed in the Middle 
Eocene, it seems certain that they must have emigrated from the 
northern land. There is little possibility that they could have 
taken the northern route and gained the land of Asia by the 
northern connection, and then worked into South America by 
the Antarctic continent; this is further borne out by the fact that 
there are no known remains of the group from the Old World 
beyond the incompletely identified specimen of one genus. It 
seems probable that there must have been a temporary connec- 
tion between the two continents in the earliest or the Middle 
Eocene. That the forms found the conditions of life exception- 
ally favorable in the southern continent is evidenced by the 
extraordinary development both of species and of individuals. 
Cetacea.— This order bears very much the same relation to 
the land mammals that the Plesiosaurs and the Icthyosaurs bore 
to the early land reptiles. The limbs have degenerated and 
become adapted as swimming organs, the bones of the proximal 
portions becoming shorter and losing their distinctive character, 
while the phalanges become much more numerous and there 
may even be added digits. In most of these forms also the hind 
limb is lost; the teeth become simpler and disappear in some 
forms; the whole body takes on the fish-like form that seems to 
be requisite for the aquatic life ; the hair disappears and is rep- 
resented by only a few scattering bristles. These remarks are 
equally applicable to the succeeding group, the Szvenza. 
The Cetacea are generally divided into three groups, the 
Archaeocetit, the Odontoceti, and the MMysticoceti. The last two 
groups, the recent dolphins and the whales are represented in 
the fossil state by specimens from the Eocene, showing all the 
