ts numerous Edentata--the sloths,
ant-eaters, and armadillos; its rodents, such as the cavies and
chinchillas; its marsupial opossums, and its quadrumana of the family
Cebidae. Remains of extinct species of all these have been found in the
caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene
deposits of the pampas many distinct genera of these groups have been
found, some of gigantic size and extraordinary form. There are
armadillos of many types, some being as large as elephants; gigantic
sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many
others; rodents belonging to the American families Cavidae and
Chinchillidae; and ungulates allied to the llama; besides many other
extinct forms of intermediate types or of uncertain affinities.[190] The
extinct Moas of New Zealand--huge wingless birds allied to the living
Apteryx--illustrate the same general law.

The examples now quoted, besides illustrating and enforcing the general
fact of evolution, throw some light on the usual character of the
modification and progression of animal forms. In the cases where the
geological record is tolerably complete, we find a continuous
development of some kind--either in complexity of ornamentation, as in
the fossil Paludinas of the Hungarian lake-basins; in size and in the
specialisation of the feet and teeth, as in the American fossil horses;
or in the increased development of the branching horns, as in the true
deer. In each of these cases specialisation and adaptation to the
conditions of the environment appear to have reached their limits, and
any change of these conditions, especially if it be at all rapid or
accompanied by the competition of less developed but more adaptable
forms, is liable to cause the extinction of the most highly developed
groups. Such we know was the case with the horse tribe in America, which
totally disappeared in that continent at an epoch so recent that we
cannot be sure that the disappearance was not witnessed, perhaps caused,
by man; while even in the Eastern hemisphere it is the smaller
species--the asses and the zebras--that have persisted, while the larger
and more highly developed true horses have almost, if not quite,
disappeared in a state of nature. So we find, both in Australia and
South America, that in a quite recent period many of the largest and
most specialised forms have become extinct, while only the smaller types
have survived to our day; and a similar