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CHAP. XXIII.] SUMMARY AND CONCLUSION. 543
continents, even reaching the southern extremity of America.
Their extinction has probably depended more on physical than
on organic changes, and we can clearly trace their almost total
disappearance to the effects of the Glacial epoch.
Bodentia.—Eodents are a very dominant group, and a very
ancient one. Owing to their small size and rapid powers of
increase, they soon spread over almost every part of the globe,
whence has resulted a great specialisation of family types in
the South American continent which remained so long isolated.
They are capable of living wherever there is any kind of
vegetable food, hence their range will be determined rather by
organic than by physical conditions ; and the occupation of a
country by enemies or by competing forms, is probably the chief
cause which has prevented many of the families from acquiring
a wide range. The occurrence of isolated species of the South
American families, Octodontidee and Echimyidse in the Ethiopian
and Palsearctic regions, is an indication that the range of many
of the families has recently become less extensive.
Edentata.—These singular and lowly-organised animals ap-
pear to have become almost restricted to the two great Southern
lands—South Africa and South America—at an early period
and, being there free from the competition of higher forms,
developed a number of remarkable types often of huge size, of
which the Megatherium is one of the best known. The incur-
sion of the highly-organised Ungulates and Carnivora into
Africa during the Miocene epoch, probably exterminated most
of them in that continent ; but in America they continued in
full force down to the Post-Pliocene period ; and even now, the
comparatively diminutive Sloths, Ant-eaters, and Armadillos,
form a large and important portion of the fauna.
Marsupialia and Monotremata.—These are probably the
representatives of the most ancient and lowly-organised types
of mammal. They once existed in the northern continents,
whence they spread into Australia; and being isolated, and
preserved from the competition of the higher forms which soon
arose in other parts of the world, they have developed into a
variety of types, which, however, still preserve a general
;