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 . — Kodents 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, Octodontidse and Echimyidse in the Ethiopian 
and Palaearctic 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. 
Marmpialia and Monotremaia . — 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 
