chap, xxm.] SUMMAEY 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. 



Rodentia. — Rodents 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, Octodontida? 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 Monotrcmata. — 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 



