GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 489 



the Tertiary. Therefore we conclude that Australia 

 was connected with other continents during middle geo- 

 logical times, and, in common with other lands, was 

 inhabited by metatheres and prototheres, but that before 

 or about the beginning of the Tertiary it was separated, 

 and has remained so ever since. Therefore, when, by 

 struggle, migrations, and conflicts on the great theater 

 of evolution Arctogoza (Eurasia and North America), 

 eutheres were evolved at the beginning of the Tertiary, 

 Australia was already isolated, and they never got there. 



3. Africa. — The fauna of Africa south of Sahara is 

 very distinct, though less so than that of Australia. The 

 mammals of Africa are of two groups: (1) A group of 

 large, powerful animals somewhat like the Eurasians, but 

 especially like the Pliocene Eurasians. (2) A group of 

 small animals of low organization (mostly insectivores 

 and lemurs) very peculiar to Africa, but more like those 

 of Madagascar than any other. These latter we shall 

 call indigenes or natives, the former group we will call 

 invaders. 



Explanation. — In preglacial times Africa south of 

 Sahara was isolated, and inhabited only by the indigenes. 

 Then came the glacial elevation, opening Africa to mi- 

 grations from the north, and the glacial cold, driving 

 Pliocene mammals southward into Africa, where they 

 were shut up by subsequent changes in physical geog- 

 raphy. Then came the struggle between the invaders 

 and the natives. Both were sorely changed, the in- 

 vaders mainly by the new environment, the natives 

 mainly by the struggle for life. The final result was 

 the mixture of the two groups, but the invaders greatly 

 predominated. (See Wallace's Geographical Distribu- 

 tion of Animals.) 



Island Faunas.— We have spoken thus far of conti- 

 nental faunas, but island faunas are peculiarly interest- 



