68 ' THE AGE OF MAMMALS 



Marsupialia Pholidota 



Insectivora Tubulidentata 



Chiroptcra Lemuroidea 



Carnivora-Creodonta Condylarthra 



Carvornia-Fissipedia Amblypoda 



Rodentia Artiodactyla 



Tillodontia Perissodactyla 



Taeniodonta or Ganodonta Ancylopoda 



Edentata-Xcnarthra Zeuglodontia 



This imposing array may, it is true, be partially swollen by inclusion of 

 orders of mammals which were probably indigenous to South America 

 (Edentata) and possibly to Africa (Pholidota, Tubulidentata, Zeuglodontia), 

 but even taking out these possibly or probably foreign members of northern 

 society, a large residuum of mammals which probably originated in the north- 

 ern hemisphere still remains and firmly establishes this as the dominant 

 hemisphere in the evolution of the Mammalia. 



Africa, Ethiopian Region, also an Important Center of Mammalian Evolution 



Regarding Africa as a theater of mammalian evolution there have been 

 two views. First, the older view that Africa derived its original primitive 

 stock of mammals from the north and then remained passive until it received 

 a new wave of highly specialized mammals. Second, the newer view that 

 Africa was throughout the Age of Mammals a great center of mammalian 

 evolution and contributed its full quota to the world stock of modernized 

 mammals. In general it may be said that prior to 1900 the African conti- 

 nent as a great theater of adaptive radiation of the Mammalia had not been 

 sufficiently considered. This was chiefly because it had practically no dis- 

 covered fossil mammal history. It was the fashion with most writers on geo- 

 graphic distribution to speak mainly and exclusively of the invasion of Africa 

 by European types rather than of the possible invasion of Europe by African 

 types. 



Hypothesis of northern invasion of Africa. — In 1867 Rlitimeyer ^ expressed 

 the opinion that at a very early period Europe sent into Africa its wealth 

 of tropical forms. The ancient population of this continent was first fully 

 discussed l^y Alfred Russel Wallace,^ who also set forth the hypothesis of 

 northern invasion, namely: that before Pliocene times Africa was occupied 

 only by a small primitive fauna, lemurs, insectivores, edentates, and rodents, 

 and that early in Pliocene times the large mammalian fauna of Europe and 

 southern Asia (Pikermi and Siwalik Hills) were "poured into Africa and, 

 finding there a new and favorable country almost wholly unoccupied by large 

 mammalia, increased to an enormous extent, developed into new forms, 

 and finally overran the whole continent." 



' Riitimeyer, Uber die Herkunft unserer Thierwelt, 1867, pp. 42-43. 

 ^ Wallace, The Geographical Distribution of Animals, 1876, p. 288. 



