56 OSBORN. 



old worlds at this time. Eveiy year's discoveiy increases the 

 resemblance and diminishes the differences between Europe and 

 the Rocky Mountain region. Distinguishing North America, 

 however, are the T}'lopoda ; this sub-order includes the peculiar 

 Artiodact>'la of the camel-llama tribe ; these Professor Scott in 

 a recent paper considers as including all the early types of 

 American ruminants which we have been vainly endeavoring to 

 compare with European types. The radiation of the tylopod 

 phylum into a great variety of types is quite conceivable and it 

 is thoroughly consistent with the fundamental law of adaptive 

 radiation which we find operating over and over again. 



III. THEORY OF SUCCESSIVE INVASIONS OF AN AFRICAN 

 FAUNA INTO EUROPE 



In Europe there are in the upper Eocene two classes of 

 animals, first, those which have their ancestors in the older rocks ; 

 second, the class including certain highly specialized animals 

 which have no ancestors in the older rocks — among these, per- 

 haps, are the peculiar flying rodents or Anoniahiiidce, now con- 

 fined to Africa, and secondly the highly specialized even-toed 

 ruminant types — the anoplotheres, xiphodonts and others, the 

 discovery of which in the Gypsc near Paris Cuvier has made 

 famous. It is tempting to imagine that these animals did not 

 evolve in Europe but that they represent what may be called 

 the first invasion of Europe by African t>'pes from the Ethiopian 

 region. 



It is a curious fact that the African continent as a great theater 

 of adaptive radiation of ^Mammalia has not been sufficiently con- 

 sidered. It is true that it is the dark continent of palaeontology 

 for it has practically no fossil mammal history ; but it by no 

 means follows that the Mammalia did not enjoy there an exten- 

 sive evolution. 



Although it is quite probable that this idea has been advanced 

 before, most writers speak mainly or exclusively of tJie invasion 

 of Africa by Eiiropcan types. Blanford and Allen it is true have 

 especially dwelt upon the likeness of the Oriental and Ethiopian 



