316 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



ridse), then at a very recent epoch, with the intro- 

 duction of the existing African genera, Hippo- 

 potamuses and Potomacherse. 



Finally, I have to point out the important mi- 

 grations of terrestrial Mammals across the Mesogea, 

 whether in the old world or in America. The ex- 

 change of faunas between the two Americas, 

 almost permanently separated by the extension 

 of the Mesogea across the Gulf of Mexico and 

 the Isthmus of Panama, are only really verified 

 from the Pliocene epoch. It is, however, prob- 

 able that the great retreat of the sea at the 

 absolute commencement of Tertiary times, per- 

 mitted the diffusion, in the two American Continents, 

 of a few groups of ancient Mammals : i.e. the 

 flesh-eating Marsupials, Condylarthra, Creodonts, 

 Imparidigitse, and Primates. But after this ex- 

 change of primitive Mammals, terrestrial communi- 

 cations were completely interrupted, and the 

 development of the mammalian fauna proceeds 

 independently in the two Americas. We have to 

 get to the end of the Tertiary to observe that the 

 closing of the Isthmus of Panama at the Pliocene 

 and ^at the beginning of the Quaternary epoch 

 allowed the emigration towards the south of the 

 Mastodons, Tapirs, Equidse, Cervidae, Dogs, Skunks, 

 Ursidse, Cats, Machairodus, etc., etc. ; while, in 

 the contrary direction a current of Patagonian 

 types flowed towards the TJnited States, comprising 

 the great gravigrade Edentata : Megalonyx, Moro- 

 therium, and Mylodon, the Glyptodons, Tatus 

 (Chlamydoiherium), and Hystricomorphous Rodents 

 (Hydrochcerus and Amblyrhiza). 



