H. F. shorn — Mammalia in Worth America. 457 



was the cause of the wide-spread extinction of these types ? 

 So far as the ancient clawed types are concerned, their teeth 

 and feet seem to be as fully adaptive in many cases as those of 

 the later unguiculates ; the hoofed types were certainly inferior 

 in tooth evolution, for all their molars evolved on the triangular 

 basis instead of the sexitubercular; the most sweeping defect 

 of both the clawed and hoofed types, was the apparent inca- 

 pacity for brain growth, their bodies went on developing while 

 their brains stood still. Thus the stupid giant fauna, the 

 Dinocerata, which rose out of this period, gave way to the 

 small but large-brained modern types. It is noteworthy that 

 the latest survivors of this wreck of ancient life were the large- 

 brained Hysenodons. 



Some of the least specialized spurs of this radiation appear 

 to have survived and become the centers of the second or mid- 

 Tertiary radiation from which our modern fauna has evolved. 

 Yet we have not in a single case succeeded in tracing the 

 direct connection. To sum up, we hnd on the North American 

 continent evidence of the rise and decline and disappearance of 

 monotremes and marsupials, and two great periods of placental 

 radiation, the ancient radiation beginning in the Mesozoic, 

 reaching a climax in the Puerco and unknown post-Puerco, and 

 sending its spurs into the higher Tertiary, and the modern 

 radiation reaching its climax in the Miocene, and sending 

 down to us our existing types. 



Another Eocene center was lower South America, which has 

 of late dimmed the prestige of North America in yielding 

 strange forms of life. One theory of this Patagonian fauna is 

 that it was an independent center of functional radiation like 

 the Puerco and Australian, full of adaptive parallels, but not 

 yielding to Europe or America any of their older types. But 

 Ameghino, to whose energetic researches we are chiefly in- 

 debted, believes that he finds a lower Eocene life zone — a sort 

 of south polar center — which supplied both America and 

 Europe. The Puerco he believes is no older than the Santa- 

 cruzian which in turn is very much older than the Parana and 

 Pampean formations, which Burmeister has made so well 

 known. This yields the Homunculus patagonicus which paral- 

 lels Cope's Anaptomorphus in presenting a dentition as ad- 

 vanced in reduction as that of man. Ameghino finds here the 

 ancestors of the Macrauchenidas ; he believes the Homolo- 

 dontotheridse are the ancestors of the Chalicotheriidse — thus 

 mistakenly deriving a buno-selenodont from a lophodont type ; 

 the Proterotheriidas, he believes, replace the Condylarthra and 



