VERTEBRATE LIFE IN AMERICA. 691 



mammalian life has here made since the beginning of the Eocene. 

 As it is, I can only say, in summing up, that the Marsupials are clearly 

 the remnants of a very ancient fauna, which occupied this continent 

 millions of years ago, and from which the other mammals were doubt- 

 less all derived, although the direct evidence of the transformation is 

 wanting. 



Although the Marsupials are nearly related to tbe still lower 

 Monotremes, now living in the Australian region, we have as yet no 

 hint of the path by which these two groups became separated from 

 the inferior vertebrates. Neither have we to-day much light as to 

 the genetic connection existing between Marsupials and the placental 

 Mammalia, although it is possible that the different orders of the 

 latter had their origin each from a separate group of the Marsupials. 



The presence, however, of undoubted Marsupials in our lower and 

 middle Eocene, some of them related to the genus Didelphys, al- 

 though' remotely, is important evidence as to the introduction of these 

 animals into America. Against this, their supposed absence in our 

 Miocene and Pliocene can have but limited weight, when taken in con- 

 nection with the fact that they flourished in the Post-Tertiary, and 

 are still abundant. The evidence we now have is quite as strongly in 

 favor of a migration of Marsupials from America to the Old World, 

 as the reverse, which has been supposed by some naturalists. Pos- 

 sibly, as Huxley has suggested, both countries were peopled with 

 these low mammals from a continent now submerged. 



The Edentate mammals have long been a puzzle to zoologists, and 

 up to the present time no clew to their affinities with other groups 

 seems to have been detected. A comparison of the peculiar Eocene 

 mammals which I have called the Tillodontia, with the least specialized 

 Edentates, brings to light many curious resemblances in the skull, 

 teeth, skeleton, and feet. These suggest relationship, at least, and 

 possibly we may yet find here the key to the Edentate genealogy. 

 At present, the Tillodonts are all from the lower and middle Eocene, 

 while Horopus, the oldest Edentate genus, is found in the middle 

 Miocene, and one species in the lower Pliocene. 



The Edentates have been usually regarded as an American type, 

 but the few living forms in Africa, and the Tertiary species in Europe, 

 the oldest known, have made the land of their nativity uncertain. I 

 have already given you some reasons for believing that the Edentates 

 had their first home in North America, and migrated thence to the 

 southern portion of the continent. This movement could not have 

 taken place in the Miocene period, as the Isthmus of Darien was then 

 submerged ; but, near the close of the Tertiary, the elevation of this 

 region left a much broader strip of land than now exists there, and 

 Over this the Edentates and other mammals made their way, perhaps 

 urged on by the increasing cold of the glacial winters. The evidence 

 to-day is strongly in favor of such a southern migration. This, how- 



