PRESENT PROBLEMS 583 



The marsupials found their opportunity for unchecked adaptive 

 radiation in Australia, and despite the disadvantage of starting from 

 a specialized arboreal type (Huxley, 1 Dollo, 2 Bensley 3 ), through the 

 later Cretaceous and entire Tertiary a richly diversified fauna evolves, 

 partly imitating the placentals and partly inventing new and more 

 or less peculiar forms of mammals, such as the kangaroo. 



The oldest placental radiation which is fully known is that which 

 was first perceived in Europe and fully recognized by the discovery 

 in 1880 of the basal Eocene mammals of North America it may be 

 called the Cretaceous radiation. These mammals 4 are distinctly 

 antique, small-brained, clumsily built, diversified, imitative both of 

 the marsupial and of the subsequent placental radiations; and our 

 fuller knowledge of them after twenty-five years of research is at once 

 satisfying and disappointing, satisfying because it gives us proto- 

 types of the higher or modern mammals, disappointing because few 

 if any of these prototypes connect with the modern mammals. This 

 fauna is found in the Cretaceous and basal Eocene of Europe, North 

 America, and possibly in Patagonian beds of South America (Ame- 

 ghino), 5 and while giving rise to many dying-out branches, by theory 

 it furnished the original spring from which the great radiations of 

 modern mammals flowed. But practically again we await the direct 

 connections and the removal of 'many difficulties in this theory. In 

 fact, one of the great problems of the present day is to ascertain 

 whether this radiation of Cretaceous mammals actually furnished 

 the stock from which the modern mammals sprang, or whether there 

 was also some other generalized source. 



The Tertiary, or Age of Mammals, presents the picture of the dying 

 out of these Cretaceous mammals in competition with the direct 

 ancestors of the modern mammals. 6 I use the word modern advisedly, 

 because even the small horses, tapirs, rhinoceroses, wolves, foxes, and 

 other mammals of the early Tertiary are essentially modern in brain 

 development and in the mechanics of the skeleton as compared with 

 the small-brained, ill-formed, and awkward Cretaceous mammals. 



Whatever the origin, two great facts have been established: first, 



1 T. H. Huxley, On the Application of the Laws of Evolution to the Arrangement 

 of the Vertebrata, and more particularly of the Mammalia, Proceedings of the Zoolog- 

 ical Society, London, pp. 649-662. 



2 L. Dollo, Les Ancetres des Marsupiaux 6taient-ils arboricoles, Miscellanees 

 Biologiques, 1899, Paris, pp. 188-203; Le Pied du Diprotodon et I'Origine arboricole 

 des Marsupiaux, Bulletin Scientifique de la France et de la Belgique, 1900, pp. 

 275-280. 



* B. A. Bensley, On the Evolution of the Australian Marsupialia, Transactions of 

 the Linnsean Society, London, 2d ser. Zoology, vol. ix, pt. 3, 4to, London, 1903. 



4 That is, the Multituberculata, Creodonta, Tillodontia, Condylarthra, Amblypoda. 



5 Fl. Ameghino, Mammiferes cretaces de V Argentine, Bollettino del Institute 

 Geografico Argentina, tomo xvm, 1897, p. 117; Notices Preliminaires sur des 

 Mammiferes Nouveaux des Terrains Cretaces de Patagonie, Boktin de 1'Academia 

 Nacional de Ciencias de Cordoba, tomo xvn, 1902, pp. 5-68. 



8 H. F. Osborn, Ten Years' Progress in the Mammalian Paleontology of North 

 America, Comptes Rendus, Congres Internationale de ZoSlogie, Bale, 1905. 



