Progress in Mammalian Palaeontology. — Osborn. 227 
and tooth structure. Elephas columbi is analogous to the 
Elephas antiquus type of Europe; the Elephas imperator is 
rather analogous to the E. meridionalis of Europe. It is 
altogether probable that these species evolved in Eurasia 
and arrived fully formed in America. Naturally their geo- 
graphical ranges overlap ; but E imperator is never found in 
the extreme North, nor E. primigenius in the extreme 
South. 
In conclusion, the great problem of all is the time of 
arrival of man amidst the Pleistocene fauna. This event 
is of such paramount importance that we must prepare for 
it by definitely determining the chronological stages of lower 
mammalian succession. At present man appears to be a late 
arrival, but personally I have a strong presentiment that hu- 
man remains will be found in an earlier Pleistocene stage 
than is generally supposed. 
CHIEF CENTRES OF ADAPTIVE RADIATION OF THE ORDERS 
OF MAMMALS. 
I. — Jurassic Radiation (Partly Hypothetical). 
Monotremata (Hypothetical, i. e. fossil forms not yet rec- 
ognized.) 
Marsupialia (Triconodonta). 
Placentalia (Insectivora Primitiva, = Trituberculata.) 
II. — Marsupial radiation, upper Cretaceous and Tertiary. 
Australia (chief centre), Antarctica and South America. Only 
one family (Didelphyidae) certainly known in North Amer- 
ica and Eurasia. 
III. — First or lower placental radiation, upper Cretaceous and lower 
Tertiary (= Meseutheria Osborn.) 
North America (chief centre), Europe, Africa (Creodonta), 
probably extending also to South America. 
a. Orders Certainly Recognized. 
Creodonta, surviving to Lower Oligocene. 
Tillodontia, Middle Eocene (possibly related to Rodentia). 
Taeniodonta, probably related to Edentata Gravigrada. 
Condylarthra, surviving to Middle Eocene. 
Amblypoda, surviving to Upper Eocene. 
