92 H. F. Osborn — Origin of Mammals. 



Akt. XL — The Origin of Mammal* ;* by Henky F. 



Osborn. 



Since the source of the Mammalian phylum was later than 

 that of the Reptilian phylum in the Permian and possibly Car- 

 boniferous, we are sure that it extended at least very far back 

 into the Triassic, and the Triassic was apparently a time of 

 great continental connections and of consequent wide geo- 

 graphical distribution even of land types. 



In presenting a phylogenetic chart which traces the ancestry 

 of Mammals to the Upper Permian, I am quite as conscious as 

 the most conservative zoologist present that we are not on 

 sure ground, that this is a castle of cards liable to fall at any 

 moment. Yet such a chart, involving as it does numberless 

 hypotheses, is necessary to set forth certain ideas. 



Clearing the way for this discussion, this chart first replaces 

 the general view defended by Huxley in 1880, of a genetic 

 succession between three sub-classes of mammals, which has 

 become a matter of creed with many zoologists. In the last 

 eighteen years not a scintilla of evidence has arisen to show 

 that Placentals are descended from Marsupials, and of late, 

 evidence has been coming in directly against this view. In 

 fact zoo-palseontology now indicates that Marsupials and 

 Placentals are parallel phyla, arising from a common stock, 

 while Monotremes are so different that they may even be con- 

 sidered diphyletic, or derived independently from the Reptilia, 

 as maintained by Mivart among others, and latterly by Seeley. 



So far as major classification is affected by this conception, 

 it appears therefore that we must revert to Gill's divisions of 

 1872, constituting only two sub-classes of Mammals, namely : 

 A — JEutheria and B — Prototheria. 



Marsupials, Pla- Monotremes. 



centals. 



Marsupials are less primitive than the placental Insectivores, 

 and Marsupials and Placentals are certainly far nearer each 

 other than either are to the Monotremes. 



To guide our speculation in the unknown pre-Tertiary 

 period, we may gather certain positive principles from the 

 known evolution of the Tertiary mammalia. First, we know 

 that adaptive radiation, characteristic of all vertebrates, and 

 beautifully illustrated among Reptilia, is in a very high degree 

 distinctive of Mammalia, because of their superior plasticity. + 



* Opening the discussion before the International Congress of Zoologists at 

 Cambridge. (In conjunction with Professor H. G Seeley.) 



f We know nothing of Africa, whether it enjoj 7 ed a radiation of its own or bor- 

 rowed its fauna from other continents. 



