4l6 APPENDIX C. 



directly against this view. In fact zoo-palaeontology now indicates 

 that Marsupials and Placentals are parallel phyla, arising from a 

 common stock, while Monotremes are so different that they may 

 even be considered 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. Eiitheria and B. PrototJieria. 



Marsupials, Monotremes. 



Placentals. 



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 ^ There is the (I) Marsupial radiation 

 of Australia (Meteutheria-) — now passing its prime, then the 

 (II) Tertiary Placetital radiation of the Northern Hemisphere (Cen- 

 eutheria), and another quite independent (III) Tertiary Placental 

 radiation in South America (rendered less pure in course of the 

 Tertiary period by migration from I and II). (IV) There is the 

 entirely distinct and archaic Ci'etaceoics Placental radiatio7i of 

 the Northern Hemisphere (Meseutheria), which extends into the 

 Tertiary, and may have given origin to II and III, although 

 as yet we have no direct proof of it. 



We mark the fact that the above radiations are all of ordinal 

 rank, for the Marsupial radii, although termed families, are adap- 

 tively equivalent to several Placental orders. 



If we apply these same principles to the Jurassic, we apparently 

 have evidence of a more fundamental, (V), Sub-class radiation of 

 Placentals, Marsupials and probably Monotremes of world-wide 

 distribution. (This involves a controverted question, to which 

 I shall revert, for by some zoologists these animals of the Purbeck 

 Clays, Como Beds, and Stonesfield Slates are considered exclusively 

 Marsupials and Mortotremes.) Finally far back in the Perm-Trias 

 we certainly observe (VI), the TJieromoiph or Theriodont Reptilian 

 radiation, spurs of which may have given rise to the Mammalia. 



T\\Q focal-types, or most primitive forms of the radiations, I — IV, 



1 We know nothing of Africa, whether it enjoyed a radiation of its own or borrowed 

 its fauna from other continents. 



^ The Eutheria may embrace the Meteutheria or Marsupials, the Meseutheria or 

 primitive Mesozoic Placentals and Ceneutheria or Tertiary Placentals. 



