H. F. shorn — Origin of Mammals. 93 



There is the (I) Marsupial radiation of Australia (Meteu- 

 theria)* — now passing its prime, then the (II) Tertiary Placen- 

 tal radiation of the Northern Hemisphere (Ceneutheria), and 

 another quite independent (III) Tertiary placental radiation 

 in South America, (rendered less pure in course of the Ter- 

 tiary period by migration from I and II). IV, There is the 

 entirely distinct and archseic Cretaceous Placental Radiation 

 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 

 adaptively equivalent to several Placental orders. 



If we apply these same principles to the Jurassic, we appar- 

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

 radiation of Placentals and Marsupials, and probably Mono- 

 tremes, of world-wide distribution. (This involves a controverted 

 question, to which I shall revert, for by some zoologists these 

 animals of the Pnrbeck Clays, Como Beds, and Stonesfield 

 Slates are considered exclusively Marsupials and Monotremes.) 

 Finally far back in the Perm-Trias we certainly observe (VI), 

 the Theromorph or Theriodont Reptilian radiation, spurs of 

 which may have given rise to the Mammalia. 



The focal-types, or most primitive forms of the radiations, 

 I — IY, were certainly small, terrestrial, clawed, insectivorous 

 or omnivorous forms. It is noteworthy that in the evolution 

 of each radiation, so far as we know at present, land types and 

 organs are invariably primitive, and water types and organs are 

 secondary, exactly as we find it among the Peptilia. In fact 

 we have not found a single instance in which a mammal or 

 reptile series is known to be transforming from a water into a 

 land type ; it is always the reverse. There is certainly no evi- 

 dence for a cetoid (Albrecht) stem of the Mammals. Again it 

 is obvious that neither carnivorous nor herbivorous types with 

 highly specialized or reduced teeth and feet can be so central 

 as insectivorous and omnivorous types. In fact the Insecti- 

 vores among Placentals, and Opossums among Marsupials, are 

 the only animals which have preserved the dental prototype 

 close to that of the Promammal. 



The backward convergence of all Tertiary mammalia to a 

 Creodont stem, as indicated in the chart, affords the clearest 

 demonstration of the existence of an earlier insectivorous stem, 

 and it not only accords with the above principles, but we are 

 thus carried a step further to realize that the Creodont ancestor 

 must have been a generalized Insectivore. 



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

 or primitive Meaozoic Placentals, the Ceneutheria or Tertiary Placentals. 



