364 PALEONTOLOGY AND EVOLUTION xi 



I have already said, contains Cheiroptera, Insecti- 

 vora, Eodentia, Ungulata, Carnivora, and Cetacea. 

 But the Cheiroptera are extreme modifications 

 of the Insectivora, just as the Cetacea are extreme 

 modifications of the Carnivorous type ; and there- 

 fore it is to my mind incredible that monodelphous 

 Insectivora and Carnivora should not have been 

 abundantly developed, along with Ungulata, in 

 the Mesozoic epoch. But if this be the case, 

 how much further back must we go to find the 

 common stock of the monodelphous Mammalia? 

 As to the Didelphia, if we may trust the evidence 

 which seems to be afforded by their very scanty 

 remains, a Hypsiprymnoid form existed at the 

 epoch of the Trias, contemporaneously with a 

 Carnivorous form. At the epoch of the Trias, 

 therefore, the Marsupialia must have already 

 existed long enough to have become differentiated 

 into carnivorous and herbivorous forms. But the 

 Monotremata are lower forms than the Didelphia 

 which last are intercalary between the Ornitho- 

 delphia and the Monodelphia. To what point of 

 the Palaeozoic epoch, then, must we, upon any 

 rational estimate, relegate the origin of the 

 Monotremata ? 



The investigation of the occurrence of the 

 classes and of the orders of the Sauropsida in time 

 points in exactly the same direction. If, as there 

 is great reason to believe, true Birds existed in 

 the Triassic epoch, the ornithoscelidous forms by 



