ROSS ON EVOLUTION. 415 



probable that the earliest representatives of some existing Orders 

 may have left no remains, especially as we have seen that the earliest 

 fishes were devoid of any internal indurated tissue, and in the case 

 of one of the earliest known Orders the Selachians, there was not 

 much well indurated tissue in the exo-skeleton ; so that it is proba- 

 ble that it will ever remain impossible to trace back the various 

 Orders of Fishes until they approximate so closely as do the earliest 

 known representatives of the Sauropsidia or the Mammalia, 



The Lahyrinthodontia^ an Order of extinct Amphibians w^hich 

 flourished abundantly throughout the Carboniferous Period, combine 

 characteristics of existing Orders of Amphibians with those of the 

 early Ganoids, while the Ichthyosauria, the Plesiosauria, the 

 Pterosaui'la^ and the Dinosauria^ are extinct Orders of Reptilians, 

 wliich connect together the various Orders of existing Reptiles, 

 and these again with Amphibians on the one side and birds on the 

 other, so that all non-Mammalian Vertebrates are thus connected, 

 and considering how imperfect the Geological Record as now 

 known is, not only from its necessary imperfection, but also from 

 the limited character of explorations yet made, enough is known to 

 suggest, if not to warrant, the opinion that originally the differ- 

 ences were only ** Generic" or even '* Specific" in value. 



The interval which separates the non-Mammalian Vertebrates 

 from \k\^Maminalia as found on the great Continents, that is Asia, 

 Europe and Africa, is wide indeed, for of the three Sub-classes into 

 which, from their structure the Mammalia are naturally divided, 

 only one — the farthest removed — is found there, the Monodelphia 

 or true Mammalia. Of the two remaining Sub-classes Didelphia 

 or Ma7'supialia, and Ornithodelphia ; the first, though once 

 abundantly represented on each of the Continents, is now nearly 

 extinct in America, and is found abundantly represented only in 

 Australia, where its isolated position has protected it from the 

 results which elsewhere' have followed its contact with the more 

 diiFerentiated and with the more highly organized tribes of the 

 greater Continents ; and it is here also that the surviving represen- 

 tatives — the Ornithorhyncus and the Echidna — of the Ornitho- 

 delphia are found. 



If the interval separating the Marsiipialia from the Saw^opsidia 



