MESOZOIC TIME — TRIASSIC AND JUKASSIC. 797 



is probable, therefore, that this highly specialized type ranked above all other 

 Eeptilian types of the Jurassic. 



3. Degeneration. — Progress in a type from toothed jaAvs to toothless must 

 be viewed as a decline, although there may be true progress in other respects. 

 Among the Ehynchocephalians — which, in the Permian genus Pcdceohatteria, 

 have numerous formidable teeth — occur later species having a horn-covered 

 extremity of the jaws like the beak of a turtle. Again, the Dinosaurians 

 vary from many-toothed, tiger-mouthed species, to those with few teeth. 



The Plesiosaurians are supposed to be degenerate land Reptiles, whose 

 limbs, even in the Triassic, had become paddles, with fingers multiplicate 

 in number of phalanges ; and the Ichthyosaurs, species of some other 

 Eeptilian type, carried downward to a still lower urostheaic stage, in which 

 the pelvic girdle had become nearly obsolete, and the fingers sometimes 

 excessive in number, as well as multiplicate in segments. Turtles are other 

 degenerate forms of the Triassic as well as of the Jurassic period. 



Such facts make it manifest that through geological time progress in the 

 Vertebrate type, as in the Invertebrate, was downward as well as upward ; 

 that degeneration, while it may make obsolete, may also return a species to 

 a low multiplicate condition, in which the multiplicate characteristic extends 

 to the number of vertebrae, to the teeth, to the fingers, to the number of 

 finger bones, and to other parts of the structure. It is atavism under some 

 physiological law deeper than atavism, bringing back characters, not of the 

 earlier Eeptiles, but of the earliest Vertebrates, the Pishes, yet not without 

 any loss of the fundamental characteristics of Eeptiles. 



Considering the very long time that Fishes were in the seas before the 

 rise in grade to the terrestrial type of the Amphibian, and the relatively 

 short time for the much greater rise from the Amphibian to the Eeptile, Bird, 

 and Mammal, there is no reason to believe that any of the upward successional 

 lines passed through the water. Through the water, for terrestrial Verte- 

 brates, as many examples show, was a quick way down in grade, not a 

 possible way up. 



4. A fragment of the Triassic world. — Australia is often spoken of as a 

 Triassic continent. As the world in Triassic time had only Marsupials 

 and Monotremes for its Mammals, so Australia has now, man's encroach- 

 ments excluded, Marsupials and Monotremes for its only Mammals. The 

 existence there of a species of Bat, and of some Mice and Eats, is hardly an 

 exception to be considered. But although thus restricted in its modern 

 fauna, its Mammals are not of few kinds ; for, as Wallace states, " some are 

 carnivorous, some herbivorous ; some arboreal, others terrestrial ; there are 

 insect eaters, fruit eaters, honey eaters, leaf or grass feeders ; some resemble 

 wolves, others marmots, weasels, squirrels, flying squirrels, dormice, or 

 jerboas." Moreover, one of the last four species of Cestraciont Sharks, a 

 tribe of Mesozoic and Paleozoic affinities, the Cestracion Philippi, or Port 

 Jackson Shark, lives in Australian seas ; and one of the last three species of 

 the Dipnoans, the Ceratodus, Carboniferous and Triassic in type, inhabits its 



