408 PALEONTOLOGY 



similar mark, under Cetacca, refers to the fossil, probably 

 washed out of an Upper Oolitic bed, referred to at p. 321. The 

 Marsupialia recur, under distinct generic forms, in the eocene 

 strata, and, according to actual knowledge, present their fullest 

 development in pliocene and modern times, more especially 

 in Australia. The orders Bruta, Perissodactyla, and Carnivora, 

 have become reduced in numbers ; the Proboscidia still more 

 so ; the representatives of the singular group Toxodontia have 

 wholly disappeared. 



The sum of the evidence which has been obtained seems 

 to prove that the successive extinction of Microlcstes, Amplii- 

 theria, Simlacothcria, Trieonodons, and other mesozoic forms of 

 mammals, has been followed by the introduction of much more 

 numerous, varied, and higher-organized forms of the class, 

 during the tertiary periods. 



It may be, however, objected that negative evidence cannot 

 satisfactorily establish the proposition that the mammalian 

 class is of late introduction, nor prevent the conjecture that it 

 may have been as richly represented in primary and more 

 ancient secondary as in tertiary times, could we but get 

 remains of the terrestrial fauna of the continents. To this 

 objection it may be replied: in the palteozoic strata, which, 

 from their extent and depth, indicate, in the earth's existence 

 as a seat of organic life, a period as prolonged as that which 

 has followed their deposition, no trace of mammals has been 

 observed. Were mammals peculiar to dry land, such nega- 

 tive evidence would weigh less in producing conviction of 

 their non-existence during the Silurian and Devonian seons, 

 because the explored parts of such strata have been deposited 

 from an ocean, and the chance of finding a terrestrial and air- 

 breathing creature's remains in oceanic deposits is very remote. 

 But in the present state of the warm-blooded, air-breathing 

 viviparous class, no genera and species are represented by 

 such numerous and widely dispersed individuals, as those of 



