ADVANCE OF THE ANIMAL KACES. 9 



the vertebrata. . The earliest kno\Yn vertebrate remainj, .tro 

 those of Fishes ; and Fishes are the most homogeneous of 

 the vertebrata. Later and more heterogeneous are Rep- 

 tiles. Later still, and more heterogeneous still, are Mam- 

 mals and Birds. If it be said, as it may fairly be said, that 

 the Palaeozoic deposits, not being estuary deposits, are not 

 likely to contain the remains of terrestrial vertebrata, which 

 may nevertheless have existed at that era, we reply that we 

 are merely pointing to the leading facts, such as they are. 

 But to avoid any such criticism, let us take the mam- 

 malian subdivision only. The earliest known remains of 

 mammals are those of small marsupials, which are the low- 

 est of the mammalian type ; while, conversely, the highest 

 of the mammahan type — Man — is the most recent. The 

 evidence that the vertebrate fauna, as a whole, has become 

 more heterogeneous, is considerably stronger. To the 

 argument that the vertebrate fauna of the Palaeozoic period, 

 consisting, so far as we know, entirely of Fishes, was less 

 heterogeneous than the modern vertebrate fauna, which 

 includes Reptiles, Birds, and Mammals, of multitudinous 

 genera, it may be replied, as before, that estuary deposits 

 of the Palgeozoic period, could we find them, might contain 

 other orders of vertebrata. But no such reply can be made 

 to the argument that whereas the marine vertebrata of the 

 Palaeozoic period consisted entirely of cartilaginous fishes, 

 the marine vertebrata of later periods include numerous 

 genera of osseous fishes ; and that, therefore, the later 

 marine vertebrate faunas are more heterogeneous than the 

 oldest known one. Nor, again, can any such re23ly be made 

 to the fact that there are far more numerous orders and 

 genera of mammalian remains in the tertiary formations than 

 in the secondary formations. Did we wisix merely to make 

 out the best case, we might dwell upon the opinion of Dr. 

 Carpenter, who says that " the general facts of Palaeontol- 

 ogy appear to sanction the belief, that the same plan may 

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