x.] PERSISTENT TYPES OF LIFE. 193 



Or among the Teleostei in what respect is the Beryx of the 

 Chalk more embryonic, or less differentiated, than Beryx lineatus 

 of King George s Sound ? 



Or to turn to the higher Vertebrata in what sense are the 

 Liassic Chelonia inferior to those which now exist ? How are 

 the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less 

 embryonic, or more differentiated, species than those of the 

 Lias ? 



Or lastly, in what circumstance is the Phascolotherium more 

 embryonic, or of a more generalized type, than the modern 

 Opossum; or a Lophiodon, or a Palceotherium, than a modern 

 Tapirus or Hyrax ? 



These examples might be almost indefinitely multiplied, but 

 surely they are sufficient to prove that the only safe and unques 

 tionable testimony we can procure positive evidence fails to 

 demonstrate any sort of progressive modification towards a less 

 embryonic, or less generalized, type in a great many groups of 

 animals of long-continued geological existence. In these groups 

 there is abundant evidence of variation none of what is 

 ordinarily understood as progression ; and, if the known geological 

 record is to be regarded as even any considerable fragment of the 

 whole, it is inconceivable that any theory of a necessarily pro 

 gressive development can stand, for the numerous orders and 

 families cited afford no trace of such a process. 



But it is a most remarkable fact, that, while the groups which 

 have been mentioned, and many besides, exhibit no sign of pro 

 gressive modification, there are others, co-existing with them, 

 under the same conditions, in which more or less distinct indica 

 tions of such a process seems to be traceable. Among such 

 indications I may remind you of the predominance of Holostome 

 Gasteropoda in the older rocks as compared with that of 

 Siphonostome Gasteropoda in the later. A case less open to 

 the objection of negative evidence, however, is that afforded by 

 the Tetrabranchiate Cephalopoda, the forms of the shells and of 

 the septal sutures exhibiting a certain increase of complexity in 

 the newer genera. Here, however, one is met at once with the 



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