222 LAY SERMONS, ADDRESSES, AND REVIEWS. [x. 



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, Plesio- 

 sauria, or Pterosauria less embryonic, or more differ- 

 entiated, 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 Lophiodoii, or a Palceotherium, 

 than a modern Ta/pirus or Hyvax ? 



These examples might be almost indefinitely multi- 

 plied, but surely they are sufficient to prove that the 

 only safe and unquestionable 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 progressive 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 progressive modification, there are 

 others, co-existing with them, under the same conditions, 

 in which more or less distinct indications of such a 

 process seem 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 



