390 PERSISTENT TYPES OF LIFE 



(affecting at most the proportions of the body and fins, 

 and the character and sculpture of the scales), notwith- 

 standing their enormous range in time. In all the essentials 

 of its very peculiar structure, the Macropoma of the Chalk 

 is identical with the Coslacanthus of the Coal. Look at 

 the genus Lepidotus, again, persisting without a modification 

 of importance from the Liassic to the Eocene formations 

 inclusive. 



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 Phascololherium 

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

 modern Opossum ; or a Lophiodon, or a Paleoiherium, 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 unquestionable testimony we can procure positive 

 evidence fails to demonstrate any sort of progressive 

 modification towards a less embryonic, or less generalised, 

 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 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 Gastero- 

 poda in the later. A case less open to the objection of 

 negative evidence, however, is that afforded by the Tetra- 

 branchiate Cephalopoda, the forms of the shells and of 



