X.] PERSISTENT TYPES OF LIFE. 225 



The late able ichthyologist Heckel pointed out the 

 fact, that, while the Pycnodonts never possess true ver- 

 tebral centra, they differ in the degree of expansion and 

 extension of the ends of the bony arches of the vertebras 

 upon the sheath of the notochord ; the Carboniferous 

 forms exhibiting hardly any such expansion, while the 

 Mesozoic genera present a greater and greater develop- 

 ment, until, in the Tertiary forms, the expanded ends 

 become suturally united so as to form a sort of false ver- 

 tebra. Hermann von Meyer, again, to whose luminous 

 researches we are indebted for our present large know- 

 ledge of the organization of the older Labyrinthodonts, 

 has proved that the Carboniferous Arcliegosaiirus had 

 very imperfectly developed vertebral centra, while the 

 Triassic Mast?clonsaurus had the same parts completely 

 ossified. 1 



The regularity and evenness of the dentition of the 

 Anoplotheriiim, as contrasted with that of existing 

 Artioclactyles, and the assumed nearer approach of the 

 dentition of certain ancient Carnivores to the typical 

 arrangement, have also been cited as exemplifications of 

 a law of progressive development, but I know of no 

 other cases based on positive evidence which are worthy 

 of particular notice. 



"What then does an impartial survey of the positively 

 ascertained truths of palaeontology testify in relation to 

 the common doctrines of progressive modification, which 

 suppose that modification to have taken place by a ne- 

 cessary progress from more to less embryonic forms, or 

 from more to less generalized types, within the limits of 

 the period represented by the fossiliferous rocks ? 



It negatives those doctrines ; for it either shows us no 



1 As this Address is passing through the press (March 7, 1862), evidence 

 lies before me of the existence of a new Labyrinthodont {Pholidog aster), 

 from the Edinburgh coal-field, with well-ossified vertebral centra. 



