19(5 LAY SERMONS, ESSAYS, AND REVIEWS. [x. 



had very imperfectly developed vertebral centra, while the 

 Triassic Mastodonsaurus had the same parts completely ossified. 1 



The regularity and evenness of the dentition of the Anoplothe- 

 rium, as contrasted with that of existing Artiodactyles, 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 necessary 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 evidence 

 of any such modification, or demonstrates it to have been very 

 slight ; and as to the nature of that modification, it yields no 

 evidence whatsoever that the earlier members of any long- 

 continued group were more generalized in structure than the 

 later ones. To a certain extent, indeed, it may be said that 

 imperfect ossification of the vertebral column is an embryonic 

 character ; but, on the other hand, it would be extremely incor 

 rect to suppose that the vertebral columns of the older Vertebrata 

 are in any sense embryonic in their whole structure. 



Obviously, if the earliest fossiliferous rocks now known are 

 coeval with the commencement of life, and if their contents give 

 us any just conception of the nature and the extent of the earliest 

 fauna and flora, the insignificant amount of modification which 

 can be demonstrated to have taken place in any one group of 

 animals, or plants, is quite incompatible with the hypothesis that 

 all living forms are the results of a necessary process of progres- 



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

 lies before me of the existence of a new Labyrinthodont (Pholidogaster), 

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



