528 ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 
degree of expansion and extension of the ends of the bony arches of 
the vertebree upon the sheath of the notochord; the Carboniferous 
forms exhibiting hardly any such expansion, while the Mesozoic 
genera present a greater and greater development, until, in the 
Tertiary forms, the expanded ends become suturally united so as to 
form a sort of false vertebra. 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 Archegosaurus had very imperfectly de- 
veloped vertebral centra, while the Triassic Wastodonsaurus had the 
same parts completely ossified.? 
The regularity and evenness of the dentition of the Azoplothe- 
riumt as contrasted with that of existing Artiodactyles, and the 
assumed nearer approach of the dentition of certain ancient Carni- 
vores to the typical arrangement, have also been cited as exempli- 
fications 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 paleontology 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 incorrect 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 
1 As this Address is passing through the press (March 7, 1862), evidence lies before me 
of the existence of a new Labyrinthodont (Pho/dogaster), from the Edinburgh coal-field, 
with well-ossified vertebral centra. 
