*J PERSISTENT TYPES OF LIFE. 219 



and the younger Fishes ; the former, the Devonian 

 Ganoids, bcin^ almost all members of the same sub-order 

 as JPoli/pterus, while the Mesozoic Ganoids are almost 

 all similarly allied to Lepidosteus} 



Again, what can be more remarkable than the singular 

 constancy of structure preserved throughout a vast period 

 of time by the family of the Pycnodonts and by that 

 of the true Ccelacanths : the former persisting, with but 

 insignificant modifications, from the Carboniferous to the 

 Tertiary rocks, inclusive ; the latter existing, with still 

 less change, from the Carboniferous rocks to the Chalk, 

 inclusive ? 



Among Reptiles, the highest living group, that of the 

 Crocodilia, is represented, at the early part of the Mesozoic 

 epoch, by species identical in the essential characters o^ 

 their organization with those now living, and differing 

 from the latter only in such matters as the form of the 

 articular facets of the vertebral centra, in the extent 

 to which the nasal passages are separated from the 

 cavity of the mouth by bone, and in the proportions 

 of the limbs. 



And even as regards the Mammalia, the scanty 

 remains of Triassic and Oolitic species afford no founda- 

 tion for the supposition that the organization of the 

 oldest forms differed nearly so much from some of those 

 which now live as these differ from one another. 



It is needless to multiply these instances ; enough has 

 been said to justify the statement that, in view of the 

 immense diversity of known animal and vegetable forms, 

 and the enormous lapse of time indicated by the accumu- 

 lation of fossiliferous strata, the only circumstance to be 

 wondered at is, not that the changes of life, as exhibited 



1 " Memoirs 01 the Geological Survey of the United Kingdom. — Decade x. 

 Preliminary Essay upon the Systematic Arrangement of the Fishes of the 

 Devonian Epoch." 



