388 PERSISTENT TYPES OF LIFE 



Polypterus, while the Mesozoic Ganoids are almost all simi- 

 larly 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 Coelacanths : 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 of 

 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 foundation 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 

 by positive evidence, have been so great, but that they 

 have been so small. 



Be they great or small, however, it is desirable to attempt 

 to estimate them. Let us, therefore, take each great 

 division of the animal world in succession, and, whenever an 

 order or a family can be shown to have had a prolonged 

 existence, let us endeavour to ascertain how far the later 

 members of the group differ from the earlier ones. If 

 these later members, in all or in many cases, exhibit a 

 certain amount of modification, the fact is, so far, evidence 

 in favour of a general law of change ; and, in a rough way, 



* " Memoirs of the Geological Survey of the United Kingdom. 

 Decade x. Preliminary Essay upon the Systematic Arrangement 

 of the Fishes of the Devonian Epoch." 



