1 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



of the same suborder as PoJypterus, while the Mesozoic Ganoids are 

 almost all similarly allied to Lepidosteiis^. 



Again, what can be more remarkable than the singular constancj'' 

 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 Carbonife- 

 rous 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 accumulation 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 ascer- 

 tain, 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, the rapidity of that 

 change will be measured by the demonstrable amount of modification. 

 On the other hand, it must be recollected that the absence of any 

 modification, while it may leave the doctrine of the existence of a 

 law of change without positive support, cannot possibly disprove all 

 forms of that doctrine, though it may afford a sufiieient refutation 

 of many of them. '.^ 



The Protozoa. — The Protozoa are represented throughout the whole 

 range of geological series, from the Lower Silurian formation to the 

 present day. The most ancient forms recently made known by 

 Ehrenberg are excessively like those which now exist : no one has 

 ever pretended that the difference betwxen any ancient and any 

 modern Poraminifera is of more than generic value ; nor are the 



* •Memoirs of the Geological Survey of the United Kingdom. — Decade x. 

 Preliminary Essay upon the Systematic Arrangement of the Pishes of the Devo- 

 nian Epoch.' 



