524 ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 
of the Pycnodonts and by that of the true Ccelacanths ; the former 
persisting, with but insignificant modifications, from the Carboni- 
ferous 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 sufficient refutation 
of many of them. 
The Protoszoa.—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 between any ancient and any 
