Mr. H. Seeley on the Fossils of the Hunstanton Red Rock. 279 
naturally ; for if the existence and origin of species should be 
due to the continuous action of physiological laws, then, seeing © 
that differentiation goes on in a sort of increasing geometrical 
proportion with every successive elaboration of fundamental 
organic structures, it will be evident that (supposing groups to 
be always founded on characters equally important) the duration 
of the genus or species in time will be directly as its degrada- 
tion. Consequently species of Vertebrata equal in value with 
species of Mollusca would mark the age with greater certainty. 
Hence until characters are coordinated and the relative duration 
of species worked out, no very determinate conclusion will ensue 
from the counting of heads. 
And there is nothing to show that, because the agencies which 
accumulated strata in a given area ceased, therefore the life in 
that area became extinct; for the superposition of a distinct 
deposit can never necessitate a different set of fossils. And as no 
physical change can operate simultaneously over more than a 
part of the globe, there must always be a portion of the circum- 
ference of the disturbed area where the forms of life will be 
scarcely if at all affected. And just as, in modern migrations of 
animals in space, instances occur where some are cut off from 
the main body and retained in what now seems an unnatural 
habitat, so must it sometimes in olden times have happened 
that a smaller or larger body, or all the forms of life of an 
area, became land-locked, and therefore the species elsewhere 
characteristic of different deposits would sometimes occur 
mixed in the same stratum. Hence in cases where fossils 
hitherto peculiar to any given bed occur in new combinations, 
their value in fixing the age of the stratum must generally be 
dubious. 
In every class a majority of the fossils was previously known 
from the Upper Greensand ; so it is evident that the fossils indi- 
cate a greater affinity with that stratum than with any other. 
But as there are Gault fossils, and they occur at the base, it is 
possible that the base of the bed may be older than ordinary 
Greensand, and bridge over the interval indicated by the change 
of the Gault to Greensand. Similarly, as there are Chalk fossils, 
it is possible that the upper part of the bed may be newer than 
the Greensand elsewhere, and bridge over the gap between that 
deposit and the Chalk-marl. So the Hunstanton Rock might 
probably be the most perfect exhibition of the Upper Greensand 
that is known. Of the named fossils, 58 are Upper Greensand 
forms, 35 occur in the Chalk, and 21 in the Gault. 
But, to see the real value of numbers like those of Gault Ce- 
phalopods and Chalk bivalves in the table, it must be seen how 
