Mr. H. Seeley on the Fossils of ttie 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 diflferentiation 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 Grecnsand ; 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 



