130 MANTTAL OF THE MOLLTISCA. 



been thirty times submerged, and bas formed part of the sea- 

 bed during two-tbirds of all tbo past geological time, — there 

 will be no difficulty in accounting for the migration of sea-shells, 

 or the diffusion of marine genera. 



On the other hand, it may be inferred that every part of tho 

 present sea has been dry land many different times ; on an 

 average not less than thirty times, — amounting to one-third • 

 of the whole interval since the Cambrian epoch. 



The average duration of the marine species has been assumed 

 at only one-third the length of a geological period, and this 

 harmonises with the fact that so few (either living or extinct) 

 have a world-wide distribution. 



The life of the land-snails and of the fresh-water shells has 

 been of longer average extent, enabling them to acquire a wide 

 range, notwithstanding their tardy migrations. 



But when we compare the estimated rate of change in physical 

 geography with the duration of genera and families of shells, we 

 not only find ample time for their diffusion by land or sea over 

 large portions of the world, but we may perceive that such 

 transferences of the scene of creation must have become in- 

 evitable. 



Method of Geological Investigation. — In whatever way geo- 

 logical history is written, its original investigators have only 

 one method of proceeding — from the known to the unknown — 

 or backwards in the course of time. 



The newest and most superficial deposits contain the remains 

 of man and his works, and the animals he has introduced. 



Those of pre-historic date, but still very modern, contain 

 shells, &c. , of recent species, but in proportions different from 

 those which now prevail (pp. 89, 90, 93). Some of the species 

 may be extinct in the immediate neighbourhood of the deposits, 

 but still living at a distance. 



In the harbour of New Bedford are colonies of dead shells of 

 the Pholas costata, a species living on the coast of the Southern 

 States. At Bracklesham, Sussex, there is a raised sea-bed 

 containing 35 species of sea-shells living on the same coast, 

 and 2 no longer living there, viz. — Peden polymorphus, a Medi- 

 terranean shell ; and Lutraria rugosa, still found on the coasts 

 of Portugal and Mogador. 



Tertiary Age. — If any distinction is to be made between 

 *' Tertiary" and *' Post- tertiary " strata, the former term should 

 be restricted to those deposits which contain some extinct species. , 

 And the newest of these, in Britain, contain an assemblage of 

 Northern shells. Professor Forbes has published a list of 1241 



