58 NUMERICAL PROPORTION OF RECENT SHELLS [Ch. V. 
three similar subdivisions, both of the Miocene and Pliocene 
epochs. In that case, the formations of the middle period 
must be considered as the types from which the assemblage of 
organic remains in the groups immediately antecedent or sub- 
sequent will diverge. 
The Recent strata form a common point of departure in all 
countries. — We derive one great advantage from beginning our 
classification of formations by a comparison of the fossils of the 
more recent strata with the species now living, namely, the ac- 
quisition of a common point of departure in every region of the 
globe. Thus, for example, if strata should be discovered in 
India or South America, containing the same small proportion 
of recent shells as are found in the Paris basin, they also might 
be termed Eocene, and, on analogous data, an approximation 
might be made to the relative dates of strata placed in the arctic 
and tropical regions, or the comparative age ascertained of 
European deposits, and those which are trodden by our anti- 
podes. 
There might be no species common to the two groups ; yet 
we might infer their synchronous origin from the common 
relation which they bear to the existing state of the animate 
creation. We may afterwards avail ourselves of the dates 
thus established, as eras to which the monuments of preceding 
periods may be referred. 
Numerical proportion of recent shells in the different Ter- 
tiary periods. — There are seventeen species of shells discovered, 
which are common to all the tertiary periods, thirteen of which 
are still living, while four are extinct, or only known as fossil*. 
These seventeen species show a connexion between all these 
geological epochs, whilst we have seen that a much greater 
number are common to the Eocene and Miocene periods, and a 
still greater to the Miocene and Pliocene. 
We have already stated, that in the older tertiary formations, 
we find a very small proportion of fossil species identical with 
* See the Tables of M. Deshayes in Appendix I, 
