Ch. V.] 
DETERMINATION OF SPECIES. 
51 
will be seen figured in the plates, illustrative of the different 
tertiary eras ; but we were more anxious, in this work, to place 
in a clear light, a point of the greatest theoretical interest, which 
has been often overlooked or controverted, viz., the identity of 
many living and fossil species, as also the connexion of the 
zoological remains of deposits formed at successive periods. 
The value of such extensive comparisons^ as those of which 
the annexed tables of M. Deshayes give the results, depends 
greatly on the circumstance, that all the identifications have 
been made by the same naturalist. The amount of variation 
which ought to determine a species is, in cases where they ap- 
proach near to each other, a question of the nicest discrimina- 
tion, and requires a degree of judgment and tact that can hardly 
be possessed by different zoologists in exactly the same degree. 
The standard, therefore, by which differences are to be mea- 
sured, can scarcely ever be perfectly invariable, and one great 
object to be sought for is, that, at least, it should be uniform. 
If the distinctions are all made by the same naturalist, and his 
knowledge and skill be considerable, the results mav be relied 
O •* ml 
on with sufficient confidence, as far as regards our geological 
conclusions. 
If one conchologist should inform us that out of 1122 species 
of fossil testacea, discovered in the Paris basin, he has only 
been enabled to identify thirty-eight with recent species, while 
another should declare, that out of two hundred and twenty-six 
Sicilian fossil shells, no less than two hundred and sixteen be- 
longed to living species, we might suspect that one of these 
observers allowed a greater degree of latitude to the variability 
of the specific character than the other; but when, in both in- 
stances, the conclusions are drawn by the same eminent con- 
chologist, we are immediately satisfied that the relations of these 
two groups, to the existing state of the animate creation, are as 
distinct as are indicated by the numerical results. 
It is not pretended that the tables, to which we refer, com- 
prise all the known tertiary shells. In the museums of Italv 
there are magnificent collections, to which M. Deshayes had no 
E2 
