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 I 

 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 may be relied 

 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 Italy 

 there are magnificent collections, to which M. Deshayes had no 



E2 



