the age of Tertiary Deposits. 119 



in the miocene division ; while Dr. Fleming, who, for more than a quarter 

 of a century, has been an indefatigable collector of British shells, considers 

 that the proportion of recent species, in the fossils of that formation, has 

 been rather under than over rated by Deshayes ; and, among the corals of 

 the crag, he has detected a large proportion of living forms. 



" The particular one of Mr. Lyell's divisions, to which a geologist will 

 refer any given deposit, must, therefore, depend upon his own estimate of 

 the characters which constitute specific distinctions ; and which is evident- 

 ly liable to the greatest possible amount of variation." 



In Taylor's London and Edinburgh Philosophical Magazine, for Janu- 

 ary, 1837, p. 1, a paper is published by the same author, entitled, — "Ob- 

 servations on the crag, and on the fallacies involved in the present system of 

 classification of tertiary deposits" 



In this subsequent memoir, the discrepancies in the opinions of different 

 naturalists, as regards the specific identities of the crag fossils with re- 

 cent species, are more fully treated of; and the following remarks, having 

 immediate reference to that subject, occur at p. 7. 



" In the annual address delivered by the President to the Fellows of the 

 Geological Society, Mr. Lyell particularly adverts to the discordance of o- 

 pinion between two such eminent naturalists as Dr. Beck and M. Deshayes, 

 and suggests that it may probably be attributed to their difference of opi 

 nion as to the amount of variation necessary to constitute a distinct species. 

 Thus, for instance, Dr. Beck would look upon those six or eight forms which 

 M. Deshayes includes under the name of Lucina divaricata, as six or eight 

 distinct species of the genus Lucina ; while M. Deshayes would consider 

 them as varieties only. Now, this explanation is only admissible upon the 

 assumption that M. Deshayes allows the existence of as much difference 

 between the crag fossils, and what he now regards as their living analogues, 

 as there is between the six or eight varieties of Lucina divaricata. This is 

 an important consideration; for if M. Deshayes should assert the identifi- 

 cation to be complete, between the crag fossils and living shells, it is evi- 

 dent that the explanation affords no solution whatever of the difficulty. 



" From these facts, the following inference may, I think, be fairly drawn: 

 that if a series of tertiary fossils be placed before the most eminent concho- 

 logists in different countries, for the purpose of ascertaining, from the per- 

 centage of extinct species, what position, in a geological series, the forma- 

 tion should hold, from which these fossils had been obtained, that position 

 might be decided to be, eocene in Denmark, miocene in England, and plio- 

 cene in France ; and had we fifty intermediate gradations, it is very possible 

 that no two conchologists would refer the deposit in question to the same 

 position. 



" Greatly as the discordance of these results is to be lamented, as retard- 

 ing the progress of geology, it must be mainly attributed to the present im- 

 perfect condition of conchological science, and not be supposed to invalidate 

 the general course of induction pursued by Mr. Lyell. Nevertheless, it 

 must be admitted, that the practical application of the principle advocated 

 by this eminent geologist, in the classification of the supracretaceous rocks, 

 will be extremely limited in operation ; for even if we suppose that con- 

 chologists universally admit the soundness of the principle upon which the 

 present system of chronological arrangement is founded, they cannot equal- 

 ly make use of it as a means of obtaining numerical relations of affinity, 

 since the characters thought by one to constitute a distinction of species, 

 are, by another, looked upon as mere modifications of form." 



The views embodied in the observations of M. Desnoyers, so precisely ac- 

 cord with the opinions expressed in the above extracts, that the former can 

 hardly be looked upon in any other light than a translation of the latter ; 



