FOR THE ADVANCEMENT OF SCIENCE. 135 



and Norfolk, some facts have come under his observation which ap- 

 pear to him to point out sources of error to a considerable extent in 

 the application of the test recently proposed by M. Deshayes and 

 Mr. Lyell, and which is now so generally adopted in the classifica- 

 tions of tertiary formations. The crag has been referred by Mr. 

 Lyell to his older pliocene period, on the authority of Deshayes, 

 who identified among the fossil testacea of that deposit forty per 

 cent, with existing species. The correctness of this result has been 

 called in question by other eminent conchologists, particularly by 

 Dr. Beck, of Copenhagen, who has examined the crag fossils in the 

 author's collection, and considers that the whole of them are extinct. 

 In this opinion Dr. Beck is supported by Mr. G. B. Sowerby, who 

 states that he lias only met with two or three crag shells which may 

 perhaps be identified with existing species. Professor Agassiz has 

 inspected an extensive series of ichthyological remains (collected 

 from the crag, by the author), and pronounces them all to belong 

 to extinct genera and species. While a precisely similar result has 

 attended Dr. Milne Edwards's examination of the corals. Profes- 

 sor Phillips, in his Introduction to Geology, has placed the crag in 

 the miocene division ; while Dr. Flemming, 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 overrated 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 division to 

 which a geologist will refer any given deposit must, therefore, de- 

 pend upon his own estimate of the characters which constitute spe- 

 cific distinctions, and which is evidently liable to the greatest possi- 

 ble amount of variation. The author next entered upon an inquiry 

 respecting the course which should be adopted in obtaining the 

 relations of analogy presented by the fossils of different deposits to 

 one another, or to the races in existence at the present period. 

 The effect of the method now made use of is to class as contempora- 

 neous those deposits which respectively furnish the same per cen- 

 tage of extinct forms, without the slightest reference to the greater 

 or less degrees of approximation which such forms exhibit when 

 compared with living types. Those conchologists who agree with 

 Dr. Beck cannot, by means of the per centage test, express the dif- 

 ference in the amount of approximation presented by the testacea 

 of the crag and London clay to those now existing, because they 

 would consider all the fossils of both these formations extinct, and 

 consequently refer them both to the eocene division. Here the rela- 

 tions of analogy can only be obtained by a general estimate of the 

 amount of resemblance borne to existing species by the entire series 

 of crag and London clay fossils taken collectively. This modifica- 

 tion of the principle introduced by Mr. Lyell, when applied to the 

 fossils of those formations which, from the presence of living spe- 

 cies, can also be subjected to the per centage test, will, under some 

 circumstances., furnish results that clearly establish a fallacy in one 



