538 On Mr, LyelVs Classification 



temporaneous those deposits which respectively furnish the 

 same per-centage of extinct forms, without the slightest refer- 

 ence to the greater or less degrees of approximation which 

 these forms exhibit when compared with living types. The 

 conchologists who agree with Dr. Beck cannot, by means of 

 the per-centage test, express the difference 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. 



In this instance, the relations of analogy can only be ob- 

 tained by a general estimate of the amount of resemblance 

 borne to existing species by the entire series of crag or London 

 clay fossils, taken collectively. This mode of procedure may, 

 at first, appear only a different adaptation of the numerical 

 plan adopted by Mr. Lyell. It will, however, be found an 

 important modification of his principle ; for, when applied to 

 the fossils of those formations which, from the presence of 

 living species, can also be subjected to the per-centage test, 

 it will, under some circumstances, furnish results that clearly 

 establish a fallacy in one of the two methods. For instance, 

 Deshayes finds that the fossils of the red and coralline crag 

 contain equal proportions of extinct species, and consequently, 

 if tried by the per centage test, these separate deposits present 

 an equal degree of approximation to the Testacea now inha- 

 biting the German Ocean. But, taking Dr. Beck as the 

 authority, the per centage test cannot be applied, and these 

 fossils are, therefore, examined with reference solely to the 

 totality of analogy which each entire series presents. Now 

 the forms most remote from existing species occur in the coral- 

 line crag ; this bed will, therefore, appear the older. 



The author then changes his line of argument, and, assum- 

 ing that there is a general agreement among conchologists as 

 to the characters which should be depended upon in discri- 

 minating species, and also that the per-centage test is the 

 true method of obtaining relations of analogy, he proceeds to 

 enquire whether the association of organic remains in fossil- 

 iferous deposits implies their previous contemporaneous ex- 

 istence. The evidence drawn from this source appears to the 

 author to be by no means so conclusive as it has been gene- 

 rally considered ; and his opinions have been formed princi- 

 pally from an attention to the causes now in operation upon 

 the earth's surface. 



* " The small portion of this island occupied by the crag 

 formation is intersected in one spot with several estuaries, 



* The portion between inverted commas is given at full length. 



