236 On computing the relative Age of Deposits 



common to both, and those which are peculiar to each, is 

 proportionate to the total resemblance between the two fauna 1 , 

 so that, when the number of identical species is great, the 

 similarity of the remainder will be great, and vice versa. 

 Hence, the degree in which the fauna of any tertiary deposit 

 resembles the existing creation may be arithmetically expressed 

 by the number of living species which it contains, and its 

 relative antiquity may be inferred accordingly. 



Mr. Charlesworth, however, objects to this numerical test 

 of Mr. Lyell, because he finds that hardly any two naturalists 

 agree in their estimate of specific differences. This is cer- 

 tainly an important, but I trust not a fatal, objection. It 

 seems to be rather a proof of human infirmity than of im- 

 perfection in the per-centage principle*, which is shown above 

 to be not essentially unphilosophical. I would rather suppose 

 that this variation of opinion proceeds from our imperfect 

 acquaintance with specific distinctions, and our inaccurate 

 estimate of their importance, than from those characters being 

 themselves really mutable and indefinite. In proportion, 

 therefore, as our knowledge of zoology advances, the utility 

 of the per-centage test will increase, and though mathematical 

 accuracy is not to be expected from it, it will probably be 

 found to indicate the zoological similarity of different deposits, 

 and, consequently, their relative age, with more certainty than 

 any other rule which can be laid down.* ^ e^» 



I now proceed to Mr. ChaHesworth's second objection to 

 Mr. Lyell's principle, grounded on the liability of fossils of 

 different ages to become mixed up in the same deposit. This 

 is a subject which has not yet excited the attention which it 

 deserves, and important errors may arise from oversight on 

 this point. Mr. Charlesworth considers that many fossils of 

 the coralline crag have in this way been enveloped in the 

 red crag, whence they are again removed by the waves, and 

 mingled with the present inhabitants of the German Ocean. 



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

 retarding the progress of geology, it must mainly be attributed to the 

 imperfect 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 

 equally 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." 

 (Charlesivorth in London and Edinburgh Phil. Mag. for 1837, P. 7.) — 



