from the Characters of their embedded Fossils. 235 



tical application * ; and that, although they may diminish, they 

 do not destroy, its utility. 



Mr. Lyell's proposition may be stated thus : — " The ratio 

 of extinct to existing species of animals in the tertiary de- 

 posits is proportionate to their antiquity." To this Mr. 

 Charlesvvorth objects, first, that naturalists differ greatly in 

 their estimate of specific characters, and, therefore, that we 

 cannot make accurate enumerations of species ; and, secondly, 

 that fossils of different ages are liable to become mixed in the 

 same stratum, and that we may thus be misled in inferring 

 their contemporaneous existence. We will consider each of 

 these objections separately. 



The essential point in Mr. Lyell's test is this : that fossil 

 forms recede from existing ones in a degree proportionate to 

 their antiquity. This general proposition is admitted by all 

 who are conversant with geological facts. Observations made 

 in all parts of the world, upon the whole fossiliferous series, 

 tend to establish this truth ; and, though a few isolated ex- 

 ceptions may exist, they do not affect its general application. 

 The difficulty is to find a correct measure of the resemblance 

 or disagreement between living and extinct forms. In the 

 secondary strata, where the fossil species are admitted to be 

 wholly extinct, we have, at present, no other measure than the 

 general estimate of resemblance and disagreement which per- 

 sons conversant with organic forms may be capable of making. 

 We cannot, I think, venture upon numerical comparisons of 

 extinct and existing genera, for these groups are, as yet, little 

 more than vague human generalisations, to which arithmetical 

 calculations cannot be correctly applied. 



But in the tertiary series, where existing species are sup- 

 posed to make their first appearance, Mr. Lyell conceived 

 that a measure of zoological resemblance might be found in 

 the numerical proportion of these existing species to the 

 extinct ones which accompany them. Now, those persons 

 who, unlike the Lamarckian school, believe in the reality 

 and permanence of specific distinctions, must admit that this 

 is a very logical path to the desired truth ; for, on comparing 

 the faunas, or entire assemblage of animals of two geo- 

 logical periods or geographical regions, we shall commonly 

 find that the numerical ratio between the species which are 



* " These observations have no reference to the principles which Mr. 

 Lyell has advocated in determining the age of tertiary deposits ; they apply 

 exclusively to the errors which may arise in the application of those prin- 

 ciples." (Charlesworth, on certain Errors which may affect the Determina- 

 tion of the Age of Tertiary Deposits. In London and Edinburgh Philoso- 

 phical Magazine for 1835,' p. 468.) — Edit. 



