Professor Pictet on the Distribution of Fossils. 267 



these with perfect accuracy. If, on the contrary, some of 

 these bodies are peculiar, and others common to many for- 

 mations, it follows that only part of them can furnish any 

 results ; and hence arises a source of uncertainty and the 

 risk of error. Geologists who have not admitted the speci- 

 alty of fossils, and who have been conscious, at the same 

 time, that these bodies must be taken into account in deter- 

 mining formations, have distinguished fossils as characteris- 

 tic, that is to say, those whose existence may be regarded as 

 a certain criterion for fixing the age of a formation, and 

 fossils non-characteristic^ such as cannot be employed for 

 that purpose. Naturalists, on the contrary, who admit the 

 specialty of fossils, regard the whole of them as character- 

 istic, and as furnishing results equally certain, provided 

 they can be clearly determined. 



In discussing this important law, palaeontologists have not 

 always taken up the same ground. M. Defrance, in parti- 

 cular, has thought it necessary to adopt a particular method 

 for the study of fossil shells. In comparing them, he dis- 

 tinguishes three degrees of resemblance ; those of which the 

 individuals, when compared, present no difference whatever, 

 he names identical shells ; such as differ in characters of the 

 same order as those which, in a recent state, constitute 

 varieties, and which may be ascribed to the more or less 

 prolonged influence of heat, locality, &c., he calls analojous 

 species : suhanalogous species are those which have only a 

 remote analogy, and surpass the limits assigned to varieties 

 of the same species. He applies the name, lost species^ to 

 such as present no degree of resemblance to living species. 



This method of comparison has been favourably received 

 by many geologists and conchologists ; and I do not deny 

 that it has had a favourable influence, by directing attention 

 to the different degrees of resemblance between fossil shells 

 and those still living. But it appears to me that it is a use- 

 less complication of the question,* and that instead of four 

 categories of differences and resemblances, it is more simple, 



* 1 here speak only of the principal question ; there are secondary 

 questions, and of inferior importance, where it ma}' be interesting to de- 

 termine the analogy of shells. 



