258 Professor Pictet on the Distribution of Fossils. 



as well as more logical and natural, to admit only two. I 

 think that the object is not to determine whether shells are 

 identical, analogous, subanalogous, or extinct, but rather 

 whether they are or are not of the same species. 



If indeed we examine with some attention the distinction 

 established by M. Defrance, we will perceive that the cate- 

 gory of analogous shells is not comprehended within clear 

 and well-defined limits. If this skilful naturalist considers 

 as analogous, only such species as differ from each other by 

 characters of such a kind, that if both were alive, we would 

 regard them as varieties of one species, then there is no real 

 interest in distinguishing analogous and identical shells, in- 

 asmuch as absolute identity never exists, and both the one 

 and the other differ only in such slight characters, that there 

 is nothing to prevent us admitting the possibility of their 

 having proceeded from the same stock. Between those 

 trifling variations which the naturalist neglects, and those 

 which lead him to distinguish a certain type under the name 

 of a variety, there are insensible shades and transitions, 

 which are wholly effaced when confronted with^the essential 

 fact, that the shells which present them ought to be referred 

 to the same species. 



But if M. Defrance understands by analogous species, 

 shells which differ in characters a little more considerable 

 than the varieties of the same living species; and if he admits, 

 at the same time, that these differences may have been pro- 

 duced by the influence of the changes of climate, or by geo- 

 logical causes, his distinction becomes much more dangerous, 

 for it prejudges a doubtful question, and rests on the action 

 of unknown and ill-defined forces. For the solution of so 

 difficult a question, we must reason only on certain grounds, 

 which the study of living nature alone can furnish ; to admit 

 more extensive influences, is gratuitously to renounce posi- 

 tive facts for hypothesis. If two species differ in charac- 

 ters which cannot be explained by the influence of exte- 

 rior agents, limited as we regard it in the present day, the 

 palaeontologist ought to establish their difference on the prin- 

 ciples of modern science. In this manner he will bring to- 



