PICTET S PALEONTOLOGY. 45 



he distinguishes three degrees of resemblance, denominating those 

 identical of which the individuals compared present not the smallest 

 difference; describing as analogous species those which differ by- 

 such peculiarities as in existing species would mark varieties, and 

 which may be attributed to local or climatal influence ; and recog- 

 nising as suhanalogous those which have but an imperfect analogy, 

 passing beyond the limits assigned to varieties of the same species. 

 He only considers as extinct species those which have none of these 

 degrees of resemblance with existing forms. 



" This method of comparison has been favourably received by many 

 geologists and conchologists, nor would I deny that it has exercised 

 a happy influence, by directing attention to the various degrees of 

 resemblance that exist between fossil and living shells. But it seems 

 to me that it introduces unnecessary complication into the question 

 now under discussion* ; and that instead of four kinds of difference 

 and resemblance, it is more simple, more logical and more natural to 

 admit but two. I think that the point is, not to determine whether 

 shells are identical, analogous, suhanalogous or extinct, but to settle 

 whether or not they are of the same species. 



" If we, in fact, consider attentively the distinctions established 

 by M. Defrance, we shall see that the group of analogous shells is 

 by no means brought within clear and well-defined limits. If this 

 skilful naturalist only considers as analogous those forms, between 

 "Which the differences are such as would allow us to unite them as 

 varieties of the same species, if they were both recent, there is in 

 fact no real distinction between identical and analogous species, 

 since absolute identity never exists, and the difference is confined 

 to unimportant characters, not sufficient to prevent our recognising 

 the two forms as resulting from the same stock. Between the mi- 

 nute variations which the naturalist entirely neglects and those 

 which induce him to designate a certain form as a variety, there are 

 insensible gradations, which entirely disappear in comparison with 

 the important fact of identity of specific character in the species 

 which exhibit them. 



" But if M. Defrance understands by analogous species those 

 which differ by characters a little more considerable than such as are 

 met with in the varieties of recent species, and admits at the same 

 time that such differences may have been produced by the influence 

 of climatal changes or by geological causes, his distinction becomes 

 more suspicious, for he prejudges a doubtful question, assuming the 

 agency of causes, the extent of whose action is unknown and ill-de- 

 fined. For the determination of so nice a question, we are only at 

 liberty to reason from positive data, which the study of existing na- 

 ture can alone furnish ; and to admit other influences is in fact to 

 give up gratuitously positive facts for hypotheses. If two species 

 differ by characters which cannot be explained by the influence of 

 external agents, limited as we know them to be at present, tiie pa- 



* " I speak here only of the main question. There are others secondary and of 

 smaller importance, where the degree of resemblance between different shells is a 

 matter of considerable interest." 



