of Carboniferous Species. 211 



as a variety of Dielasma {Terebratula) sacculus, and simulating 

 D. sufflata, than as belonging to that species." Now, what is 

 Prof. King's opinion on this question ? Is it that T. sufflata is 

 identical with the Boliand shell, aud so recurrent from Carboni- 

 ferous strata ? or that it is a distinct species, and merely simu- 

 lated in form by the Boliand shell ? Both these opinions are 

 given in the same paper ; it is therefore impossible to see which 

 expresses the views of its author. The former is, of course, that 

 which I feel satisfied to be the correct one. 



It would thus appear, in respect to the identification of the 

 Permian and Carboniferous Brachiopods just noticed, that Prof. 

 King in three cases is of the same opinion as Mr. Davidson and 

 myself, or, at least, that he allows three of the species to have 

 had a Carboniferous and Permian existence, and that in two 

 cases he has no decided opinion, while in one he disputes the 

 identification. But notwithstanding that he only in one instance 

 clearly differs from us in respect to the recurrency of the spe- 

 cies, he occupies the conclusion of his paper in arguing against 

 the method or " plan" of determining species that has been fol- 

 lowed in identifying these Carboniferous and Permian fossils. 

 Prof. King seems to consider that our views of species are too 

 comprehensive, and that in some instances we include two or 

 more distinct forms in one specific group. And in support of 

 this opinion, he refers to the evidence to be derived from the 

 study of recent shells, many of which, he asserts, would have 

 had their specific identity ignored had they been determined on 

 this plan as palaeozoic fossils. 



I am not aware myself of there being anything novel about 

 the method on which these species have been determined, it 

 having long been used by naturalists ; and it, moreover, seems 

 to be the only one that it is possible to use, on the commonly 

 received opinion of the individuality of species. For the sake 

 of precision, however, I will briefly state what our method of 

 determining species is ; and in doing so I speak for Mr. David- 

 son as well as myself, our views on this subject being the 

 same. 



By species we understand groups of individuals characterized 

 in common by peculiar features. These features or characters 

 are considered to be persistent, and special to the groups they 

 distinguish. Characters which are not persistent, but which are 

 subject to essential modification, are looked upon as only of in- 

 dividual value. The great difficulty of palaeontologists, as well 

 as of other naturalists, is to distinguish in all cases between 

 these two kinds of features or characters ; for the latter kind 

 often approach the former in apparent value; and it must be 

 allowed that it is not always an easy matter to decide whether a 



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