176 THE SUCCESSION OF ANIMAL FORMS 



their succession, in possession of which we may return to 

 attack the mystery of their origin. 



First, it must strike every observer that there is a great same- 

 ness of plan throughout the whole history of marine inverte- 

 brate life. If we turn over the pages of an illustrated textbook 

 of geology, or examine the cases or drawers of a collection of 

 fossils, we shall find extending through every succeeding for- 

 mation, representative forms of Crustaceans, Mollusks, Corals, 

 etc., in such a manner as to indicate that in each successive 

 period there has been a reproduction of the same type with 

 modifications ; and if the series is not continuous, this appears 

 to be due rather to abrupt physical changes ; since sometimes, 

 where two formations pass into each other, we find a gradual 

 change in the fossils by the dropping out and introduction of 

 species one by one. Thus, in the whole of the great Palaeozoic 

 Period, both in its Fauna and Flora, we have a continuity and 

 similarity of a most marked character. 



It is evident that there is presented to us in this similarity 

 of the forms of successive faunas and floras, a phenomenon 

 which deserves very careful sifting as to the question of identity 

 or diversity of species. The data for its comprehension must 

 be obtained by careful study of the series of closely allied 

 forms occurring in successive formations, and the great and 

 undisturbed areas of the older rocks in America seem to give 

 special facilities for this, which should be worked, not in the 

 direction of constituting new species for every slightly diver- 

 gent form, but in striving to group these forms into large 

 specific types ^ 



There is nothing to preclude the supposition that some of 

 the groups mentioned in the note are really specific types, with 



' The Rynchonellse of the type of R. plena, the Orthids, of the type of 

 O. testtidinaria, the Strophomenifi of tlie types oi S. alternata and S. Rhom- 

 boidalis, the Atrypas of the type of A. reticularis^ furnish cases in point 

 among the Brachiopods. 



