334 Viscount d'ARCHiAC and M. de Verneuil 



space of time probably much more considerable than that during which the se- 

 condary deposits were accumulated, show that there was not any sudden and 

 complete change in the organization of the animals which peopled those seas ; 

 and that the three Palseozoic systems are combined together zoologically by a 

 small number of species common to all. Nevertheless, the modifications were such, 

 that not only the greater part of the species, but also of the genera, successively 

 ceased to live, in order that they might be, successively also, replaced by others 

 which have disappeared in their turn ; while some are persistent through all the 

 periods and still live in our present seas, as if to allow us to compare more exactly 

 the most extreme productions of creation. 



If then some few species were capable of resisting all the changes which modified 

 external circumstances during the three ancient periods, it may be supposed that 

 the same was the case during the formation of the less mighty secondary beds ; and 

 it will be difiicult to admit that the uprisings, which, during the deposition of these 

 latter, changed the face of nature in certain countries, could have occasioned at the 

 same time the complete destruction of animals which then lived in the seas most 

 distant from the theatre of those great phaenomena. 



The examination and the nature of the review which we have taken of the Palae- 

 ozoic Fauna appear to us to modify a little the scientific opinion long entertained, 

 that the organic beings of the ancient seas had a simplicity of structure and a 

 uniformity in their distribution, which entirely distinguished them from those cha- 

 racters which the more modern deposits present. This assertion derived its greatest 

 strength from the fact, that the more easily studied secondary and tertiary beds 

 have presented much higher numerical results than the others, and have done it 

 much sooner. If the numerous circumstances which will for a long time, and per- 

 haps for ever, prevent the creation of the ancient periods from being as well known 

 as that of the most recent, be taken into account, it will be seen that species were 

 already very varied and very numerous in the Silurian, Devonian, and Carboni- 

 ferous seas, and that the general uniformity of their distribution is less than has 

 been stated. We have noticed many genera which lived at only certain epochs and 

 in very limited areas, thus constituting local faunas, comparable to what is observed 

 in the more recent deposits ; but this would not prevent other genera or other spe- 

 cies which lived in the same localities from existing at the same time at the most 

 distant parts of the earth, and in the most various latitudes. 



If certain divisions, such as the Gasteropoda, the Monomyaria, the Dimyaria, 

 and the Annelides, had comparatively few representatives in the ancient seas, 

 others, such as the Polyparia and the Cephalopoda, are not less plentiful than in 

 any of the following periods ; and some, such as the Brachiopoda and the Crinoidea, 

 are infinitely more varied. Lastly, if the development of Palseozoic organism be 



