Professor Pictet on the Distribution of Fossils. 261 



authors having often grouped many allied species under the 

 same name, and which were separated by their successors. 

 In like manner with fossils ; the most exact observers, or 

 those who had most leisure, have detected differences where 

 none were previously noticed. Hundreds of instances might 

 be mentioned, where species at first united had afterwards to 

 be separated, and have thus served to demonstrate the truth 

 of a law which they had, at first, the tendency to make us 

 regard as false. 



Recent investigations have taught us how far this law ex- 

 tends. Already, most palaeontologists admit it for the four 

 great periods, and even for the formations into which we 

 have immediately subdivided them. Thus we can scarcely 

 any longer deny that, in the secondary epoch, for example, 

 the fossils of the triassic, Jurassic, and cretaceous forma- 

 tions, are completely different from each other. But it is 

 probable that we ought to go still further, and that the stages 

 into which we have divided these formations, have likewise 

 their special fauna. The best recent works seem to shew, 

 for example, in the case of the cretaceous formation, that no 

 species of the fossils contained in the neocomian, albian, and 

 white chalk, is found, at the same time, in any two of these 

 formations. M. D'Orbigny goes even further, and proves 

 that he found no species common to the formations into which 

 he subdivides these stages, that is to say, to the turonian 

 and senonian in regard to the white chalk, the neocomian 

 and aptian in the neocomian stage. The results to which 

 M. D'Orbigny has come, are facts fitted to inspire perfect 

 confidence ; it remains for future investigations to shew where 

 the generality of these differences terminates, and to modify 

 the classification of formations according to these considera- 

 tions. 



I cannot conclude the discussion of this law, one of such 

 essential importance, without making one observation. I 

 have said that the demonstration of the law must result from 

 the knowledge of facts. There are cases, of rare occurrence 

 it is true, where these facts may be interpreted in a different 

 manner, according to the preconceived opinion which the 

 palaeontologist who observes them shall have formed of the 



