Professor Pictet on the Distribution of Fossils. 263 



tion still more different, which require the formation of new 

 families or orders. 



This law holds true in regard to all classes of animals, but 

 it presents some differences in its application. The classes 

 which have appeared from the first, and which consequently 

 have representatives in the most ancient formations, contain 

 forms which have undergone little variation during very long 

 periods. In those, on the contrary, whose appearance is 

 comparatively recent, the law becomes applicable, as it were, 

 in a more rapid manner, and the forms vary at less remote 

 intervals. If we compare, for example, the mollusca and the 

 mammifera, we will perceive that the former, which have ex- 

 isted in the most ancient eras of which we have any know- 

 ledge, have scarcely undergone any change of form since the 

 termination of the cretaceous epoch, and that the shells of 

 the tertiary formation almost all belong to the same genera 

 as modern shells ; while the mammifera, which have appeared, 

 for the first time, at the commencement of the tertiary epoch, 

 present us with forms which render it necessary to create 

 many new genera. In the most ancient formations of this 

 epoch we find, along with shells of the same genera as our 

 own, the anoplotherium^ anthracotherium, and the palceotherium, 

 which are extinct types ; and we must come to the most re- 

 cent tertiary formation, and the diluvian epoch, before we 

 can find a mammiferous fauna of which the majority of species 

 can be referred to existing genera. 



At the same time, however real may be the facts ge- 

 nerally expressed by this law, we must not exaggerate it, 

 from a desire to render it too precise. It holds true when 

 we compare faunas with each other, as a whole ; but it 

 would be a serious error to imagine that it extends to all 

 the details. The ancient formations, in which a great part 

 of the animals present very different forms from those of 

 existing beings, and the fauna of which has a general aspect 

 which clearly distinguishes it from the most recent faunas, 

 likewise offer to our notice many species very nearly allied 

 to such as live in our own times. If, for example, the 

 cephalopod molluscs are represented in the ancient forma- 

 tions by the lituites, the orthoceratites, and other extinct 



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