Pictet on the Age of Fossil Groups. 275 



PICTET ON THE METHOD OF DETERMINING THE 

 AGE OF FOSSIL GROUPS. 



M. F. J. Pictet, under the title of " Discussion de quelques 

 points Palseontologiques," publishes, in the Bibliolheque Uni- 

 verselle of Geneva, some very important comments on a dis- 

 course delivered by Professor Agassiz, in reference to the 

 classification of the Museum at Cambridge, United States, in 

 which the last named philosopher observes : — 



" Until now, geologists, in identifying the horizons of the 

 successive deposits which form the crust of our globe, have 

 started with the idea, universally admitted, that animals of the 

 same geological age are either identical, or closely related over 

 wide geographical extents. Nothing is farther from the truth 

 than this hypothesis, and it suffices to compare the fauna of the 

 present period in distant continents to see how much they 

 differ. If the remains of ancient times, belonging" to the same 

 geological periods, have, in general, appeared identical or closely 

 related, that arises principally from the fact that they have 

 been studied in the same geographical zones. Actually we find 

 the same resemblance between the animals that live in the tem- 

 perate zones of Europe, Asia, and North America ; but when 

 we pass to other climates the scene changes completely. It 

 was the same in past ages, as Ave are taught by the tertiary 

 mammalia in Southern Africa and in Australia, and I have no 

 doubt this fact would be confirmed by more ancient formations 

 as yet incompletely known. The specific differences between 

 remains of the same age, found in deposits remote from each 

 other, are more clearly demonstrated every day. Since I began 

 to compare the fossils of America with those of Europe, I have 

 been led by degrees to infer that we should probably never be 

 able to establish the specific identity of animals that lived at 

 great distances from each other, although they were contempo- 

 raneous. The doctrine of the identity of fossils of the same 

 age requires great modifications. I am already certain that 

 species of the same family, belonging to different epochs, but 

 found in corresponding latitudes, are often more nearly related 

 than species of the same age belonging to different zones. The 

 time is rapidly approaching when zoological affinity alone will 

 not be considered a sure criterion of contemporaneity ; nor will 

 the most striking zoological differences be held sufficient 

 proof of difference of geological age. I have arrived at this 

 result, unexpected, and perhaps painful, to geologists, by a 

 careful comparison of numerous ancient faunas, arranged in the 

 manner which I have already explained. If this discovery 

 renders, on one hand, the determination cf formations by means 



