2 78 Plctet on the Age of Fossil Groups. 



it is very probable that it was extinguished in. the north, and 

 probably" in the centre. We might find many analogous 

 examples from which we should draw the conclusion that 

 identical faunas, separated by great geographical spaces, may 

 indicate identical climates which were not contemporaneous. Let 

 us only remark, that as analogous causes produce similar 

 effects, it will ordinarily happen that in different regions the 

 series of faunas will themselves be identical. The identity of 

 faunas, insufficient to prove absolute contemporaneity, will thus 

 serve to show that they had a similar relative age in the series 

 to which they belong. 



"If we now occupy ourselves with the case in which the 

 faunas differ from each other, we find ourselves confronting an 

 investigation a little more delicate and a little more difficult. 



" In the case where the geographical distances between the 

 faunas are not considerable, their difference will most often 

 result from the circumstance that they were formed at different 

 epochs. This is one of those facts which stratigraphy has so 

 often put in evidence that we need not insist upon it. In a 

 given region we frequently find different faunas superimposed in 

 an identical order that proves their regular succession in time. 

 But this rule is subject to important exceptions. Just as in 

 natural seas, the association of different species follows the 

 nature of the sea-bed, that of the waters, their depths, etc. ; so, 

 during the same geological periods, different faunas may have 

 been deposited on muddy beds, on rocky banks, in profound 

 depths, etc. Geologists and palaeontologists have for a long 

 time demonstrated these facts, and have given the names of 

 'muddy facies/ 'coralline facies/ etc., to deposits in which 

 contemporary, but dissimilar fauna have been preserved. 

 Every one knows the curious researches of Edward Forbes, on 

 the different associations which the existing seas present under 

 analogous circumstances; and, more recently, M. Alphonse 

 Milne-Edwards has given a new extension to these facts by the 

 discovery of species altogether new and unknown, obtained from 

 very great depths. We may therefore say that, according to 

 circumstances, the difference between faunas of the same geo- 

 graphical region may sometimes correspond with the same 

 epoch, sometimes with a different epoch. 



"It remains to be seen if, besides the stratigraphical 

 evidence, which is alone incontestable, the palaeontologist is 

 completely disarmed when he endeavours to deal with these par- 

 ticular cases. We do not think so, and we believe, on the con- 

 trary, that the nature and composition of the faunas, generally 

 bear with them the answer to these questions. Two dissimilar 

 parallel faunas are ordinarily characterized by biological differ- 

 ences, manifested by the existence of certain genera, and the 



