Picteton the Age of Fossil Groups. 279 



absence of others. Thus we easily recognize a fauna deposited 

 in a muddy bottom by the presence of genera which live buried 

 in nmd, and by the absence of others which have need of 

 naked rocks, such as corals and perforating mollusks. Inverse 

 characters distinguish a coralline and a litoral fauna. We 

 might say that these dissimilar parallel faunas form the com- 

 plement one of the other. 



" Two dissimilar successive faunas present inverse characters. 

 If we take them together in their entirety, and over a certain 

 extent, we shall see that in general they do not exhibit 

 biological differences, that they are composed of the same 

 genera; but that the species have been raodified, although 

 retaining the same sort of life. We shall easily comprehend 

 these facts on comparing two successive faunas of the same 

 facies — muddy, or coralline. 



" It is evident that there is no general rule for the practical 

 resolution of these difficulties, and that these directions, dic- 

 tated by a judicious method, presuppose an ample collection of 

 paleeontological and stratigraphical facts. 



" Lastly, there remains the case in which dissimilar faunas are 

 separated by great geographical spaces, and here we recognize 

 the truth of the opinion expressed by M. Agassiz. There is a 

 greater difference between two contemporaneous faunas sepa- 

 rated by great geographical distances, than between two faunas 

 of the same region, but of different age, provided the epochs are 

 not very remote. This fact is incontestable, and may be proved 

 by comparisons drawn from all periods. An example, taken 

 from recent epochs, will suffice to make its bearing known. 

 The tertiary fauna of Australia is much like the modern fauna 

 of that country, and not at all like the tertiary fauna of 

 America and Europe. It is the same with these last, and we 

 find, particularly in the fauna, so abundant and so remarkable, 

 that occupied the American continent before the present 

 period, all the types that were precursors of the fauna that we 

 find there to-day, such as the edentata, the apes with thirty-six 

 teeth, etc. In each country the fauna of one epoch derives its 

 characteristics from two factors : the one resulting from that 

 constant law of modification of which every part of palaeontology 

 supplies the proof; the other, and less powerful, is the condition 

 of the organization of the preceding fauna that served for a 

 point of departure." 



1 ' It is not necessary that we should call the attention of our 

 readers to the importance of these facts, in reference to the 

 explanations we seek in order to elucidate the cause by which 

 the succession of faunas takes place/'' 



