PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 375 



Pachyderrnata* The other families present too few num- 

 bers to pronounce with certainty on this point. The follow- 

 ing table will better explain these proportions : — 



If now we turn to a general consideration of the genera 

 of the extinct fauna with reference to their agreement with 

 the present, we find that out of the thirty-two genera (lea- 

 ving out the fossil Rodent above-mentioned, which I have not 

 yet been able to identify) which the extinct class of mam- 

 mals has hitherto afforded us, eighteen still inhabit this di- 

 strict, while fourteen are no longer found here. We thus see 

 that the deeper we examine the subordinate divisions of the 

 system, the greater is the agreement between the ancient 

 and the recent faunas ; so much so, that more than half the 

 genera are identical for the two periods. The genera com- 

 mon to both may be conveniently divided into two groups, 

 which are of very unequal importance in developing the 

 history of our globe. Under the first group I class those 

 genera that are common to both the Old and New World, 

 and which are therefore not adapted to throw any light on 

 the peculiar types of animal life belonging to this continent 

 in former periods. These genera are only six; namely, 

 Cervus, Tapirus, Felis, Canis, Lejms, and Mus. The 

 second group consists of the genera which are at present 

 confined to this quarter of the globe. It is more numerous 

 than the first, and contains the following twelve genera : — 

 Myrmecophaga, Dasypus, Dicotyles, Eyrara, Nasua, Di- 

 delphis, Echimys, Synoetheres, Ancema^ Dasyprocta, CceIo- 

 genys, and Hydrochcerus. The first glance at this group is 

 sufficient to show that '^ the fauna which inhabited the 

 tropical highlands of Brazil previous to the last re-construc- 

 tion of our earth, was in its fundamental types the same as 

 is now found there." This result is of vast importance with 

 reference to the theory of the relations of development in 

 the organic productions of our planet ; and I reserve for a 



