PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 313 



I was unable to distinguish accurately more than two, or at 

 the most three species, whereas an examination of the Na- 

 tural History of the district soon convinced me of the exist- 

 ence of six species in these parts. If we then suppose the 

 same number of species to have existed in the former epoch, 

 which we know to exist now (a supposition evidently not too 

 over-stretched for a period that is proved to have been so rich 

 in animal forms), we have already a greater number of species 

 for that period than for the present ; and we are emboldened 

 to extend to this family also, the result to which the consi- 

 deration of all the other families has led us, namely, the su- 

 periority of the ancient fauna over the present, with reference 

 to variety of species. This conclusion, which must be con- 

 sidered as established for the family taken as a whole, is also 

 true for many of its genera, as for instance, Cutia, Capivar, 

 and Paca. We have already observed the same fact in one 

 family, in the instance of the genus Dicotyles ; and we are 

 thus conducted to the remarkable result, that not only are all 

 the families we have hitherto examined to be consider- 

 ed as mere fragments of what they were in former times, 

 but that also many of their genera are similarly circum- 

 stanced. A closer examination of these genera acquaints us 

 also with two other facts, not less important, namely, first 

 that they are all such as are now peculiar to the new world ; 

 and secondly, that such of their specific forms as have re- 

 presentatives in the existing fauna, are distinguished by a 

 smaller bulk than those which have no such representatives 

 now ; so that we are induced to regard the existing fauna as 

 a repetition of the extinct, on a diminished scale, with refer- 

 ence both to numbers and size. 



Family of Marsupials, 



Of this family there is only a single existing genus in this 

 district : it is, however, tolerably abundant in species. These 

 admit of two subdivisions according to their size ; one com- 

 prising the larger species, which both in habits and magni- 

 tude may be compared to our martens and polecats ; the 

 other the smaller, that scarcely exceed our mice and rats. 

 I am acquainted with two species in the first division, Didel- 

 phis aurita, Pr. Max., and Did. alhiventer, mihi, and three 

 in the latter. Did. mwina, Lin., Did. hrachyura, Pall., and 

 Did. pusilla, Desm. I find the fossil remains of species be- 

 longing to both these divisions, which, for the present, I 

 refer to only two species ; at the same time applying to this 

 genus the observations I made respecting the number of spe- 



