PRKVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 379 



guide, we ought to suppose for the other few species that are 

 in the same circumstances as were the fossil species of Paca, 

 until complete comparisons enable us to settle the question, 

 a similar condition, that is a specific difference from the 

 living species^ to which they show a greater or less con- 

 formity. 



I think, therefore, that I am supported by the highest de- 

 gree of probability, approaching in most points to certainty, 

 in confidently laying down this result, that the pre-existing 

 race of mammals in South America, as far as regards spe- 

 cies, was entirely different from that which now inhabits 

 this same continent. Cuvier was led to the same result by 

 his examination of the extinct fauna of the Old World ; and 

 the more this important conclusion has been doubted and 

 combated by later naturalists, the more am I gratified in 

 being enabled, by my researches in this quarter of the globe, 

 to corroborate it. 



Having thus cast a cursory glance at the extinct mammals 

 which last existed in this district, and having next considered 

 more closely their relation to the modern fauna that has suc- 

 ceeded them on the same spot, we will now advance from 

 this foundation, and by the help of these new facts, where- 

 with science has been enlarged, endeavour to elucidate seve- 

 ral important points in the history of our globe. It had 

 been a firmly received maxim in science, that the tropical 

 zone, at least in its lower portions, was either entirely unin- 

 habited in the period that immediately preceded the present 

 state of things, or at any rate was very thinly inhabited. 

 The present inquiry has, on the contrary, proved that this 

 zone, far from having been uninhabited at that time, dis- 

 played a richness and variety in its animal kingdom, which 

 seem far to surpass what nature is able to maintain there in 

 our days. We have seen that this position is certain for the 

 greater portion of the families composing the class of mam- 

 mals, and that it is true for the whole of them, as far as 

 genera are concerned ; but that it may be equally so for 

 them, with reference also to species, no one surely will doubt 

 when he thinks of the great number of species that have at 

 once been discovered, upon the very first glance we have 

 cast behind the curtain of that extinct fauna, a number so very 

 little inferior to that of the living races. This probability must 

 undoubtedly appear to every one so strong by itself, that I 

 cannot but consider it almost superfluous to corroborate it 

 by the following considerations. The extinct fauna is in the 

 natural course of things withdrawn from our observation : 

 only favourable circumstances, and luckily conducted explora- 



