PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 311 



have already remarked in the genera Cutia and Capivar ; 

 namely that the ancient fauna possessed two subordinate 

 forms of this genus, of which one shows a striking agreement 

 with the existing species, while the other repeats it on a mag- 

 nified scale. The first of these subordinate forms comprises 

 two species in the genus Paca ; whereas in the two last- 

 named genera, we are only acquainted w^ith a single species 

 of each, most probably in consequence of the imperfect state 

 of our materials. How far this subordinate form is to be 

 considered as specifically identical with the existing animal, 

 we have been able to decide with certainty in the genus 

 Paca ; and we are entitled, at least for the present, to extend 

 this result not only to the other genera of this family, which 

 are found similarly circumstanced, but still more to those out 

 of it, in which the resemblance between extinct and living 

 forms is much less obvious. With reference to the numeri- 

 cal relations of this family in the present and former periods, 

 we see that only one of the existing genera is absent from 

 the list of the fossil genera, namely that of squirrels. The 

 contrary could scarcely have been expected. The squirrel 

 is an animal whose habits and haunts entirely protect it from 

 the pursuit of the larger predaceous beasts ; and that it evi- 

 dently also very seldom becomes the prey of birds, I con- 

 clude from the circumstance of my having never seen ajiy 

 trace of it in those heaps of bones that are formed in the 

 caves from the remnants of the food of the Strix perlata. 

 The absence hitherto of this genus, therefore, from the list of 

 fossil species, does not warrant us in concluding that this 

 form was in reality non-existent in those former times : on 

 the contrary we may suppose, with a high degree of proba- 

 bility, that inasmuch as this family presented so perfect an 

 agreement in those ages with what it now contains, neither 

 was this form wanting. 



In addition, there appears on the list of the extinct fauna 

 a genus which is not now found here ; whence it seems that 

 the greater variety or richness of generic forms, which for- 

 merly characterized all the families we have yet considered, 

 also extends to this. Of the nine genera whereof this family 

 now consists, six are peculiar to this continent ; the Capi- 

 var, Paca, Cutia, Perea, Synetheres, and Echimys. All 

 these genera are again found in the extinct fauna of this dis- 

 trict : and we thus obtain one of the most beautiful and con- 

 clusive arguments for a result we have already frequently 

 insisted on, namely, that the extinct fauna of this continent 

 was a true prototype of the existing races. Before I proceed 

 to compare the number of species of this family for the two 



