272 Professor Pictet on the Distribution of Fossils, 



neither more nor less perfect. But these intermediate faunas 

 differ from those which have preceded them, because the 

 vertebrates are represented by reptiles and didelphi; and 

 they are distinguished from our own in this, that they do 

 not possess monodelphous mammifera. 



We perceive, therefore, that neither the radiata, articu- 

 lata, molluscs, nor fishes, have gone on improving in the 

 geological ages, and that, from their first appearance, the 

 species belonging to these classes possessed the same degree 

 of perfection as those that live in our days. It is conse- 

 quently false to say that the primitive faunas have been gene- 

 rally composed of more imperfect animals than recent faunas. 



It will be seen, at the same time, that the superior term 

 of organization becomes elevated with the series of geologi- 

 cal eras. In the ancient formations fishes formed the upper 

 limit of perfection ; in the following epochs reptiles have 

 been created ; after these, in more recent periods, the mam- 

 mifera. It is to these very restricted facts that we must 

 reduce the law of which we are now speaking. It does not 

 hold true, if we apply it to organization considered in its 

 whole extent. It expresses an ascertained fact, if we regard 

 only the epoch of the appearance of superior degrees in this 

 organization. 



The law thus restricted, cannot, then, be of much service 

 in establishing a great superiority in recent faunas ; and in 

 support of this manner of regarding it, I shall conclude with 

 two observations. 



The first is, that we ought not, perhaps, to be too hasty in 

 asserting the absence of certain more perfect types in the 

 ancient faunas, because they have not hitherto been found in 

 them. We have scarcely any acquaintance with these faunas 

 except in relation to marine animals; and in. the present 

 state of the globe terrestrial species in general exhibit a 

 superior organization. May it not be the case, that there 

 may likewise have existed, in the primitive ages, terrestrial 

 animals more perfect than the marine, and that their re- 

 mains have not been preserved, or are so rare, that they have 

 not yet been discovered I The existence of didelphous mam- 

 mifera in the Jurassic epochs has been ascertained, by the 



