Species of the Secondary and Tertiary Formations ? 249 



although it is not always thus with the generic type. But 

 in order to comprehend the whole value of these differences, 

 it is necessary to enter into some details on this point. 



The greater part of the species of geological times differ 

 more or less from the existing generations, and cannot be 

 confounded with them. In the opinion of many palaeontolo- 

 gists, among whom it is sufficient to name MM. Agassiz and 

 Deshayes, there is perhaps no species identical between these 

 two creations. MM. F. J. Pictet and D'Orbigny have also 

 adopted the same opinion in their palseontological writings. 



Although the general diversity of the beings of the ancient 

 generations from those that compose the new, is not alto- 

 gether demonstrated, it appears at all events so much the 

 more striking, as we compare species belonging to the most 

 opposite ages. It is particularly among the species of 

 the most remote times, that the dissimilarity to the living 

 races is most conspicuous. That these generations have 

 fewer points of analogy is probably owing to this, that there 

 is little parity between the climateric circumstances and ex- 

 terior media to whose influence they were subjected. 



Accordingly, organic species present similar characters 

 only where the climateric conditions (comprehending in these 

 conditions influences of every kind, which exert some action 

 on their life) are nearly the same. We can thence conceive 

 why we discover fossil species approaching the living races 

 only among the last geological periods. But is it certain 

 that these species are really identical with the latter \ We 

 must examine into this matter. 



In the earliest times of the study of fossils, a multitude of 

 living species were associated with them, owing to the want 

 of strict examination, and also, perhaps, in consequence of 

 mental indolence, which disposes us to simplify the objects 

 of our study. More attentive comparisons have proved to 

 us, that there may not exist a single species of the secondary 

 formations like the presently existing races. They have even 

 led us to presume, that such also is the case with those be- 

 longing to the most recent epoch of the tertiary formations. 



As an example of these facts, we may remind our readers, 

 that M. Agassiz, to whom we are indebted for such beautiful 



