86 



OUTLINES OF GEOLOGY. 



animal life having existed. These, which belong in the 

 higher rocks to genera now existing, become more and more 

 different from those of the present day, and it is in this class 

 that we now find the most marked evidence of the changes 

 in the inhabitants of the globe; for it seems to be proved^ 

 that the same species is never common to two different 

 orders, and rarely to two subformations even of the same 

 group. 



The shells of molluscous animals are more numerous than 

 the remains of any other of the grand divisions of the ani- 

 mal kingdom ; they have, therefore, hitherto furnished the 

 most obvious distinction between the several formations. In 

 the diluvium they differ from those of the present day, prin- 

 cipally in furnishing evidence of the extension of such a cli* 

 mate, as we now find within the tropics, to the temperate 

 zone. In the Pliocene, Miocene, and Eocene "groups, they 

 gradually change their character, until less than half the 

 fossil shells of the latter belong to existing genera. In the 

 higher formations of the supermedial order, an entire change 

 in the fossil shells is observed, so that it would appear as if 

 the deposit of the last layers of chalk was accompanied by 

 the entire extinction of all the species, and most of the gene- 

 ra then living, which were succeeded by a new creation. 



193. It is unnecessary to dwell longer on the detail of 

 these changes in animal life ; it is sufficient for our purpose to 

 state, that in proceeding from the lower to the higher orders, 

 there appears to have been a gradual development of the 

 powers and functions of animal life ; for, if the grand divi- 

 sions whose organization is least perfect continue to exist up 

 to the present day, although in new families, genera, and 

 species ; there are added to them at each successive step, 

 families, orders, and grand divisions of more and more per- 

 fect organization, until the scale is terminated by man, supe- 

 rior in this respect to any other animal. 



