16 ON THE LAW WHICH HAS REGULATED 



to it, and becomes the representative group of the 

 former one in another geological formation. It may, 

 however, happen, that though later in time, the new 

 series of species may never attain to so high a 

 degree of organization as those preceding it, but 

 in its turn become extinct, and give place to yet 

 another modification from the same root, which may 

 be of higher or lower organization, more or less 

 numerous in species, and more or less varied in form 

 and structure than either of those which preceded it. 

 Again, each of these groups may not have become 

 totally extinct, but may have left a few species, the 

 modified prototypes of which have existed in each 

 succeeding period, a faint memorial of their former 

 grandeur and luxuriance. Thus every case of ap- 

 parent retrogression may be in reality a progress, 

 though an interrupted one : when some monarch of 

 the forest loses a limb, it may be replaced by a 

 feeble and sickly substitute. The foregoing remarks 

 appear to apply to the case of the Mollusca, which, 

 at a very early period, had reached a high organi- 

 zation and a great development of forms and species 

 in the testaceous Cephalopoda. In each succeeding 

 age modified species and genera replaced the former 

 ones which had become extinct, and as we approach 

 the present sera, but few and small representatives of 

 the group remain, while the Gasteropods and Bi- 

 valves have acquired an immense preponderance. In 

 the long series of changes the earth has undergone, 

 the process of peopling it with organic beings has 



