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 wra, 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 
