LIFE ON THE EAKTH. 173 



in all the countless ages which have rolled away 

 since the branches of Zamia were blown into the 

 lagoon of Stonesfield, the amount of organic change 

 has been small in each group of plants and animals ; 

 that a similar amount of change affected the unlike 

 inhabitants of land and sea ; that Mollusca and 

 Sharks, and Turtles and Crocodiles, have all been 

 modified by differences of a small description in pass- 

 ing from Oolitic to modern times, while not only hosts 

 of Ammonites and Belemnites have perished in the 

 experiment, but many new forms, as Oliva, Mitra, 

 Triton, Struthiolaria, unknown in the earlier period, 

 have come into view in the latter! But let it be 

 adopted. What follows? These small differences 

 then, accomplished in all that prodigious range of 

 elapsed time, under all that variety of physical 

 changes and removals, these are all the mutations 

 which have been possible under the constant ten- 

 dency of hereditary descent to perpetuate similar 

 forms with modification. 



One of these genera, that of Trigonia, is known 

 to be in the fossil state rich in species ; supposing 

 them to have all come from one original typical form, 

 the differences which they shew in strata of the same 

 system, deposited within the same grand period, and 

 under much similarity of conditions, argues a facility 

 in giving variations ; let this operation be supposed 



