Chap. X.] IN ANY SINGLE FORMATION. 281 



those which lived on naked submarine rocks, would be embedded : 

 and those embedded in gravel or sand would not endure to a distant 

 epoch. Wherever sediment did not accumulate on the bed of the 

 tea, or where it did not accumulate at a sufficient rate to protect 

 organic bodies from decay, no remains could be preserved. 



Formations rich in fossils of many kinds, and of thickness 

 sufficient to last to an age as distant in futurity as the secondary 

 formations lie in the past, would generally be formed in the archi- 

 pelago only during periods of subsidence. These periods of 

 subsidence would be separated from each other by immense inter- 

 vals of time, during which the area would be either stationary or 

 rising ; whilst rising, the fossiliferous formations on the steeper 

 shores would be destroyed, almost as soon as accumulated, by 

 the incessant coast-action, as we now see on the shores of South 

 America. Even throughout the extensive and shallow seas within 

 the archipelago, sedimentary beds could hardly be accumulated of 

 great thickness during the periods of elevation, or become capped 

 and protected by subsequent deposits, so as to have a good chance 

 of enduring to a very distant future. During the periods of sub- 

 sidence, there would probably be much extinction of life ; during 

 the periods of elevation, there would be much variation, but the 

 geological record would then be less perfect. 



It may be doubted whether the duration of any one great period 

 of subsidence over the whole or part of the archipelago, together 

 with a contemporaneous accumulation of sediment, would exceed the 

 average duration of the same specific forms ; and these contingen- 

 cies are indispensable for the preservation of all the transitional 

 gradations between any two or more species. If such gradations 

 were not all fully preserved, transitional varieties would merely 

 appear as so many new, though closely allied species. It is also 

 probable that each great period of subsidence would be interrupted 

 by oscillations of level, and that slight climatal changes would inter- 

 vene during such lengthy periods ; and in these cases the inhabitants 

 of the archipelago would migrate, and no closely consecutive record 

 of their modifications could be preserved in any one formation. 



Very many of the marine inhabitants of the archipelago now 

 range thousands of miles beyond its confines ; and analogy plainly 

 'cads to the belief that it would be chiefly these far-ranging species, 

 though only some of them, which would oftenest produce new 

 varieties ; and the varieties would at first be local or confined 

 to one place, but if possessed of any decided advantage, or when 

 farther modified and improved, the}'' would slowly spread and 

 supplant their parent-forms. When such varieties returned tc> 



