Chap. IX. IN ANY SINGLE FORMATION. 285 



the secondary formations lie in the past, -wonkl f^cncrally 

 be formed in the archipehigo only during poriods of subsi- 

 dence. These periods of subsidence would be separated 

 from each other by immense intervals of time, during which 

 the area would be either stationary or rising ; while rising, 

 the fossiliferous formations on the sleeper 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 witiiin 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 subsidence, there would probably be much ex- 

 tinction of life ; during the periods of elevation, there would 

 be much variation, but the geological record would then be 

 least 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 contingencies 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 and distinct species. It is also probable 

 that each great period of subsidence would be interrupted 

 by oscillations of level, and that slight climatal changes 

 would intervene during such lengthy periods ; and in these 

 cases the inhabitants of the archipehigo 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 leads 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 generally be local or confined to one place ; 

 but if jiossessed of any decided advantage, or when further 

 modified and improve<l, they would slowly spread and supplant 

 their parent-forms. ^V^hen such varieties returned to their 

 ancient homes, as they would difTcr from their former state, 

 in a nearlv-unifonn, though ]>orhaj)s extremely-slight degree. 



