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Show 300 IMPERFECTION OF THE CHAP. IX. whole world in organic beings; yet if .all the species were to be collected which have ever hved there, how imperfectly would they represent the natural history of the world! But we have every reason to believe that the terres-trial productions of the archipelago w~uld be preser.ved in an excessively imperfect manner m the formatiOns which we suppose to be there accumulating. I suspect that not many of the strictly littoral animals, or of 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 sea, or where it did not accumulate at a sufficient rate to protect organic bodies from decay, no remains could be pre-served. In our archipelago, I believe that fossiliferous forma-tions could be formed of sufficient thickness to last to an age, as distant in futurity as the secondary formations lie in the past, only during periods of subsidence. These periods of subsidence would be separated from each other by enormous intervals, during which the area would be either stationary or rising ; whilst rising, each fossiliferous formation would be destroyed, almost as soon as accumulated, by the incessant coast-action, as we now see on the shores of South America. During the periods of subsidence there would probably be much extinction of life ; during the periods of elevation, there would be much variation, but the geological rec:ord 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 ac?umulation of sediment, would exceed the average duratwn of the same specific forms; and these contingencies are CHAP. IX. GEOLOGICAL RECORD. 301 indispensable for the preservation of all the transitional gradations between any two or more species. If such gradations were not fully preserved, transitional varieties would merely appear as so many distinct species. It is, also, probable that each great period of subsidence would be interrupted by oscillations of level, and that slight cli~atal chan?es would interve~e during such lengthy periods ; and In these cases the Inhabitants of the archipelago would have to 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 leads me to believe that it would be chiefly these far-ranging species which would oftenest produce new varieties ; and the varieties would at first generally be local or confined to one place, but if possessed of any decided advantage, or when further modified and improved, they would slowly spread and supplant their parent-forms. When such varieties returned to their ancient homes, as they would differ from their former st~te, in a nearly uniform, though perhaps extremely slight degree, they would, according to the principles followed by many palooontologists, be ranked as new and distinct species. If then, there be some degree of truth in these remarks, we have no right to expect to find in our geological formations, an infinite number of those fine transitional forms, which on my theory assuredly have connected all the past and present species of the same group into one long and branching chain of life. We ought only to look for a few links, some more closely s?me more distantly related to each other ; and thes~ links, let them be ever so close, if found in different stages of the same formation, would, by most palooonto- |
