Chap. IX. 
GEOLOGICAL RECOKD. 
301 
in tlie archipelago, of thickness sufficient 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 accu¬ 
mulated, by the incessant coast-action, as we now see on 
the shores of South America. 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 
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 accumu¬ 
lation 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 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 
climatal changes would intervene during such lengthy 
periods; and in these cases the inhabitants of the archi¬ 
pelago 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 archipe^ 
lago 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 flrst gene-? 
rally be local or confined to one place, but if possessed 
