ified descendants;
and the periods during which species have undergone modification, though
long as measured by years, have probably been short in comparison with
the periods during which they retained the same form. It is the dominant
and widely ranging species which vary most frequently and vary most, and
varieties are often at first local--both causes rendering the discovery
of intermediate links in any one formation less likely. Local
varieties will not spread into other and distant regions until they are
considerably modified and improved; and when they have spread, and are
discovered in a geological formation, they appear as if suddenly created
there, and will be simply classed as new species. Most formations have
been intermittent in their accumulation; and their duration has probably
been shorter than the average duration of specific forms. Successive
formations are in most cases separated from each other by blank
intervals of time of great length, for fossiliferous formations
thick enough to resist future degradation can, as a general rule, be
accumulated only where much sediment is deposited on the subsiding bed
of the sea. During the alternate periods of elevation and of stationary
level the record will generally be blank. During these latter periods
there will probably be more variability in the forms of life; during
periods of subsidence, more extinction.

With respect to the absence of strata rich in fossils beneath the
Cambrian formation, I can recur only to the hypothesis given in the
tenth chapter; namely, that though our continents and oceans have
endured for an enormous period in nearly their present relative
positions, we have no reason to assume that this has always been the
case; consequently formations much older than any now known may lie
buried beneath the great oceans. With respect to the lapse of time not
having been sufficient since our planet was consolidated for the assumed
amount of organic change, and this objection, as urged by Sir William
Thompson, is probably one of the gravest as yet advanced, I can only
say, firstly, that we do not know at what rate species change, as
measured by years, and secondly, that many philosophers are not as yet
willing to admit that we know enough of the constitution of the universe
and of the interior of our globe to speculate with safety on its past
duration.

That the geological record is imperfect all will admit; but that it is
imperfect to the degree