whether or not
these doubtful forms are varieties? As long as most of the links between
any two species are unknown, if any one link or intermediate variety be
discovered, it will simply be classed as another and distinct species. Only
a small portion of the world has been geologically explored. Only organic
beings of certain classes can be preserved in a fossil condition, at least
in any great number. Widely ranging species vary most, and varieties are
often at first local,--both causes rendering the discovery of intermediate
links less likely. Local varieties will not spread into other and distant
regions until they are considerably modified and {465} improved; and when
they do spread, if discovered in a geological formation, they will 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, I am inclined to believe, has been shorter than the average
duration of specific forms. Successive formations are separated from each
other by enormous blank intervals of time; for fossiliferous formations,
thick enough to resist future degradation, can 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 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 fossiliferous formations beneath the lowest
Silurian strata, I can only recur to the hypothesis given in the ninth
chapter. That the geological record is imperfect all will admit; but that
it is imperfect to the degree which I require, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that all species have changed; and they have changed in the manner
which my theory requires, for they have changed slowly and in a graduated
manner. We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other, than
are the fossils from formations distant from each other in time.

Such is the sum of the several chief objections and difficulties which may
justly be urged against my theory; and I have now briefly recapitulated the
answers and explanations which can be given to them. I have felt these
difficulties far too heavily during many year