Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great
number. Many species when once formed never undergo any further change but
become extinct without leaving modified 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.
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