Every species seems to come into being at
a certain definite point of time, and to finally disappear at
another definite point; though there are few instances indeed,
if there are any, in which our present knowledge would permit
us safely to fix with precision the times of entrance and exit.
There are, moreover, marked differences in the actual time during
which different species remained in existence, and therefore
corresponding differences in their "vertical range," or, in other
words, in the actual amount and thickness of strata through which
they present themselves as fossils. Some species are found to
range through two or even three formations, and a few have an
even more extended life. More commonly the species which begin
in the commencement of a great formation die out at or before its
close, whilst those which are introduced for the first time near
the middle or end of the formation may either become extinct, or
may pass on into the next succeeding formation. As a general rule,
it is the animals which have the lowest and simplest organisation
that have the longest range in time, and the additional possession
of microscopic or minute dimensions seems also to favour longevity.
Thus some of the _Foraminifera_ appear to have survived, with
little or no perceptible alteration, from the Silurian period
to the present day; whereas large and highly-organised animals,
though long-lived as _individuals_, rarely seem to live long
_specifically_, and have, therefore, usually a restricted vertical
range.
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