Exceptions to this, however, are occasionally to be found
in some "persistent types," which extend through a succession
of geological periods with very little modification. Thus the
existing Lampshells of the genus _Lingula_ are little changed
from the _Linguloe_ which swarmed in the Lower Silurian seas; and
the existing Pearly Nautilus is the last descendant of a clan
nearly as ancient. On the other hand, some forms are singularly
restricted in their limits, and seem to have enjoyed a comparatively
brief lease of life. An example of this is to be found in many of
the _Ammonites_--close allies of the Nautilus--which are often
confined strictly to certain zones of strata, in some cases of
very insignificant thickness.
Of the _causes_ of extinction amongst fossil animals and plants,
we know little or nothing. All we can say is, that the attributes
which constitute a _species_ do not seem to be intrinsically
endowed with permanence, any more than the attributes which
constitute an _individual_, though the former may endure whilst
many successive generations of the latter have disappeared. Each
species appears to have its own life-period, its commencement,
its culmination, and its gradual decay; and the life-periods
of different species may be of very different duration.
From what has been said above, it may be gathered that our existing
species of animals and plants are, for the most part, quite of modern
origin, using the term "modern" in its geological acceptation.
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