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.
Measured by human standards, the majority of existing animals (which are capable of being preserved as fossils) are known to have a high antiquity; and some of them can boast of a pedigree which even the geologist may regard with respect.