re 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. Not a few of
our shellfish are known to have commenced their existence at
some point of the Tertiary period; one Lampshell (_Terebratulina
caput-serpentis_) is believed to have survived since the Chalk; and
some of the _Foraminifera_ date, at any rate, from the Carboniferous
period. We learn from this the additional fact that our existing
animals and plants do not constitute an assemblage of organic
forms which were introduced into the world collectively and
simultaneously, but that they commenced their existence at very
different periods, some being extremely old, whilst others may be
regarded as comparatively recent animals. And this introduction of
the existing fauna and flora was a slow and _gradual_ process, as
shown admirably by the study of the fossil shells of the Tertiary
period. Thus, in the earlier Tertiary period, we find about 95
per cent of the known fossil shells to be species that are no
longer in existence, the remaining 5 per cent being forms which are
known to live in our present seas. In the middle of the Tertiary
period we find many more recent and still existing species of
shells, and the extinct types are much fewer in number; and this
gradual introduction of forms now living goes on steadily, till,
at the close of the Tertiary period, the proportions with which
we started may be reversed, as many as 90 or 95 per cent of the
fossil shells being forms still alive, while not more than 5 per
cent may have disappeared.

All known animals at the present day may be divided into some
five or six primary divisions, which are known technically as
"_sub-kingdoms_." Each of these sub-kingdoms [9] may be regarded
as representing a certain type or plan of structure, and all
the animals comprised in each are merely modified