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 forms of this common type.