as individuals, rarely seem to live long specifically, and have, therefore, usually a restricted vertical range.  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.  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.