Representing Mollusks, Sponges, Crustaceans, Worms and Sea-Weeds.] But of the causes that have led to the extinction of plants and animals, little or nothing is known. All that can be affirmed, in our present knowledge, is that the attributes constituting a species do not seem to be intrinsically endowed with permanence, any more than those constituting an individual, though the former may endure whilst many successive generations of the latter have disappeared from the earth. Each species, it would seem, has its own life-period--its beginning, culmination and decay--the life-periods of different species being of very different duration. From all that has been said, it may be gathered that our existing plants and animals are for the most part of modern origin, using the term modern in its geological acceptation. Measured by human standards, many of our existing animals, those which are capable of being preserved as fossils, are known to have a high antiquity. Not a few of our shell-fish commenced their existence at some time in the Tertiary, while one species of Lampshell--_Terebratulina caput-serpentis_--is believed to have survived since the Chalk, and a number of the Foraminifera date from the Carboniferous Period. Thus, we learn the additional fact that our existing flora and fauna do not constitute an aggregation of organic forms which were introduced into the world collectively and simultaneously, but that they commenced their existence at very different times, some being extremely ancient, whilst others are of comparatively recent origin. And this introduction of existing plants and animals, as admirably shown by the study of the fossil shells of the Tertiary Period, was a slow and gradual process. Ninety-five per cent. of the known fossil shells in the earliest Tertiary are found to be species no longer in existence, the remaining 5 per cent. being forms that are known to live in our present seas. In the Middle Tertiary, the extinct types are much fewer in number, while at the close of the Period the proportion with which we started may be reversed, not more than 5 per cent. being extinct types. [Illustration: CARBONIFEROUS TIMES. Animals and Plants That Prevailed.]