which live at the present time.  That is the case with one of the cretaceous lamp-shells (Terebratula), which has continued to exist unchanged, or with insignificant variations, down to the present day.  Such is the case with the Globigerinae, the skeletons of which, aggregated together, form a large proportion of our English chalk.  Those Globigerinae can be traced down to the Globigerinae which live at the surface of the present great oceans, and the remains of which, falling to the bottom of the sea, give rise to a chalky mud.  Hence it must be admitted that certain existing species of animals show no distinct sign of modification, or transformation, in the course of a lapse of time as great as that which carries us back to the Cretaceous period; and which, whatever its absolute measure, is certainly vastly greater than thirty thousand years.

There are groups of species so closely allied together that it needs the eye of a naturalist to distinguish them one from another.  If we disregard the small differences which separate these forms and consider all the species of such groups as modifications of one type, we shall find that, even among the higher animals, some types have had a marvellous duration.  In the chalk, for example, there is found a fish belonging to the highest and the most differentiated group of osseous fishes, which goes by the name of Beryx.  The remains of that fish are among the most beautiful and well preserved of the fossils found in our English chalk.  It can be studied anatomically, so far as the hard parts are concerned, almost as well as if it were a recent fish.  But the genus Beryx is represented, at the present day, by very closely allied species which are living in the Pacific and Atlantic Oceans.  We may go still farther back.  I have already referred to the fact that the Carboniferous formations, in Europe and in America, contain the remains of scorpions in an admirable state of preservation, and that those scorpions are hardly distinguishable from such as now live.  I do not mean to say that they are not different, but close scrutiny is needed in order to distinguish them from modern scorpions.

More than this.  At the very bottom of the Silurian series, in beds which are by some authorities referred to the Cambrian formation, where the signs of life begin to fail us—­even there, among the few and scanty animal remains which are discoverable, we find species of molluscous animals which are so closely allied to existing forms that, at one time, they were grouped under the same generic name.  I refer to the well-known Lingula of the Lingula flags, lately, in consequence of some slight differences, placed in the new genus Lingulella.  Practically, it belongs to the same great generic group as the Lingula, which is to be found at the present day upon your own shores and those of many other parts of the world.