Licence: Public Domain Mark
Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
95/748 (page 69)
![(6) That many fossiliferous rocks have been so altered in their nature that all traces of their origin and their fossils have disappeared. With all these causes operating against the likelihood of preservation, and of finding those forms that may have been preserved, it is litde wonder if the geological record is incomplete; but such as it is, it is in general agreement with what the other evidence, theoretical and actual, leads us to expect as to the relative age of the great types of animal life. We have reason to wonder, not at the incompleteness of the geological record, but rather that the traces of past history are not even more fragmentary than they are. Probabilities of fossils in the varioits classes of animals.— But it will be useful to note the probabilities of a good representation of extinct forms in the various classes of animals. Thus among the Protozoa the Infusoria have no very hard parts, and have therefore almost no chance of pre- servation, and the same may be said of forms like amoebje ; while the Radiolaria and the Foraminifera, having hard structures of lime or silica, have been well preserved. The Sponges are well represented by their spicules and skeletons. Of the Ccelenterates, except an extinct order known as Graptolites, only the various forms of coral had any parts readily capable of preservation, and remains of these are very abundant in the rocks of many ancient seas. But, strange as it may seem, some beautiful remains of jellyfish have been discovered. Of the great series of worms, only the tube-makers have left actual remains, the others are only known by their tracks, while of any that may have lived on the land there is no evidence. The Echinoderms, because of their hard parts, are well represented in all their orders except the Holothurians, where the calcareous structures characteristic of the class are at a minimum. The Crustacea, being mostly aquatic, and in virtue of their hard skin, are fossilised in great numbers. The Arachnida and the Insects, owing to their air-breathing habit, are chiefly represented by chance individuals that have been drowned, or enclosed within tree stumps and amber. The Mollu.sca and Brachiopods are ])erhaps better pre-](https://iiif.wellcomecollection.org/image/b21958671_0095.jp2/full/800%2C/0/default.jpg)
