have had a long line of ancestors from whom they inherited their good qualities. The mollusca of the Cambrian include the Nautilus, which is a very highly organized animal, something like an octopus in a shell. The whole collection of animals, therefore, of these extremely old beds show that the strata laid down previously to them must have contained organic remains, but we have not yet found them. Possibly the ancestors of the Cambrian animals all possessed soft bodies unprotected with shells or armour, and therefore left no trace in the sediments when they died; or, again, all the beds that contained them have suffered such squeezing and alteration that any organic remains that might have been in them may have become unrecognizable. Neither of these cases, however, is probable; the beds containing the remains of the ancestors of the Cambrian animals will one day be found, though it may be that the greater part of them are under the sea or covered with later sediments.
The second point is that though whole groups of animals have at one time flourished on the face of the earth or in the waters covering it, and have died out completely, like the Nummulites, Ammonites, Belemnites, Trilobites, and Graptolites and the same is true to a much greater extent in the genera and species of various classes of animals yet there are cases in which animals have persisted unaltered from remote times to the present day. The most striking example is the little tongue-shaped brachiopod, Lingula, which is abundant in the Cambrian and still exists in the sea. The Nautilus of the Cambrian, again, is closely allied to the modern pearly nautilus; and, lastly, the Ceratodus, or Australian mud fish of the present day, is found in
