I have not reproduced any of the attempted restorations of these old forms, as usually given in the text-books, because all such restorations possess a large element of fancy, due to the personal bias of the observer. I have put in Rohon's idea of the general shape of Tremataspis (Fig. 17) in order to draw attention to the lamprey-like appearance of the fish according to his researches (_cf._ Fig. 18). [Illustration: FIG. 17.--RESTORATION OF _Tremataspis_. (After ROHON, slightly modified.)] [Illustration: FIG. 18.--_Ammocoetes._] The argument, then, from geology, like that from comparative anatomy and from the consideration of the importance of the central nervous system in the upward development of the animal race, not only points directly to the arthropod group as the ancestor of the {33}vertebrate, but also to a distinct ancient type of arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. It follows, therefore, that for the following up of this clue, Limulus especially must be taken into consideration, while Branchipus and Apus are always to be kept in mind. AMMOCOETES RATHER THAN AMPHIOXUS IS THE BEST SUBJECT FOR INVESTIGATION. It is not, however, Limulus that must be investigated in the first instance, but the vertebrate itself; for it can never be insisted on too often that in the vertebrate itself its past history will be found, but that Limulus cannot reveal the future of its race. What vertebrate must be chosen for investigation? Reasons have been given why our attention should be fixed upon the king-crab rather than on the lobster on the invertebrate side; what is the most likely animal on the vertebrate side? From the evidence already given it is manifest that the earliest mammal belonged to the lowest group of mammals; that the birds on their first appearance presented reptilian characteristics, that the earliest reptiles belonged to a low type of reptile, that the amphibians at their first appearance were nearer in type to the fishes than were the later forms. As each of these groups advances in number and power, specialization takes place in it, and the latest developed members become further and further removed in type from the earliest. So also it must have been with the origin of fishes: here too, in the quest for information as to the structure and nature of the first-formed fishes, we must look to the lowest rather than to the highest living members of the group.