This long-continued free-living existence in the larval or Ammocoetes stage makes the lamprey, more than any other type of lowly organized fish, invaluable for the present investigation, for throughout the animal kingdom it is recognized that the larval form approaches nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate. Not only are the tissues formed during the stages which are passed through in a free-living larval form, serviceable tissues comparable to those of adult life, but also these stages proceed at so much slower a rate than do those in the embryo _in utero_ or in the egg, as to make the larval form much more suitable than the embryo for the investigation of ancestral problems. It is true enough that the free life of the larva may bring about special adaptations which are not of an ancestral character, as may also occur during the life of the adult; but the evidence is very strong that although some of the peculiarities of the larva may be due to such coenogenetic factors, yet on the whole many of them are due to ancestral characters, which disappear when transformation takes place, and are not found in the adult. Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more resemblance to the fish than the frog. If {36}it be supposed that the arthropod arose from the segmented worm, the caterpillar bears out the suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod group, the larval form of the fish is most likely to give decisive information on the point. For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocoetes, the larval form of the lamprey--a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate. COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND ARTHROPOD WITHOUT REVERSAL OF SURFACES. So far different lines of investigation all point to the origin of the vertebrate from arthropods, the group of arthropods in question being now extinct, the nearest living representative being Limulus; also to the fact that of the two theories of the origin of vertebrates, that one which is based on the resemblance between the central nervous systems of the Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this evidence than the other, which is based mainly on the supposed possession of a notochord among certain animals.