A series of papers on "The Origin of Vertebrates, deduced from the study of Ammocœtes," in the Journal of Anatomy and Physiology, as follows:—
| 1898. | Part | I. | "The Origin of the Brain," vol. xxxii., p. 513. |
| " | II. | "The Origin of the Vertebrate Cranio-facial Skeleton," vol. xxxii., p. 553. | |
| " | III. | "The Origin of the Branchial Segmentation," vol. xxxiii., p. 154. | |
| 1899. | " | IV. | "The Thyroid, or Opercular Segment: the Meaning of the Facial Nerve," vol. xxxiii., p. 638. |
| 1900. | " | V. | "The Origin of the Pro-otic Segmentation: the Meaning of the Trigeminal and Eye-muscle Nerves," vol. xxxiv., p. 465. |
| 1900. | " | VI. | "The Old Mouth and the Olfactory Organ: the Meaning of the First Nerve," vol. xxxiv., p. 514. |
| 1900. | " | VII. | "The Evidence of Prosomatic Appendages in Ammocœtes, as given by the Course and Distribution of the Trigeminal Nerve," vol. xxxiv., p. 537. |
| 1900. | " | VIII. | "The Palæontological Evidence: Ammocœtes a Cephalaspid," vol. xxxiv., p. 562. |
| 1901. | " | IX. | "The Origin of the Optic Apparatus: the Meaning of the Optic Nerves," vol. xxxv., p. 224. |
| 1902. | " | X. | "The Origin of the Auditory Organ: the Meaning of the VIIIth Cranial Nerve," vol. xxxvi., p. 164. |
| 1903. | " | XI. | "The Origin of the Vertebrate Body-cavity and Excretory Organs: the Meaning of the Somites of the Trunk and of the Ductless Glands," vol. xxxvii., p. 168. |
| 1905. | " | XII. | "The Principles of Embryology," vol. xxxix., p. 371. |
| 1906. | " | XIII. | "The Origin of the Notochord and Alimentary Canal," vol. xl., p. 305. |
CHAPTER I
THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM
Theories of the origin of vertebrates.—Importance of the central nervous system.—Evolution of tissues.—Evidence of Palæontology.—Reasons for choosing Ammocœtes rather than Amphioxus.—Importance of larval forms.—Comparison of the vertebrate and arthropod central nervous systems.—Antagonism between cephalization and alimentation.—Life-history of lamprey: not a degenerate animal.—Brain of Ammocœtes compared with brain of arthropod.—Summary.
At the present time it is no longer a debatable question whether or no Evolution has taken place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming that all zoologists look upon this question as settled, and desire now to find out the manner in which such evolution has taken place. Here two problems offer themselves for investigation, which can be and are treated separately—the one dealing with the question of those laws of heredity and variation which have brought about in the past and are still causing in the present the evolution of living beings, i.e. the causes of evolution; the other concerned with the relationship of animals, or groups of animals, rather than with the causes which have brought about such relationship, i.e. the sequence of evolution.
It is the latter problem with which this book deals, and, indeed, not with the whole question at all, but only with that part of it which concerns the origin of vertebrates.
This problem of the sequence of evolution is of a twofold character: first, the finding out of the steps by which the higher forms in any one group of animals have been evolved from the lower; and secondly, the evolution of the group itself from a lower group.
In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is possible to speak of the progress from lower to higher members of the group, and it is conceivable, given time to work out the details, that the natural relationships between the members of the whole group will ultimately be discovered.
Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace the progress from the lowest fishes to man, and already the discoveries of palæontology and anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile, from reptile to mammal on the one hand, and to bird on the other. That the different members of the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt; the connected problems are matters of detail, the solution of which is certain sooner or later. The same may be said of the members of any of the other great natural groups, such as the Arthropoda, the Annelida, the Echinodermata, etc.