The Origin of Vertebrates

by Walter Holbrook Gaskell

Still, as already said, even if the theory advanced to explain the facts be discredited, the facts remain the same; and still to this day an explanation is required as to why such extraordinary resemblances should exist between the two nervous systems, unless there is a genetic connection between the two groups of animals. An explanation may still be found, and must be diligently sought for, which shall take into account the strong evidence of this relationship between the two groups, and yet not necessitate any reversal of surfaces. It is the object of this book to consider the possibility of such an explanation.

What are the lines of investigation most likely to meet with success? Is it possible to lay down any laws of evolution? It is instructive to consider the nature of the investigations which have led to the two theories just mentioned, for the fundamental starting-point is remarkably different in the two cases. The one theory is based upon the study of the vertebrate itself, and especially of its central nervous system, and its supporters and upholders have been and are essentially anatomists, whose chief study is that of vertebrate and human anatomy. The other theory is based upon the study of the invertebrate, and consists especially of an attempt to find in the invertebrate some structure resembling a notochord, such organ being considered by them as the great characteristic of the vertebrate; indeed, so much is this the case, that a large number of zoologists speak now of Chordata rather than of Vertebrata, and in order to emphasize their position follow Bateson, and speak of the Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use Masterman's term), as Diplo-chordata.

The upholders of this theory lay no stress on the nature of the central nervous system in vertebrates, they are essentially zoologists who have made a special study of the invertebrate rather than of the vertebrate.

Of these two methods of investigating the problem, it must be conceded that the former is more likely to give reliable results. By putting the vertebrate to the question in every possible way, by studying its anatomy and physiology, both gross and minute, by inquiring into its past history, we can reasonably hope to get a clue to its origin, but by no amount of investigation can we tell with any certainty what will be its future fate; we can only guess and prophesy in an uncertain and hesitating manner. So it must be with any theory of the origin of vertebrates, based on the study of one or other invertebrate group. Such theory must partake rather of the nature of prophecy than of deduction, and can only be placed on a firm basis when it so happens that the investigation of the vertebrate points irresistibly to its origin from the same group; in fact, "never prophesy unless you know."

The first principle, then, I would lay down is this: In order to find out the origin of vertebrates, inquire, in the first place, of the vertebrate itself.

Importance of the Central Nervous System.

Does the history of evolution pick out any particular organ or group of organs as more necessary than another for upward progress? If so, it is upon that organ or group of organs that special stress must be laid.

Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific mind that 'the fittest' may be produced in two diametrically opposite ways: either by progress upwards to a superior form, or by degeneration to a lower type of animal. The principle of degeneration as a factor in the formation of groups of animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking example is to be found in the widely distributed group of Tunicata, which live, in numbers of instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life, propagate by budding, have lost all the characteristics of higher forms, and yet are considered to be derived from an original vertebrate stock. Such degenerate forms remain degenerate, and are never known to regenerate and again to reach the higher stage of evolution from which they arose. Such forms are of considerable interest, but cannot help, except negatively, to decide what factor is especially important for upward progress.

At the head of the animal race at the present day stands man, and in mankind itself some races are recognized as higher than others. Such recognition is given essentially on account of their greater brain-power, and without doubt the great characteristic which puts man at the head is the development of his central nervous system, especially of the region of the brain. Not only is this point most manifest in distinguishing man from the lower animals, but it applies to the latter as well. By the amount of convolution of the brain, the amount of grey matter in the cerebral hemispheres, the enlargement and increasing complexity of the higher parts of the central nervous system, the anthropoid apes are differentiated from the lower forms, and the higher mammals from the lower. In the recent work of Elliot Smith, and of Edinger, most conclusive proof is given that the upward progress in the vertebrate phylum is correlated with the increase of brain-power, and the latter writer shows how steady and remarkable is the increase in substance and in complexity of the brain-region as we pass from the fishes, through the amphibians and reptiles, to the birds and mammals.

The study of the forms which lived on the earth in past ages confirms and emphasizes this conclusion, for it is most striking to see how small is the cranium among the gigantic Dinosaurs; how in the great reptilian age the denizens of the earth were far inferior in brain-power to the lords of creation in after-times.