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 {16}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."