The difficulties in the way of accepting such reversal of surfaces have proved insuperable, and another school has arisen which suggests that evolution has throughout proceeded on two lines, the one forming groups of animals in which the central nervous system is pierced by the food-channel and the gut therefore lies dorsally to it, the other in which the central nervous system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the highest products of the evolution have become markedly segmented animals, in the former, annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is based is the existence of low forms of animals, known as the _Enteropneusta_, the best known example of which is called _Balanoglossus_; they are looked upon as aberrant annelid forms by many observers. This theory does not attempt to explain the peculiarities of the tube of the vertebrate central nervous system, or to account for the extraordinary resemblance between the structure and arrangement of the central nervous systems of vertebrates and of the highest invertebrate group. Neither of these theories is satisfactory or has secured universal acceptance. The problem must be considered entirely anew. What are the guiding principles in this investigation? {65}The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form--man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration. If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group--the Arthropoda--for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group.