Theories of the origin of vertebrates.--Importance of the central nervous system.--Evolution of tissues.--Evidence of Palæontology.--Reasons for choosing Ammocoetes 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 Ammocoetes 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; {9}thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Coelenterata, 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.