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 Palaeontology. — Reasons for 

 choosing- Animocoetes 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 Animocoetes 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 accu- 

 mulation 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 ; 



