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 Paleontology.—Reasons for 
choosing Ammoccetes 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 Ammoccetes 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, 1.¢. 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 ; 
