944. REPORT—1896. 
changed, and there emerges a higher animal, the Petromyzon, which can now be 
compared organ for organ, structure for structure, with the larval form of the 
Amphibian ; and so through the medium of these larval forms we can trace upwards 
without a break the evolution of the vertebrate from the ancient king-crab form. 
On the other hand, Amphioxus and the Tunicates are distinctly degenerate ; it is 
easier to look upon either of them as a degenerate Ammocete than as giving a 
clue to the ancestor of the Ammoccete. It is to my mind surprising how difficult 
it appears to be to get rid of preconceived opinions, for one still hears, in the 
assertion that Petromyzon as well as Amphioxus is degenerate, the echoes of the 
ancient myth that the Elasmobranchs are the lowest fishes, and the Cyclostomata 
their degenerated descendants, 
The characteristic of the vertebrate central nervous system is its tubular 
character ; and it is this very fact of its formation as a tube which has led to the 
disguising of its segmental character, and to the whole difficulty of connecting 
vertebrates with other groups of animals. The explanation of the tubular 
character of the central nervous system is the keystone to the whole of my theory 
of the origin of vertebrates. The explanation which I have given differs from all 
others, in that I consider the nervous system to be composed of two parts—an 
internal epithelial tube, surrounded to a greater or less extent by a segmented 
nervous system ; and I explain the existence of these two parts by the hypothesis 
that the internal epithelial tube was originally the alimentary canal of an 
arthropod animal, such as Limulus or Eurypterus, which has become surrounded 
to a greater or less extent by the nervous system. 
Any hypothesis which deals with the origin of one group of animals from 
another must satisfy three conditions :— 
1. It must be in accordance with the phylogenetic history of each group. It 
must therefore give a consistent explanation of all the organs and tissues of the 
higher group which can be clearly shown not to have originated within the group 
itself. At the same time, the variations which have occurred on the hypothesis 
must be in harmony with the direction of variation in the lower group, if not 
actually foreshadowed in that group. 
This condition may be called the Phylogenetic test. 
2. The anatomical relation of parts must be the same in the two groups, not 
only with respect to coincidence of topographical arrangement, but also with 
respect to similarity of structure, and, to a large extent, also of function. 
This condition may be called the Anatomical test. 
3. The peculiarities of the ontogeny or embryological development of the higher 
group must receive an adequate explanation by means of the hypothesis, while at 
the same time they must help to illustrate the truth of the hypothesis. 
This condition may be called the Ontogenetic test. 
I hope to convince you that all these three conditions are satisfied by my 
hypothesis as far as the head region of the vertebrate is concerned. I speak only 
of the head region at present, because that is the part which I have especially 
studied up to the present time, and also because it is natural and convenient to 
consider the cranial and spinal nerves separately ; and I hope to demonstrate to you 
that not only the nervous system and alimentary canal of such a group of animals 
as the Gigantostraca—i.e. Limulus and its allied forms—is to be found in the head 
region of Ammoceetes, but also, as must logically follow, that every part of the 
head region of Ammoccetes has its homologous part in the prosomatic and 
mesosomatic regions of Limulus and its allies. I hope to convince you that our 
brain is hollow because it has grown round the old cephalic stomach; that our 
skeleton arose from the modifications of chitinous ingrowths ; that the nerves of 
the medulla oblongata—z.e. the facial, glosso-pharyngeal, and vagus nerves—arose 
from the mesosomatic nerves to the branchial and opercular appendages of 
Limulus, while the nerves of the hind brain are derived from the nerves of the 
prosomatic region of Limulus ; that our cerebral hemispheres are but modifications 
of the supra-cesophageal ganglia of a scorpion, while our eyes and nose are the 
direct descendants of its eyes and olfactory organs. 
In the first place, I will give you shortly the reasons why the central nervous 
