TRANSACTIONS OF SECTION I. 9493 
Origin of Vertebrates,’ feeling sure that the evidence which has appealed to me as 
a physiologist will be of interest to the Physiological Section; while at the same 
time, as I have invited also the Sections of Zoology and Anthropology to be 
present, I request that this address may be considered as opening a discussion on 
the subject of the origin of vertebrates. I do not desire to speak ex cathedrd, 
and to suppress discussion, but, on the contrary, I desire to have the matter 
threshed out to its uttermost limit, so that if I am labouring under a delusion the 
nature of that delusion may be clearly pointed out to me. 
The central pivot on which the whole of my theory turns is the central nervous 
system, especially the brain region. There is the ego of each animal; there is the 
master-organ, to which all the other parts of the body are subservient. It is to my; 
mind inconceivable to imagine any upward evolution to be associated with a 
degradation of the brain portion of the nervous system, The striking factor of 
the ascent within the vertebrate phylum from the lowest fish to man is the steady 
increase of the size of the central nervous system, especially of the brain region. 
However much other parts may suffer change or degradation, the brain remains 
intact, steadily increasing in power and complexity. If we turn to the inverte- 
brate kingdom, we find the same necessary law: when the metamorphosis of an 
insect tales place, when the larval organs are broken up by a process of histolysis, 
and new ones formed, the central nervous system remains essentially intact, and 
the brain of the imago differs from that of the larva only in its increased growth 
and complexity. 
A striking instance of the same necessary law is seen in the case of the 
transformation of the larval lamprey, or Ammoccetes, into the adult lamprey, or 
Petromyzon; here also, by a process of histolysis, most of the organs of the head 
region of the animal undergo dissolution and re-formation, while the brain remains 
intact, increasing in size by the addition of new elements, without any sign of 
preliminary dissolution. On the other hand, when, as is the case in the Tunicates, 
the transformation process is accompanied with a degradation of the central nervous 
system, we find the adult animal so hopelessly degraded that it is impossible to 
imagine any upward evolution from such a type. 
It is to my mind perfectly clear that, in searching among the Invertebrata for 
the immediate ancestor of the Vertebrata, the most important condition which such 
ancestor must fulfil is to possess a central nervous system, the anterior part of 
which is closely comparable with the brain region of the lowest vertebrate. It is 
also clear on every principle of evolution that such hypothetical ancestor must 
resemble the lowest vertebrate much more closely than any of the higher vertebrates, 
and therefore a complete study of the lowest true vertebrate must give the best 
chance of discovering the homologous parts of the vertebrate and the invertebrate. 
For this purpose I have chosen for study the Ammoccetes, or larval form of the 
lamprey, rather than Amphioxus or the Tunicates, for several reasons. 
n the first place, all the different organs and parts of the higher vertebrates 
can be traced directly into the corresponding parts of Petromyzon, and therefore of 
Ammoceetes. Thus, every part of the brain and organs of special sense—all the 
cranial nerves, the cranial skeleton, the muscular system, &c., of the higher 
vertebrates can all be traced directly into the corresponding parts of the lamprey. 
So direct a comparison cannot be made in the case of Amphioxus or the 
Tunicates. 
Secondly, Petromyzon, together with its larval form, Ammoccetes, constitutes an 
ideal animal for the tracing of the vertebrate ancestry, in that in Ammoccetes we 
have the most favourable condition for such investigations, viz. a prolonged larval 
stage, followed by a metamorphosis, and the consequent production of the imago or 
Petromyzon—a transformation which does not, as in the case of the Tunicates, lead 
to a degenerate condition, but, on the contrary, leads to an animal of a distinctly 
higher vertebrate type than the Ammoccetes form. As we shall see, the Ammo- 
ccetes is so full of invertebrate characteristics that we can compare organ for 
organ, structure for structure, with the corresponding parts of Limulus and its 
allies. Then comes that marvellous transformation scene during which, by a 
process of histolysis, almost all the invertebrate characteristics are destroyed or 
