THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 35 
The next lowest group of living fishes is the Marsipobranchii which 
include the lampreys and hag-fishes. To these naturally we must turn 
for a clue as to the organization of the earliest fish, for here we find 
all the characteristics of the vertebrates represented: a well-formed 
brain-region, well-developed eyes and nose, cranial nerves directly 
comparable with those of other vertebrates, and even the commence- 
ment of vertebre. 
Among these forms the lamprey is by far the best for investiga- 
tion, not only because it is easily obtainable in large quantities, but 
especially because it passes a large portion of its existence in a larval 
condition, from which it emerges into the adult state by a wonderful 
process of transformation, comparable in extent with the transforma- 
tion of the larval caterpillar into the adult imago. So long does the 
lamprey live in this free larval condition, and so different is it in 
the adult stage, that the older anatomists considered that the two 
states were really different species, and gave the name of Am- 
mocates branchialis to the larval stage, while the adult form was 
called Petromyzon planeri, or Petromyzon fluviatilis. 
This long-continued free-living existence in the larval or Am- 
moccetes stage makes the lamprey, more than any other type of 
lowly organized fish, invaluable for the present investigation, for 
throughout the animal kingdom it is recognized that the larval 
form approaches nearer to the ancestral type than the adult form, 
whether the latter is progressive or degenerate. Not only are the 
tissues formed during the stages which are passed through in a 
free-living larval form, serviceable tissues comparable to those 
of adult life, but also these stages proceed at so much slower a rate 
than do those in the embryo im utero or in the egg, as to make 
the larval form much more suitable than the embryo for the investi- 
gation of ancestral problems. It is true enough that the free life of 
the larva may bring about special adaptations which are not of an 
ancestral character, as may also occur during the life of the adult; 
but the evidence is very strong that although some of the peculi- 
arities of the larva may be due to such ccenogenetic factors, yet on 
the whole many of them are due to ancestral characters, which dis- 
appear when transformation takes place, and are not found in the 
adult. 
. Thus if it be supposed that the amphibian arose from the fish, 
the tadpole presents more resemblance to the fish than the frog. If 
