FOREBRAIN MORPHOLOGY 435 
The skeleton is an organic system of the greatest importance for 
vertebrate phylogeny, as it can be studied not only in living, 
but also in extinct animals, and our idea concerning vertebrate 
phylogeny is thus grounded chiefly on the comparative anatomy 
of this system. 
The living species of the groups of vertebrates represent the 
upper tips of the widely branched vertebrate phylum, and thus 
they are not to be derived from each other. In the same way it is 
impossible to derive the brain structures of one of these groups 
from those of another. This is a rule against which comparative 
neurology has sinned very much. Many times the brain of a 
Petromyzon, a shark, a teleost, an urodele, a frog, a reptile, a bird, 
and a mammal have been combined together as a typological 
series which has served also for demonstrating the phylogenetic 
process. The ground for the phylogeny of the brain structures 
must be the genealogic tree, constructed on the knowledge of the 
extinct animals also. Such a genealogic tree has many times 
been built up, and it might in broad lines have the aspect of the 
diagram, figure 42. 
In this diagram stem-groups, as crossopterygians, stegocepha- 
lians, cotylosaurians, are marked with circles and the groups of 
living vertebrates, probably derived from these directly or indi- 
rectly, are drawn up from the periphery of the circles. This is 
done in order to avoid mistakes in the closer arrangement of the 
branches in relation to each other. The full-drawn lines show the 
chronological distribution of the groups, based upon palaeonto- 
logical discoveries. In this diagram there are two hypothetical 
ancestral groups lettered « and y. The x-form represents the 
common ancestor to the cyclostomes and all other vertebrates, 
the y-form the common ancestor of selachians-holocephalians and 
other vertebrates. 
The forebrain of extinct crossopterygians 
It is a very delicate affair to try to get an idea of the brain of 
an extinct animal group, and I wish to accentuate that what here 
will be said has more the character of a keen experiment of thought 
than a well-grounded theory. But in the light of the structure of 
