162 JoURNAL OF CoMPARATIVE NEUROLOGY. 
anticipated. The cerebellum is marked by an organization out 
of all proportion to that of adjacent regions, an organization, 
moreover, far more highly differentiated than is presented by 
the cerebellum of either the amphibian or the reptile. The 
oblongata, on the contrary, has retained the plan of structure 
of the primitive neural tube without the intervention of pro- 
found changes. It is due to this fact that homologies between 
the oblongata and the spinal cord are so readily traced in this 
animal. A more extreme degree of simplicity is found in the 
interbrain, the thalamus having such a slight differentiation as 
to make comparisons between it and higher thalami somewhat 
difficult. Finally, the forebrain is far in advance of the fore- 
brains of other fishes. Contrast the membranous pallium of 
the teleost or the ganoid with the nervous pallium of Mus- 
telus, which, as has been pointed out in the preceding pages, 
anticipates the olfactory associations of higher brains to a 
noteworthy degree. 
These illustrations clearly point to an underlying princi- 
ple. The organization of the brain is the expression of the 
adjustment which has constantly taken place between the race 
of animals and the stimuli to which they have been subjected. 
This relationship between nervous organization and peculiarities 
in the environment is such a close one that the degree of devel- 
opment of the several parts of the brain may be very unequal 
indeed. And hence it is that the cerebellum of Mustelus is so 
highly organized, for this is the correlative of the powerful 
swimming capacity of the animal, requiring an adequate mechan- 
ism of equilibration. The forebrain, with its luxurious devel- 
opment of neurones, has arisen in connection with the large 
place occupied by olfactory impressions in the Selachii. 
Morphological data in neurology must necessarily provide 
the foundation for all physiological work, but it is none the less 
important that morphological facts should have the check of 
experimental evidence wherever this is possible. Nowhere is 
there to-day a more urgent need for careful observations of this 
character than in the group of the fishes, where nervous proc- 
esses are of such a simple order as to introduce relatively few 
