February, 1897. HAVE FISH A MEMORY ? I19 



the great ganglion-cells, whence arise the motor nerves, and also 

 nerve-cells which seem from the distribution of their processes fitted to 

 fulfil an associative function between the neuro-muscular apparatus of 

 adjacent segments or metameres. A precisely similar fundamental 

 mechanism is present in the spinal cord of all vertebrates from the 

 lowest to the highest, and this mechanism must alone suffice for all the 

 ■various and often purposeful movements which a decapitated frog, 

 duck, or rabbit can perform, and indeed for the whole range of 

 capacities possessed by Amphioxiis. A direct influence of the brain 

 upon the affairs of the cord does not exist in the lowest vertebrates, 

 though already in sharks association-tracts connect the cerebellum 

 and mid-brain with the cord. In passing upwards, such association- 

 tracts develop more and more, till in mammals we find the 

 pyramidal tracts connecting the cord with the cerebral cortex itself. 

 At the upper end of the cord, where the cranial nerves arise, the 

 association-tracts are immensely developed, and this, with the size 

 and importance of the nerve-nuclei there situated, gives rise to the 

 medulla oblongata. 



The cerebellum first appears in cyclostomes (the lamprey and 

 hag-fish), but its development depends chiefly on physiological causes : 

 great swimmers have great cerebella, e.g., sharks; and swimming 

 reptiles have one twice as big as have reptiles which live on land. 

 The lateral lobes are comparatively late acquisitions. 



Next to the spinal cord, the mid-brain is the most constant in the 

 whole animal series : in all embryos, and permanently in lower 

 vertebrates it is a large and conspicuous organ, and it receives the 

 main part of the terminations of the optic nerve, as well as the 

 terminations of most of the sensory association-tracts from the cord. 

 In animals with large eyes, e.g., bony fishes and birds, it has an 

 unusual development. 



The fore-brain consists essentially of an olfactory apparatus, a 

 basal ganglion, and a pallium. The olfactory apparatus is always 

 basal, and has already reached a high grade of development in fishes, 

 though in different classes it varies with the mode of life, being almost 

 absent in birds. The function of the corpus striatum, though as yet 

 unknown, is manifestly important. It is the pallium which is the 

 important region from the point of view of psychical development : 

 the evolution of other parts of the brain is insignificant in comparison 

 with that here to be seen, ending in the colossal cerebral hemispheres 

 of man. For simple movements, for the primary appreciation of 

 sensory impressions, a simple apparatus serves everywhere alike, but 

 higher associative mental activity first becomes possible with the 

 development of this part of the fore-brain. In earliest rudiment, it 

 exists as a thin plate of epithelium in selachians, ganoids and 

 teleosteans. In amphibians it is definitely of nervous texture, and in 

 reptiles it first takes on the form of a distinct cerebral cortex. The 

 primitive cortex seems to have demonstrable relations with the 



