378 COMPARATIVE ZOOLOGY OF CHORDATES 



is, they occur in all members of a species, just as they all have 

 kidneys or livers. But because instinct is determined by the 

 hereditary pattern of the neurons, such behaviour is not easily 

 modified to meet unusual circumstances. A good example of 

 such shortcomings is to be found in the meadow pipit, a bird 

 which is parasitised by the cuckoo. In the pipit's nest the 

 cuckoo lays an egg, which hatches into a young cuckoo. This 

 young parasite proceeds to eject the young pipits from the 

 nest. It was observed on one occasion that the young pipit 

 so ejected remained just outside the nest, under the mother- 

 bird's nose, where it lay helpless and squeaking. It never 

 occurred to the mother-bird to put it back in the nest under 

 her, and so the young one died. The situation was novel and 

 had not presented itself to the bird before, and it could not 

 rise to the occasion. The necessary correlation of neurons 

 could not be made ; and if it could, the bird would probably 

 not have been able to act on the experience of a similar previous 

 occasion. The corpus striatum is not well adapted for such 

 powers of individual adaptability, though it is very suitable 

 for ready-made correlations which make the species as a whole 

 well adapted to a particular routine of life. It is interesting 

 to note that the behaviour of birds resembles that of insects 

 in this respect, and that both the brain of the insect and the 

 corpus striatum of birds are solid compact masses of neurons. 

 For really effective and unusual correlations such an arrange- 

 ment appears to be ill suited. The cerebral cortex which 

 fulfils this very function is shaped not as a solid mass, but as 

 a layer of neurons, the number of which is augmented by 

 increasing the area of the layer. The hollow tubular nerve- 

 cord of vertebrates is very suitable for such an arrangement, 

 and it is probable that its possession enabled vertebrates to 

 evolve as they have done, while its absence from insects 

 prevented them from progressing any further. 



The cerebral cortex is a layer of grey matter near the surface 

 of the end-brain. It is scarcely represented in the fish, and 

 in the amphibia most of the neurons remain in the primitive 

 position for grey matter ; that is, near the central cavity. 

 Some neurons, however, migrate towards the surface. At 



