Edinger[1] has shown that to the primitive division of the brain of the vertebrate (or the paleencephalon), which is the bearer of the reflexes and the instincts, there has been gradually added with peculiar adjustments a newer division (the neencephalon), which corresponds to the cerebrum, in which alone is centered the power of association, and of forming memory images. In proportion as the cerebral cortex increases, the primitive brain recedes, while in a corresponding ratio intelligence rises, and purblind instinct wanes.
A very interesting fact for us, as Edinger has further shown, is that while the cerebral cortex of the bird is more highly developed than in reptiles, the far greater bulk of the brain is mainly due to an enlargement of the primitive division, the parts of which reach a size and proportion nowhere else seen, while at the same time they are very generally connected with the cerebrum. From these facts alone we are warranted to infer that while birds are intelligent and able to form associations of some sort freely, they must be animals in which the instincts are developed to an extraordinary degree of perfection, comparable in large measure with those of the social insects. All this is amply proved by their behavior, and in describing the activities of birds it seems best to discriminate as sharply as possible their instinctive activities from the operations of intelligence, assuming, until the contrary is proved, that their reflexes and instincts pertain exclusively to the primitive division of the brain, as already stated, while the power of association is lodged in the cerebrum alone.
Not only does the bird's brain possess great basal ganglia of which the huge optic lobes are most prominent, but a large cerebellum, and very diminutive, possibly rudimentary olfactory lobes. These facts find their clear counterpart in behavior. Large optic nerves, optic tracts and lobes, are to be expected in animals like the birds, which possess the keenest eyes of any vertebrates known, and which depend so largely upon vision for finding their food and for detecting their enemies and their friends.
The wonderful powers of flight, possessed by birds as a class, may not only be long sustained, as in the golden plover which is supposed to make the journey from Nova Scotia to the West Indies, a distance of 1,700 miles, in a single flight, but is often so rapid that fatal results would follow were the control of direction less precise. Their movements clearly demand an organ for the most perfect coordination of their skeletal muscles, and such is undoubtedly found in their large cerebellum, the action of which is purely reflex. Thus swallows are often seen to enter a barn, where they have their nests, at a perilously rapid rate, through cracks or holes, barely large enough to admit their
- ↑ Edinger, Ludwig, "The Relations of Comparative Anatomy to Comparative Psychology" (translated by H. W. Rand), Journ. of Comp. Neurology and Psychology, Vol. XVIII., No. 5, 1908.
