Page:Popular Science Monthly Volume 7.djvu/65

Rh increases and a larger range of actions is performed, this power to predict how any individual will act is gradually lost.

Advancing step by step upward in the mammalian series, certain changes in the development of the cerebrum occur, accompanied in each instance by the introduction of new faculties or the improvement of old ones. Throughout the lower orders—fishes, reptiles, and birds—the cerebrum, though constantly increasing in relative size from mere nodules of gray matter less in size than the optic ganglia, up to masses of much greater bulk than the sensorium, still retains the same general appearance—two smooth oval bodies, one on each side of the median line, and gradually approaching each other as they enlarge until they meet, then extending forward and backward so as more and more to cover in and hide the sensorium. These represent the hemispheres; and this is the condition of brain which is found in the lower mammalians. Associated with this cerebral development we find a psychical condition not very far advanced beyond that already considered as pertaining to birds; namely, action partly prompted by instinct, and partly controlled by intelligence, as especially seen in the rodentia, or gnawing animals. The perfection of instinct and want



of reasoning power in regard to certain actions are well illustrated in an example quoted by Dr. Carpenter. It is that of a beaver which a gentleman kept in his house, and who, notwithstanding his unfavorable surroundings, would exercise his instinct for dam-building on all occasions; for materials he appropriated brooms, warming-pan, walking-sticks, baskets, boots, and books, or in fact any thing portable within his reach, arranging them in the most approved style of beaver architecture, although he had never witnessed the process in others. He exercised the same skill, also, in preparing himself a dwelling. The absurdity of the whole process from a reasoning point of view