the brain may grow with increased exercise and larger and more frequent demands upon it like any other organ up to the point of becoming out of due proportion to the body, but still, the individuals possessing brains which exceed this limit would necessarily labor under such a disadvantage, as entire organisms, that they would in time be eliminated from the species. Now these are not simple deductive conclusions, as we shall soon see.
We have now cleared the ground for a statement of the actual sequential physical development accompanying increased mental activity after the head has attained such size that the cranial capacity can not be further enlarged without serious disadvantage to the individual. It is well known that the exterior surfaces of the brains of the lower vertebrates have, as a rule, comparatively smoother surfaces where they come in contact with the cranium, showing that the capacity of such heads as are in best proportion to their bodies is quite sufficient (without any such convolutional development as we find in the case of man) for all the mental and emotional activity and brain work demanded in the lives they lead.
But there are other animals that have thumbs opposed to four fingers like our own, and they can use these hands as we use ours, they can pick up stones and sticks, fruits, seeds, and nuts, catch insects, break branches and throw them down; they can pick and pull things in pieces, examine, inspect, and experiment with them, and reach conclusions (such as they are) about almost everything with which they come in contact. They can also unite their strength and their ingenuity, and can thus render services to each other. All this naturally multiplies their needs, and much more their desires, renders the exigencies of their daily experience more frequent and important, and their lives more diversified and complex. Thus the mental and emotional activities of all the quadrumana are so much increased by the use of a thumb and fingers that a head which is in due proportion to a body such as would be best adapted to springing, climbing, jumping, swinging, and other arboreal habits requiring great activity and expertness, has not sufficient cranial capacity for a brain with so much mental and emotional work to do. Here, then, we find for the first time a variation which characterizes more or less the whole family of quadrumana according to the degrees of intelligence of the several species. It consists of slight corrugations on the exterior of the brain, thus increasing the superficial area for the gray matter, without enlarging the head, the brain weight, or the cranial capacity to their disadvantage in other respects.
In the case of anthropoid apes, and proportioned to their greater intelligence, these corrugations or convolutions are increased in number. In the case of the lowest savage there is still