530 THE POPULAR SCIENCE MONTHLY. 



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 indi- 

 viduals possessing brains which exceed this limit would neces- 

 sarily 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 disadvan- 

 tage to the individual. It is well known tha' the exterior sur- 

 faces of the brains of the lower vertebrates have, as a rule, com- 

 paratively 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 de- 

 manded 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 experi- 

 ment 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 activi- 

 ties 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 intelli- 

 gence 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 in- 

 creased in number. In the case of the lowest savage there is still 



