94 THE EVOLUTION OF MAN 



development is even from the standpoint of simple numbers, 

 for from this one egg cell by division must come not simply 

 the nine billion and more cells of the cortex, but all the other 

 countless billions of cells that go to make up the rest of the 

 body. Nor is this process of cell multiplication, prodigious 

 as its results show it to be, the only remarkable feature in 

 development, for it is also equally striking that when the 

 requisite number of cells have been produced, the operation of 

 cell division stops. At least this is true of the cortex, for here, 

 as in a few other parts of the body, the neurones change very 

 little in number after birth. The brain cells with which the 

 babe is born last for the most part without renewal through 

 mature life to old age and death. What brings the operation 

 of cell multiplication to an end at the appropriate moment is 

 as little understood by embryologists as is the exciting cause of 

 the initial increase. 



When it is recalled that the 9,200,000,000 cells in the hu- 

 man cerebral cortex are the nervous elements of this organ and 

 that they collectively constitute rather less than a cubic inch 

 of protoplasm, it seems almost incredible that they should 

 serve us as they do. They are the materials whose activities 

 represent all human mental states, sensations, memories, voli- 

 tions, emotions, affections, the highest flights of poetry, the 

 most profound thoughts of philosophy, the most far-reaching 

 theories of science, and, when their action goes astray, the 

 ravings of insanity. It is this small amount of protoplasm in 

 each of us that our whole educational system is concerned with 

 training and that serves us through a lifetime in the growth of 

 personality. 



THE NERVOUS SYSTEM AND SEX 



The great importance of the nervous system for all that 

 pertains to the higher life of man cannot be denied and yet this 



