THE NATURAL HISTORY OF MAN 69 



more rapidly than the girl then the rate of growth goes on 

 more slowly up to seven years of age. This is followed by a 

 more rapid increase in growth rate up to about seventeen years 

 in the boy and sixteen years in the girl, with a prepubertal 

 acceleration present in both sexes. The growth rate then de- 

 creases to twenty-five years of age, at which time growth prac- 

 tically ceases. 



During the early years the boy is slightly taller and heavier 

 than the girl but, owing to an earlier prepubertal acceleration 

 in the girl, the two sexes are equal in height at twelve years of 

 age and for the next three years the girl is the taller. Simi- 

 larly, at thirteen years of age, the two sexes are of equal weight 

 and for the next three years, the girl is the heavier. After 

 fifteen years of age the boy surpasses the girl in height and 

 after sixteen years in weight. The average height for the 

 people in the United States is about five feet and eight inches 

 for men, and five feet and four inches for women. On the 

 other hand some of the dwarf races of central Africa scarcely 

 attain four and one half feet in height. The average weight 

 of the male is 150 pounds and of the female 125 pounds. 



Puberty. At puberty certain marked physical and psychic 

 characters, known as the secondary sexual characters, develop 

 which quite clearly distinguish the sexes. These changes, as 

 experimental work on the lower animals has shown, are 

 brought about by the internal secretions of the sex glands and 

 can be prevented by their early removal. Also some of the 

 sexual characters in the human female may be modified to 

 somewhat resemble the male after the climacteric when the 

 internal secretions of the sex glands diminish or cease. In the 

 male the growth in weight and height is usually greater, the 

 brain and face grow larger, the vocal cords elongate rapidly, 

 which causes the pitch of the voice to drop an octave, the beard 

 develops, the muscles become larger, giving a more angular 

 appearance to the surface of the body in marked contrast to 



