70 THE RATE OF C4R0WTH [ch. 



is not very apparent. It is admirably brought out, however, in 

 the acceleration-curves. Here we see that, after infancy, say 

 from three years old to eight, the velocity in the girl is steady, 

 just as in the boy, but it stands on a lower level in her case than 

 in his : the Uttle maid at this age is growing slower than the boy. 

 But very soon, and while his acceleration-curve is still represented 

 by a straight hne, hers has begun to ascend, and until the girl 

 is about thirteen or fourteen it continues to ascend rapidly. 

 After that age, as after sixteen or seventeen in the boy's case, it 

 begins to descend. In short, throughout all this period, it is a very 

 similar curve in the tw^o sexes ; but it has its notable differences, 

 in ampUtude and especially in fliase. Last of all, we may notice 

 that while the acceleration-curve falls to a negative value in the 

 male about or even a httle before the age of thirty years, this 

 does not happen among women. They continue to grow in 

 weight, though slowly, till very much later in hfe ; until there 

 comes a final period, in both sexes alike, during which weight, 

 and height and strength all ahke diminish. 



From certain corrected, or "typical" values, given for American children 

 by Boas and Wissler {I.e. p. 42), we obtain the following still clearer comparison 

 of the armual increments of stature in boys and girls : the typical stature at 

 the commencement of the period, i.e. at the age of eleven, being 135-1 cm. 

 and 136-9 cm. for the boys and gu'ls respectively, and the annual incx'ements 

 being as follows : 



Age 12 13 14 15 16 17 18 19 20 



Boys (cm.) 4-1 6-3 8-7 7-9 5-2 3-2 1-9 0-9 0-3 



Girls (cm.) 7-5 7-0 4-6 2-1 0-9 0-4 0-1 0-0 0-0 



Difference -3-4 -07 4-1 58 43 2-8 18 09 0-3 



The result of these differences (which are essentially phase- 

 difierences) between the two sexes in regard to the velocity of 

 growth and to the rate of change of that velocity, is to cause the 

 ratio between the weights of the two sexes to fluctuate in a some- 

 what complicated manner. At birth the baby-girl weighs on the 

 average nearly 10 per cent, less than the boy. Till about two 

 years old she tends to gain upon him, but she then loses again 

 until the age of about five ; from five she gains for a few years 

 somewhat rapidly, and the girl of ten to twelve is only some 

 3 per cent, less in weight than the boy. The boy in his teens gains 



