816 



SCIENCE 



[N. S. Vol. XXXVI. No. 937 



pared with a boy a year and a half older than 

 she is. 



If this view is correct, then it appears that 

 the relation in size of the two sexes persists 

 even in childhood. 



I think no better proof can be given of the 

 correctness of this view than the peculiar be- 

 havior of those parts of the body which com- 

 plete their growth at a very early time; for 

 instance, that of the head. The total amount 

 of the growth of the head from the second 

 year on is very slight. K, therefore, girls are 

 ahead of boys in their development by about a 

 year and a half or two years, the total amount 

 of growth of the head in their favor will be 

 the small amount of growth accomplished dur- 

 ing this period of a year and a half or two 

 years. If, then, there is a typical difference 

 between the size of the body of male and fe- 

 male in childhood of the same character as 

 found in adult life, then the head of the girl 

 ought to be at all periods smaller than the 

 head of the boy; and this is what actually 

 happens. The phenomenon has been inter- 

 preted as indicating a less favorable develop- 

 ment of the head of the woman; but the pre- 

 vious remarks show that it is obviously due 

 solely to the different rate of physiological 

 development of the two sexes. The results of 

 psychological tests which show very generally 

 that girls do better than boys of the same age, 

 may be another expression of the general ac- 

 celeration of their development. 



Based on these observations, we may speak 

 of a curve of growth and development of the 

 whole body and its organs which has charac- 

 teristic values for each sex and for each 

 moment in the life of the totality of indi- 

 viduals that compose a social group. Not 

 each individual, however, passes through these 

 stages of development with equal rapidity. 

 The mean square variability of the chronolog- 

 ical age at which a certain point in the physi- 

 ological development of an individual is 

 reached is contained in the following table: 



Varia- 

 Age Observations bility 



0.0 Pregnancy ± 0.04 



0.6 First incisors ± 0.21 



1.6 First molars ± 0.31 



4.2 Ossification of scaphoid bone, girls ±: 1.4 



4.2 Ossification of trapezoid bone, girls ± 1.2 



5.8 Ossification of scaphoid bone, boys ± 1.1 

 6.2 Ossification of trapezoid bone, boys ± 1.3 

 7.0 Inner permanent incisors, girls ... ±1.6 

 7.5 Inner permanent incisors, boys ... ±1.4 



8.9 Outer permanent incisors, girls ... ±2.1 



9.0 Bicuspids, girls ± 2.8 



9.5 Outer permanent incisors, boys ... ±2.1 



9.8 Bicuspids, boys ± 1.6 



11.2 Permanent canines, boys ± 1.4 



11.3 Permanent canines, girls ± 1.0 



12.7 Beginning of pubescence, boys ... ±1.6 



12.8 Second molars, girls ± 1.6 



13.2 Second molars, boys ±2.0 



14.6 Completed pubescence, boys ± 1.1 



14.9 Puberty, girls ±2.0 



19.3 Wisdom teeth, boys ±2.1 



22.0 Wisdom teeth, girls ±1.8 



35.0 Preauricular wrinkles ± 6.6 



36.5 Hair on tragus ± 8.3 



44.5 Menopause ± 5.3 



62.5 Death due to arterial diseases, men ±13.2 



It appears from this table, which may be 

 represented in the form of a curve, that the 

 variability of the physiological stages of devel- 

 opment increases very rapidly — roughly speak- 

 ing, so that its logarithm is about proportional 

 to the actual age, or, to use the term applied 

 by Dr. Crampton and Professor Eotch, to the 

 " chronological age." The causes that lead to 

 this rapidly increasing variability are so far 

 entirely unknown. It is certain, however, that 

 there must be definite causes at work which 

 bring about this phenomenon; for, if the 

 variability were due to accidental causes only, 

 it would increase considerably slower than in 

 a ratio proportional to the increasing age. 

 The study of the general curve indicating the 

 increase of variability in physiological devel- 

 opment indicates an irregularity at the time 

 of approaching maturity. At this period the 

 variability seems to increase at an unusually 

 rapid rate, and either to be stationary or to 

 decrease again at a later time. 



I have spoken here of the variability of the 

 physiological development of the body as 

 though this were a unit. In 1895, in a dis- 



