657 



older generations, also in tlie 1Ü''' year. This fact conlirms the opinion 

 already mentioned al)0\e, that the middle ut' the 10''' year is the 

 physiological threshold-age of sexual maturity in woman. 



Opposite to this very constant starting-point of the variability- 

 curve stands the most changeahle ending-point. This falls in the 

 older generations in the 21*' year, in conti-ast to the 18"' in the 

 younger generation. The top of the curve, which in the latter indi- 

 viduals lies in the 13"' year, has been shifted to a higher age in 

 the older generations and lies in the 14"' and 15''' year. 



From this it already appears that formeily the phase of sexual 

 latency, after crossing the threshold-age, lasted considerably longer 

 in a great many girls than nowadays. This also follows from the 

 fact that, as shown in Table I, more than 50 7o of the youngest 

 generation menstruates before the end of the 13''' year, while of 

 those born before 1880 this was only the case in 26 "/o- 



During the last 40 years, therefore, the period of the menarche 

 has gradually become earlier, and how much earlier can be learned 

 from both the following averages. The average menarche of the 

 persons worked out in Table I (fig. 1, curve J) of whom the greater 

 quantity was born between 1897 and 1906, is 13 years, 9 months, 

 15 days; while the mean age of the first menstruation in the per- 

 sons born before 1880 (fig. 1, curve B) is 15 years, 3 months, and 

 20 days. From this it follows that in the last decades the menarche 

 arrives a year and a half eaiiier than formerly. 



I must point out, in passing, that the last mentioned average 

 more resembles those found in literature regarding the West-Euro- 

 pean population, which depend on investigations of an older date. 



A second manner in which the earlier ap|)earance of the menar- 

 che has been pro\ed, is the comparison of the age of the menarche 

 in mothers and daughters. I arranged these data in two groups; in 

 the first I collected the data in which the age of the menarche was 

 accurately known, e\'en up to the nionlh, for both mothers and 

 daughters. To this group belong 45 mothers and 71 daughters. The 

 second group contains the data in which only the year could be 

 mentioned ; here there aie 56 mothers and 82 daughters. 



It seems to me of interest to discuss the data of the first group 

 more extensively, as one or two remarks must still be made about 

 them; they may be seen in Table V, in which the data have been 

 arranged according to the inenarche-age of the mother. 



From this table follows, in the first place, that of 71 daughters 



the menstruation of 52 begins at a younger age than in the 



mother, though, as remarked already, also in the older generation 



43 

 Proceedings Royal Acad. Amsterdam. Vol. XXVI. 



