92 HUMAN FECUNDITY 



Other figures show that Jewesses menstruate earlier than the 

 average age for menstruation in the country in which they live. 1 



What is of chief interest to us is that good "conditions also 

 influence the age at which menstruation begins. The better the 

 conditions, the earlier does it begin. Further, it is also known 

 that the mature period tends to be prolonged where conditions 

 are good. It is known, for example, that the mature period comes 

 to an end earlier among the labouring than among the richer 

 classes. 2 Therefore good conditions tend to be connected not only 

 with an earlier beginning but also with a longer duration of the 

 mature period. Thus Mayer found the average age at which 

 menstruation began among 8,000 women of the upper classes to 

 be 14-69 years, and among 3,000 poor women to be 16 ; years. 3 

 Several observers have recorded the average age for the beginning 

 of menstruation among the different classes in the population. 

 The following table is given by Krieger : 4 



5. With regard to the question as to the interval between births 

 some interesting indirect evidence is obtained from what we can 

 learn regarding the former condition of the sexual cycle in man. 

 A sketch was given in the second chapter of the main features of 

 the mammalian sexual cycle. These cycles may follow one another 

 without any prolonged interval, or there may be a lengthy period 

 of rest. In monkeys and also in man the interval is short ; but 

 the former cannot breed at every heat period ; special sexual 

 se.asons are experienced at definite times of the year when breeding 

 takes place. It is probable that ovulation is limited to this special 

 season, and that, although copulation may take place at other 

 seasons of the year, conception can only follow copulation at this 

 special season. 5 In civilized man, so far as is known, ovulation is 



1 It is a remarkable fact that the Eskimo begin to menstruate at about thirteen 

 years of age at an earlier age, that is to say, than the average for the United 

 States (Kelly, loc. cit., p. 83). 2 Marshall, Physiology of Reproduction, p. 672. 



3 Krieger, loc. cit., p. 21. Ibid., p. 24. 6 Marshall, 



Physiology of Reproduction, p. 63. There are considerable variations in the con- 

 ditions among monkeys ; the above is only generally true. 



