28o SCIENCE PROGRESS 



factor is weak and will only make itself felt when acting in con- 

 junction with the other causes. Menstruation may occur, then, 

 on any day according to the resultant effect of all the causative 

 forces acting upon the person, but in more instances than not it 

 will fall in that part of the tropical lunar period in which the 

 periodic factor is acting. 



Arrhenius suggests that the factor in question is the amount 

 of electricity in the atmosphere which he has shown to vary 

 rhythmically with a period of 27*32 days. But in addition to 

 this period the atmospheric electricity has a shorter (non- 

 lunar) one of 25-93 days. If now the same shorter period could 

 be demonstrated also in the menstruation figures, the possibility 

 of a causal connection between the two phenomena would be 

 heightened. Further investigation showed that the shorter 

 period was present in the case of menstruation. In addition, 

 the effect of the two periodicities should be to cause the average 

 length of the menstrual period to be 26'6o5 days. The average 

 for 1834 individuals in Denmark was found to be 26-68 days, 



Arrhenius demonstrated the same double periodicity for 

 epilepsy, but found it to be absent for bronchitis and mortality. 



Among plants the sole authentic cases of a lunar rhythm in 

 reproduction seem to be among the Algae. Didyota dichotoma 

 is a common marine alga found in various parts of the world. 

 At Beaufort, North Carolina, this plant produces one crop of 

 sexual cells in each lunar month (Hoyt, Bot. Gaz., 43, 1907). 

 The same species at Bangor, Plymouth (Williams, Ann. Bot., 

 19, 1905), and Naples (Lewis, Bot. Gaz., 60, 1910) has a tidal 

 reproductive rhythm, i.e. two cycles per lunation. This differ- 

 ence in behaviour in different localities has not been explained. 

 It is the more problematical since the tidal range at Beaufort 

 is small, resembling the Mediterranean conditions and differing 

 widely from the considerable tidal range on the English coasts. 

 The figures for the average tidal ranges are as follows : Bangor 

 17-9 feet, Beaufort 2-8 feet, Naples i-o feet. Sargassum, too, 

 shows a bilunar reproductive cycle (Tahara, Bot. Mag. Tokyo, 



23, 1909)- 



Popular beliefs in a favourable influence of the moon on 

 plant growth are both world-wide and ancient, although most 

 of them are probably on a par with the superstition that a wax- 

 ing moon increases, a waning moon decreases, any process such, 

 for instance, as the acquisition of wealth. Seneca, Galen, and 

 other classical authors speak of the moon as hastening the 

 maturation of fruits. Athenio is more precise, writing that 

 " cucumbers in gardens grow at full moon, showdng a visible 

 development, and so do sea-urchins." This quotation is of 

 special interest in relation to a common belief of the modern 

 Egyptians that melons, marrows, and other fruits of the Natural 





