INTRODUCTORY 9 



gate, in regard to which we must confess that we do not 

 see the r ional interpretation. And the case of the 

 mackerel f ads us to the result very valuable in inter- 

 pretative .^science that a genuine correlation may be 

 established through a circuitous nexus. It may turn out, 

 in spite of the phrase "all moonshine," that the moon 

 causes vital tides. 



In the case of long-established correlations between 

 external periodicities and internal vital changes, it seems 

 that the former may come to be needed only as liberating 

 stimuli or trigger-pullers. The internal periodicity may 

 become, in the course of ages, so thoroughly ingrained 

 constitutionally, that it requires only an appropriate touch 

 at the appropriate time to keep the business going smoothly. 

 In this connection we may refer to Professor Semon's pretty 

 experiment with young acacias (Albizzia lophanthd). They 

 had never been exposed to the normal alternation of day 

 and night, to which their race responds by expanding and 

 closing the leaves. Semon exposed them to artificial days 

 and nights of six hours' or twenty-four hours' duration, 

 but the plants exhibited the twelve-hours' cycle quite 

 unmistakably though just a little altered. After this 

 experiment, Semon exposed his plants to continuous dark- 

 ness or to continuous illumination. The twelve-hours' 

 cycle still manifested itself for a time, but gradually 

 became indistinct. Here we see the ingrained hereditary 

 periodicity struggling against inappropriate stimuli in- 

 appropriate to an extent that could hardly ever occur in 

 nature. 



It is said that the tropical African mudfish (Protopterus), 

 taken to North Europe and kept with abundant water, tends 

 to become dormant at what corresponds to the African 

 dry season, when it normally goes to sleep for half the year. 

 It is said that migratory birds in cages become restless at 



