i6o THE WONDER OF LIFE 



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 

 (precise facts would be very valuable) that migrant birds 

 in cages become restless in autumn — at the proper time 

 for southward flight — although they are living in conditions 

 of apparent comfort. It is certain that many birds begin 

 their autumnal migration with notable punctuality at a 

 time when the external changes have not yet begun to 

 be in any sense compelhng. 



The point may be further illustrated by reference to 

 Professor Semon's suggestive experiments with young 

 Acacias [Alhizzia lophantha). They had never been 

 exposed to the normal alternation of day and night, to 

 which acacias are wont to respond by expanding or closing 

 the leaves. Semon exposed them to artificial days and 

 nights of six hours' or twenty- four hours' duration, but the 

 young 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, and he had the satis- 

 faction of seeing the twelve-hours' cycle still manifestLag 

 itseK for a httle. It gradually became indistinct, as the 

 plants gave up asserting themselves against ' times out 

 of joint '. At first, however, the experiments showed very 

 beautifully how the ingrained hereditary periodicity may 

 struggle against inappropriate external conditions. 



It is interesting to consider the diversity of ways in 

 which animals meet the difiiculties involved in the winter- 

 conditions in North Temperate countries. Many birds, 

 ' inteUigent of the seasons ', as Milton has it, escape the 



