68 LIFE : OUTLINES OF GENERAL BIOLOGY 



in April. The females, with males in attendance, wriggle tail fore- 

 most into the sand and lay about two thousand eggs several inches 

 below the surface, enclosed in a kind of capsule. The fertilised eggs 

 develop in the sand, but the fry are not liberated until the capsule 

 is washed out by the high tides associated with the next full moon. 

 This is a very adaptive rhythm that works out profitably for the 

 race of (irunions. 



A third group of rhythms may be defined to include cases where 

 there is more or less indejx^ndence of the external punctuation, 

 as is illustrated by the migratory impulse in many animals. Very 

 interesting is the rhythmic behaviour of the tiny green worm, 

 Convoluta. of the beach at Roscoff, in I^rittany, that comes to the 

 surface of the sand when the tide goes down, and retreats again at 

 the first splash of the incoming flow. It will continue this rhythm for 

 a week at least in a tideless aquarium. There has been some internal 

 registration that lasts for a short time even when there is no ebb 

 and flow to emphasise the external gravitational periodicity. 



Everyone is familiar with the rise and fall of leaves in the morning 

 and in the evening respectively. The regular movements are well 

 seen in plants like clovers, French beans, and acacias, but they are 

 of general cKcurrence ; and it is interesting to find that their regu- 

 larity sometimes lasts although the external stimulus of day and 

 night is no longer operative. That is to say, the regular change from 

 the day position to the night position takes place for a time even 

 wlien the plants are kept in constant darkness or in constant illu- 

 mination. The tune goes on, so to sjx^ak, though the conductor has 

 ceased to beat time. 



Semon reared acacias in an alternation of 12 hours' light and 

 12 hours* darkness, and then shifted them to periods of 6 hours' 

 (or of 24 hours') light and darkness. He found that they exhibited 

 an interesting compromi.se, as if the establi.shed rhythm was 

 struggling with a much altered stimulation. The seedlings of acacias 

 that had become accustomed to the 12-12 alternation were exposed 

 to artificial days and nights of six hours, but they exhibited the 

 12-hour rhythm slightly modified. Vet it was a rhythm that they 

 had not exjxTienced as .separate individuals! Some others that were 

 exposed to continuous darkness or to continuous illumination exhi- 

 bited the 12-hour rhvthm for a time. But it soon became indistinct, 

 anrl abnormal conditions set in. It would be interesting to have a 

 critical re|>etition of these experiments. 



A fourth group of rhythms may be defined to include those 

 internal regularities which have more or less lost, if they ever had, 

 any obvious connection with external changes. In illustration may 

 be mentioned the beating of the heart, the loading and unloading 

 of gland-cells, the see-saw of growing and not growing, of working 

 and resting, of waste and repair. This kind of rhythm is characteristic 



