RHYTHM IN NATURE — FLATTELY. 393 



laying of Convuluta roscoff ens-is is also periodic, and is related in a 

 remarkable way to the rhythm of the tides. Egg-laying begins with 

 the onset of the spring tides and continues for a week. The reason 

 for this is as follows : In the summer, the time of year when these 

 observations were made, the low-water of spring tides at Roscoff 

 occurs about midday and midnight. 7 When, however, the zone occu- 

 pied by the Convolutas is uncovered during the nighttime, the ani- 

 mals in the absence of light do not rise to the surface. Hence, during 

 the spring tides, the worm has an uninterrupted period of some 18 

 hours in which to lay its eggs. Experiments in the laboratory have 

 shown that egg-laying reaches its maximum when the animals are 

 not obliged to come to the surface twice in 24 hours — that is to say, 

 when they can have the longest possible spell of darkness ; in other 

 words, the conditions most favorable to egg-laying occur when the 

 moon is full or new — a remarkable example of the effect of the tides 

 on the habits of shore animals. 



Shore animals like the common periwinkle, for instance, are sub- 

 mitted to a double periodic influence — the rhythmic ebb and flow of 

 the sea and the alternation of day and night. The existence of peri- 

 winkles comprises regularly alternating periods of active life in the 

 water or moist air (at high tides) and periods of suspended anima- 

 tion within their shells. This constant reaction to the tidal rhythm 

 is not without a profound influence on the functions of the organism. 

 For instance, inert periwinkles, even in a dry environment, can be 

 reactivated by shaking; but, according to Bohn, the reactivation 

 occurs much more readily at certain times and hours. Bohn 8 states 

 that if a collection of these mollusks has been isolated for a certain 

 length of time in a laboratory, it is easily demonstrated that, at 

 periods of low tide, one has to shake much longer to produce the reac- 

 tivation than when the tide is high. That is to say, the periods of 

 inertia in the laboratory correspond to the periods of desiccation on 

 the shore. 



The impress of the external rhythm on the organism is, of course, 

 not permanent, but becomes gradually weaker with the passage of 

 time. Numerous similar cases occur, both in plants and animals, but 

 the above are sufficient for our purpose. 



It is common knowledge that in many animals the color is not 

 fixed, but varies according to the hue of their surroundings. This 

 power of color change has been investigated most minutely in the 

 case of the ^Esop prawn (Hippolyte variant). 9 The color of this 



7 Keeble, F., Plant Animals, Cambridge, University Press, 1910. 



8 Bohn, Georges, Institut Psychologique, Travail du Groupe d'fitude de Psychologie 

 zoologique. Mginoire I : Attractions et Oscillations des Animaux marina sous l'influenoe de 

 la luiniere, Paris, 1905. 



" Keeble, F., and Gamble, F, W„ Phil. Trans. Roy. Soc, B. vol. cxcvi, London, 1904. 



