28 PLANT-ANIMALS [CH. 



capsule. Suddenly they leave it, passing with ease 

 through the mucilaginous wall, though they not 

 infrequently return now and again to the capsule 

 after enjoying a short spell of activity a fact the 

 significance of which we shall have occasion to 

 comment upon later. 



In seeking an explanation of the significance of 

 the fortnightly periodicity in egg-laying, it is easy to 

 conclude that the periods chosen by C. roscoffensis 

 are the most convenient for this purpose. For in 

 summer, to which period of the year these observa- 

 tions apply, low water of spring tides occurs about 

 midday and midnight Now, as we have learned, when 

 the roscoifensis zone is uncovered during the darkness 

 of night the animals do not remain on the surface. 

 Hence, during the spring tides, C. roscoffensis has an 

 uninterrupted period of some eighteen hours in which 

 to lay its eggs. At other tidal periods, its leisure 

 would be less, for, as the tide runs off the patch, the 

 animal must come up to the light and it must re- 

 main up till the returning tide gives the signal that its 

 vigil is at an end. In short, whereas, during the 

 neap-tides, at which periods low water occurs in 

 early morning and late afternoon, C. roscoffensis has 

 two up- and two down-periods, during the springs it 

 has only one period of compulsory " upness " in each 

 twenty-four hours. 



The weak point, however, of all such teleological 



