THE BIONOMICS OP CONVOLUTA ROSCOFFENSIS. 409 



Since Convolnta is, in all probability, descended from a 

 wholly aquatic ancestor, we must regard its present stations 

 as indicating- a progress landwards. Its special equipment 

 of green cells, its high light and heat attunements, its 

 inability to expose itself to light when shaken, all indicate 

 that it can only function as a surface animal, — that is, Con- 

 volnta is not a sub-aerial, nor is it a marine creature, but is 

 one whose habitat is in a thin film of water; indeed, we find 

 that the characteristic position of the Convolnta zone marks 

 the limit of the animal's power to spread landwards. At the 

 upper edge of this zone there is at most tides a constant slow 

 stream of water. Beyond it, landward, the sand becomes dry 

 during low water. Now although Convolnta, like lichens, 

 can withstand a very considerable amount of desiccation, yet 

 its habit of swimming down stream in a slow current will 

 prevent it making any considerable headway against stream 

 even tliongh its positive pliototropism may start it in that 

 direction. Although, by i-eason of its positive phototropism, 

 it should succeed in climbing further up the beach, its chances 

 of establishing itself there are very small. For either it will 

 succumb to the combined effects of desiccation and of high 

 temperature, or during some subsequent ebbing tide it will 

 be caught whilst in its lethargic condition and swept back 

 again seaward by the drainage stream, which flows quickly 

 down this steeply sloping part of the beach. In any case, 

 animals extending landwards would be at neap tides, beyond 

 the tide-way, high and diy. Still, it is well to bear in mind 

 that a slight modification of its tropisms might convert 

 Convolnta into a thorough land aiiimal. We cannot tell 

 whether, in watching its present distribution, we are watching 

 a creature in coui'se of adaptation to a land habit, or whether 

 it has already reached the limit of its powers in this respect. 



The lower limit of the Convolnta zone is not so clearly 

 defined, and for good reasons. Local variation in the rate of 

 flow of water from the salt springs will carry patches of 

 a colony seaward. The place where these come to rest will 

 be determined by several factors. Seaward, the slope of the 



