iv] GREEN CELLS OF CONVOLUTA 119 



during the winter and to skate during the summer " ; 

 a paradox which contains a biological truth, for 

 the effects of exercise sum themselves up and write 

 the addition in our experience, not during the exer- 

 cises, but in the intervals between them. At all 

 events, it was in the winter that a scrutiny of results 

 of the previous summer's work showed that when 

 green animals appeared among larvae left with their 

 capsules in filtered sea-water, the manner of their 

 appearance was like that in which an epidemic 

 declares itself. At first, it marks down a single 

 victim, then some of the neighbours are affected till, 

 by and by, the disease is general. So it was with 

 the green-cell infection of C. roscoffensis. For 

 days after animals hatched in sea-water had become 

 quite green, those hatched from capsules kept in 

 filtered sea- water remained colourless. Then one 

 green, among many non-green, appeared. The 

 numbers of green animals increased, till, finally, 

 all became green. At once the conclusion presents 

 itself. In our experiments, the colourless stage of 

 the larvae, which lasts so long as fourteen days, is an 

 incubation period, not for the animal but for the 

 infecting organism. Here or there, in spite of many 

 washings, a single algal cell which had settled on 

 the surface of the body has remained entangled in 

 the slime covering the animal. Transferred during 

 egg-laying to the capsule, it grows and divides. It 



