MOTION 317 



calm, if caught by the wind the animal will be seen to rotate until R comes into line with C (Text- 

 fig. 6 c) ; it then drifts steadily, along a course at an angle of about 45 ° with the down-wind direction— 

 to the right in this case because the specimen is left-handed. Rotation is very rapid, because the aboral 

 half of the animal X-B, being free of appendages, only rests lightly on the surface. 



The slight drift along the axis AB, which theory shows to be inevitable, I could not detect at sea. 

 On the other hand there appeared to be a marked progression in the opposite direction, that is from 

 B to A. No doubt this is accounted for by movement of the surface layer of water past B in the 

 direction BP, and by a visual impression of movement of the animal across the wind from P to P 1 

 (Text-fig. 6d). The direction of drift of both right- and left-handed specimens is shown diagram- 

 matically in Text-fig. 5. 



Woodcock (1944) was the first to draw attention to the motion of Physalia relative to wind and water. 

 He cited observations on the asymmetry of 421 specimens in the North Atlantic and the Gulf of 

 Mexico, and put forward a hypothesis to explain the advantages to specimens in the northern hemi- 

 sphere of being driven consistently about 45 ° to the left of the down- wind direction. He also cited 

 twenty-two preserved specimens from the southern hemisphere, most of which were mirror-images 

 of the northern ones, and which would have been driven to the right according to his theory. But 

 Fontaine (1954) noticed that forms driven ashore on the southern coast of Jamaica by prevailing east 

 winds were left-handed, that is, they would have been driven to the right of the down-wind direction. 

 I will not refer in detail to Woodcock's hypothesis because it need never, I think, have been postulated. 

 My companion, Dr G. O. Mackie, and I (1956) found at Arrecife and off the island of Allegranza to 

 the north of Lanzarote that both right- and left-handed specimens appear to be common in the 

 North Atlantic. Chun (1887) had already recorded finding on the same day and in the same place in 

 the Pacific Ocean thirty-two young specimens, of which eighteen were right- and fourteen left- 

 handed. He also mentioned that both Eschscholtz and Leukart had referred to what he called this 

 'inversion'. We found that the type which comes ashore at any particular spot depends on the local 

 wind. Indeed, on one particular day, 15 March 1955, a south-east wind had blown at Arrecife and 

 was falling off at 18.00 hr., when I collected specimens that would have been driven to the left of the 

 wind. An hour or two later, when the wind had shifted to the south-west, specimens of the other 

 other hand were taken. Winds from these two quarters were less likely to have driven in Physalia of 

 the opposite hands, as can be seen from Text-fig. 7. Dr Mackie suggested to me that this disymmetry 

 may have survival value, in that it obviates the stranding of the whole mixed population of Physalia 

 in any one set of circumstances. No doubt it also ensures optimum density of the population in the 

 most favourable environment. Woodcock had already suggested that motion either to the right or left 

 of the wind would make it possible for Physalia to avoid remaining in the convergent zone caused by 

 parallel, wind-induced surface vortices ; as for example in the North Atlantic, where it could be fatally 

 trapped by sargasso weed in those areas where the weed is common. 



Wilson (1947) had accepted, as a working hypothesis, Woodcock's contention that movement will 

 always be to the left of the wind-direction in the northern hemisphere, and suggested that though a 

 fair breeze might give the full 45 inclination, a storm might tend to drive Physalia more directly 

 before it.* His table vi assumed this last suggestion. He also thought that shoals of Physalia blown 

 on to our coasts may have bred in latitudes higher than usual. If that were the case, one would expect 

 the specimens to be small instead of exceptionally large. 



After the preliminary experiments and observations on drifting Physalia made on 14 April 1955, at 

 Arrecife, I carried out some further crude experiments on 15 and 16 April off the shore and inside 



* Natzio's observations (see p. 322) tend to confirm this. 



