882 NATURE S

882 NATURE November 19, 1949 Vol. 164 tial spring tides. From an examination of living IN reply to Prof. Graham Cannon, I neither stated material, the animal was identified as S. cambrensis at the British Association, nor have I ever held, Brambell and Cole, only minor differences in colora that all characters must possess 'selection value': tion being apparent. Mr. Burdon Jones, who is precisely the contrary, since I took some care to working on the group, has seen preserved specimens explain that the spread of non-adaptive characters, and agrees with this identification. which certainly exist, cannot be responsible for It is of great interest that S. cambrensis should be evolution in wild populations. At the same time I found at Dale Fort, thus supporting the view of pointed out the danger of stating that any particular Brambell and Cole that the species might prove to character is non-adaptive, since even a I per cent be widely distributed. At Dale Fort it occurs advantage can rarely be detectable by the most in an environment similar to that described by accurate experiments, though it is considerable from Brambell and Cole, with a few minor differences an evolutionary point of view. It is genes, not which are worthy of note. The beds are at and below characters, that must very seldom be of neutral chart datum and are inaccessible during many months survival value. That is by no means to say that they of the year. Soil analysis of the surface two inches of are never so, but, as stressed at the meeting, such sand from adjacent parts of the beach have indicated genes cannot spread in a semi-isolated population in general about 95 per cent of fine sand and only so as to produce the 'Sewall Wright effect' unless it small quantities of silt, clay and organic matter. be numerically very small, of well under a thousand Whereas at Bangor the surface 8 in. were so con individuals, and such populations are unlikely to be stituted, at Dale, below two inches on the Saccoglossus sufficiently permanent units. ground, the sand was a dirty grey colour and prob It chanced that, among many other forms, I only ably contained more organic matter. No analysis referred to Drosophila in relation to two points. The of this deeper sand has yet been made. first was to mention that the chromosome inversions At Dale, S. cambrensis occurs in association with in wild populations of D. pseudo-obscura, often stressed Tellina fabula Gmelin, Chione striatula (da Costa), as an example of non-adaptive evolution, have Ensis ensis (L.), Acrocnida brachiata Montagu, recently been proved to be controlled by selection. Echinocardium cordatum (Pennant) and Peachia The second was in reply to the question, "Can some hastata Gosse. On no occasion, however, did we find of the trivial characters often used in taxonomy be either Ammodytes tobianus or Arenicola marina in regarded as non-adaptive?" My answer was that this area. they may be, but that the genes controlling them are A general account of the marine fauna in the not likely to be so. This I illustrated with reference vicinity of Dale Fort is being prepared by one of to Drosophila melanogaster merely because it is better us (R. D.P.) and will be published elsewhere in due known genetically than any other organism-animal course. or plant. That aspect of my answer was very relevant R. D. PURCHON to Prof. Cannon's query. It was this: Many of the G. T. JEFFERSON known genes are recognized only by minute and Department of Zoology, trivial effects which (bearing in mind the uncertainty University College, of the statement) we may quite reasonably think Cardiff. June 20. non-adaptive (for example, the presence or absence 1 Brambell, F. W. Rogers, and Cole, H. A., Proc. Zool. Soc. Lond., of a bristle), yet every one of these genes has a (B), 109, 211 (1939). detectable influence on the viability of the organism. • Brambell, F. W. Rogers, and Goodhart, C. B., J. JJfar. Biol. Assoc., N.S., 25, 2, 283 (1941). E. B. FORD University Museum, Oxford. Non-Adaptive Characters IN an address to Section D at the Newcastle meeting of the British Association, Dr. E. B. Ford, Torque and Angular Momentum of Centi· if I understood him correctly, stated or inferred that metre Electromagnetic Waves he considered that all characters must possess WHEN plane electromagnetic waves, which are 'selection value'. circularly polarized, are absorbed by a screen dis· From the discussion that followed, moreover, it posed perpendicularly to the direction of propaga appears that he holds this belief not only in regard tion, or are transformed with a suitable device into to animals but to plants also. In answer to questions linearly polarized waves, they produce on the screen he plainly rejected the possibility that, in the cases or device a mechanical torque, the intensity of quoted ('ringed' character in guillemots, 'spotted leaf' which for unit surface is character in such plants as Arum maculatum, and human blood groups) no adaptive character was -,s (I) involved. To him, apparently, however trivial char w acters may appear, they must have some adaptive where S is the Poynting vector and w is the angular value ; evidence for this was, so far as one could frequency of the wave. This effect was pointed out gather, derived from experimental work on Drosophila. many years ago by A. Sadowsky1 ; and later J, H. As there is no mention of this fundamental concept Poynting• gave a complete theory for it. in the summary of his paper published on p. 62 of Formula (I) may be easily proved, taking into the Journal of the meeting, may I ask Dr. Ford if I account the fact that photons have a spin h/2n have stated his views correctly and, if so, whether ± (where h is Planck's constant). he would publish a concise account of the arguments In effect, an elliptically polarized wave may bf which led him to arrive at these views? resolved into two components along the principa H. GRAHAM CANNON axis of the ellipse, namely, University, Manchester 13. Oct. 12. x = X 0 coswt, y = Y 0 sinwt, ©1949 Nature Publishing Group.

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