January 19, 1922] 



NATURE 



77 



A Curious Physiological Phenomenon. 



In Prof. Graham Brown's comments in Nature of 

 December 22 last on the letter with the above title, 

 he points out that "the peculiarity of the present 

 movement is that it is in the same direction as the 

 original one." In this it resembles the sensory pheno- 

 mena of positive optical after-images. Now these are 

 periodically repeated after fairly strong stimulation, 

 and it seemed worth while to look for such repetition 

 in the present case. This is easily found, merely by 

 increasing the time during which the hand is pressed 

 against the wall up to about a minute until the whole 

 arm feels tired. The rise and fall described occurs as 

 usual; after a brief pause the arm again rises some- 

 what less vigorously, and a third much weaker rise 

 may follow. This succession of movements does not 

 always occur. In fact, in my own case even the first 

 rise occasionally fails for no' apparent reason. 



I have found these repeated movements in the 

 cases of colleagues who were not informed in any 

 way what to expect. I am not familiar with the 

 physiological theory of fatigue, but to a physicist 

 these alternatioiis of rest and activity inevitably sug- 

 gest the accumulation and discharge of potential 

 energy. I do not know if physiologists can find place 

 for a (? cerebral) mechanism by which a thwarted 

 stimulus can store up "strain " energy of appropriate 

 kind, which after a brief interval is discharged by 

 fulfilling its particular motor or sensorv function. 

 Further accumulation and discharge might follow, 

 something after the fashion of the successive residual 

 charges and discharges of a Leyden jar. 



If this be so, positive visual after-images may have 

 a similar origin. As is well known, the intensity of 

 sensations only increases by equal increments when 

 the exciting stimuli increase by equal ratios ; thus 

 strong stimuli fail, so to speak, to produce their full 

 effect; there is inhibition and perhaps a storing of 

 "straiji "energy. Afterwards liberated, may this 

 give rise in the sensory case to after-images, or with 

 motor mechanisms to these involuntary muscular 

 movements? 



It may be added that the phenomenon can be pro- 

 duced in the leg by standing on one foot and press- 

 ing the outer side of the other against a wall, then 

 allowing the leg to fall to the vertical with the foot 

 still clear of the ground ; after a short pause the leg 

 rises laterally again. Or if the lower leg is drawn 

 back so that its calf (or Achilles tendon) presses up- 

 wards and backwards against the edge of a heavy 

 chair, and is then allowed to fall, the knee involun- 

 tarily flexes again after a very brief interval. In this 

 case there is (with me, at any rate) a strong tendency 

 for the pressure against the chair to cause acute 

 cramp in the biceps muscle of the thigh. 



,r. • T ,,, J. H. Shaxby. 



Viriamu Jones Physical Laboratory, 



University College, Cardiff, December 30. 



Structures and Habits Associated with Courtship. 



Mr. Julian Huxley's letter in Nature of Decem- 

 ber^ 29 last upon the habits of courtship brings to 

 notice some of the recently ascertained facts. From 

 these he concludes that the conspicuous colours, pat- 

 terns, and forms made use of in these displays and 

 ceremonies resemble copulatory organs in being sub- 

 servient to efficiency in securing union of the gametes, 

 and that, therefore, the problem of their evolutionary 

 origin is much simplified and similar to adaptive 

 characters in general. 



However, the argument that because thev are thus 



used they are therefore adaptations for the purpose of 

 producing sexual excitement is not justified. It may 

 well be that each animal for display makes use of 

 a bright coloration or conspicuous structure of which 

 the evolutionary origin is governed by some other 

 factor. The dog uses its hind limb to scratch its 

 back, but who would say that back-scratching con- 

 trols its evolutionary origin? 



The fact that polygamy is especially associated with 

 brilliant males, whilst in polyandry females are 

 usually the more highly coloured sex, clearly shows 

 that some other factor governs the evolutionary origin 

 of these secondary sexual differences. 



A few other antagonistic facts may be mentioned. 

 In birds it is the rule for the sexes to be similar 

 when they both take part in the rearing of the young, 

 as in the partridge ; whereas when the male takes no 

 part, as in ducks, secondary sexual differences are 

 common. 



In relatively unpalatable animals the sexes are 

 usually similar, whereas it is in palatable animals 

 that the greatest secondary sexual differences are to 

 be found. Butterflies and birds especially exhibit this 

 distribution. 



In predatory animals it is the rule for the sexes 

 to be alike. Instances of differences in coloration 

 between young and adults exactly similar to the 

 secondary sexual differences are widely distributed in 

 birds. Further special difficulties arise in the case 

 of insects, in view of their low visual acuity and 

 poor colour perception which probably precludes the 

 female from ever seeing the colour and pattern of 

 the male. Further, the study of their courtship shows 

 that scent and motion (which tends to conceal colouf 

 and pattern) are the means chiefly used to promote 

 sexual excitement. 



It is generally agreed that destructive criticism 

 should be accompanied by some alternative explana- 

 tion, but columns for correspondence do not permit of 

 lengthy expositions. I would, however, offer the ex- 

 planation that the distribution of secondary sexual 

 colorations is related to the vision of preyer and 

 preyed-upon and the necessity especially to protect the 

 female even at the expense of the male. This thesis 

 is fully expanded in J. C. Mottram's "Controlled 

 Natural Selection " (Longmans, Green and Co., 1914). 



J. C. M0TTRA.M. 

 Radium Institute, Riding House Street, 

 London, W. 



Spontaneous Ignition of Peaty Soils. 



I observe in Nature of August 25 last (p. 811), 

 which has just reached me, a letter by Mr. E. A. 

 Martin entitled "The Generation of Heath Fires," in 

 which the spontaneous ignition of peaty soils brought 

 about by exposure to the direct rays of the sun is 

 mentioned. 



It may be of interest to remark that in this part 

 of the world such examples of ignition of peaty soils 

 are quite common when the soils, in addition to being 

 exposed to the heat of the sun, are brought into a 

 condition of extreme drought. 



In certain parts of Cachar and Sylhet, where the 

 conformation of the land is that of a series of rounded 

 hillocks with intervening depressions, the depressions 

 are filled with peaty deposits, often of considerable 

 depth, known locally as "bheels." These bheels 

 have been formed in the usual manner by the con- 

 tinued growth of vegetation in a place where water 

 accumulates, and in the ordinary course of events 

 are always waterlogged. 



NO. 2725, VOL. 109] 



