35^ 



NATURE 



GUST 

 [AUGL- 



13' 1903 



The results were as follows ; the rates of leak are given 

 in scale divisions per hour, and are corrected to 30 inches 

 pressure : — 



The following table gives the relative ionisations, re- 

 ferred to air as unity. The values of the same constants 

 for the a and J3 rays formerly found are included, and also 

 measurements of relative ionisation under Rontgen rays. 

 These latter form part of an investigation not hitherto 

 published. 



Relative lontsations. 



The determinations for the 7 rays are less accurate than 

 the former ones for the a and 3 rays, on account of the 

 very much smaller rales of leak which have to be measured. 

 I think, if this be taken into account, there is no reason 

 to doubt that, within the limits of experimental error, the 

 7 rays give the same values as the j8 rays. These values 

 are nearly proportional to the density of the gas, except 

 in the case of laydrogen. The law which holds in the case 

 of Rontgen rays is totally different. 



This conclusion throws some light on the nature of the 

 j3 rays. The view seems to be gaining ground that these 

 are Rontgen rays, produced by the impact of the j8 rays 

 on the radium itself.^ This theory seems to have much to 

 recommend it. The /8 rays should, by analogy with the 

 kathode rays in a vacuum tube, produce Rontgen rays 

 when they strike a solid obstacle, and these Rontgen rays 

 should be much more penetrating than the ;8 rays them- 

 selves. The 7 rays seem at first sight to be just what 

 should be expected. But the present paper shows that in 

 one respect, at all events, the 7 rays behave quite differ- 

 ently from Rontgen rays, while, on the other hand, they 

 resemble the a and /8 rays. There seems to be a possibility 

 that they too are of a corpuscular nature, though uncharged 

 with electricity. This would account for the absence of 

 magnetic deflection. 



I do not think that the absence of conspicuous Rontgen 

 radiation is very hard to understand, if we consider that 

 the current emitted in kathode rays by a square inch of 

 intensely active radium is only lo-^' amperes; the current 

 through a focus tube is of the order 10-^ amperes, and 

 probably a great part of this is carried by the kathode 

 ravs. 



1 See, for instance, Madame Ci 

 Sciences," 1903, p. 83. 



NO. 



' Theses presentees a la Faculty des 



763, VOL. 68] 



2) and its annual 

 \ as the most 

 THE COLORATION OF THE QUA . 



TT is well known that, in different districts of th"elative to 

 -•■ the zebras of the type commonly known as Burchel^^" 

 but which, for reasons elsewhere given, I propose to calf 

 "quaggas," present distinct and easily determinable 

 colour variations, sufficiently constant in character to be 

 worthy of nominal recognition. Grant's quagga occurs in 

 North-East Africa, Crawshay's quagga in Nyasaland, 

 Selous's quagga in Rhodesia, and Chapman's quagga in 

 Angola. Still further south came Burchell's quagga, and 

 south of this again the two or more extinct types which, 

 as Mr. Lydekker has shown, pass currently as the quagga 

 proper. 



The first and last of this category are the extremes in 

 pattern variation. Grant's quagga may claim to rank as one 

 of the most completely striped of existing horses. Apart 

 from the ears, which are sometimes nearly white, and the 

 muzzle and fetlocks, which are usually black, he is a mass 

 of stripes from head to tail, from hoof to spine ; and in 

 sharpness of contrast -between the blackness of the stripes 

 and the whiteness of the interspaces, he rivals the 

 Abyssinian race of Gravy's zebra and the Angolan race of 

 the mountain species, while surpassing both in the inferior 

 extension of the stripes to the middle line of the belly. 

 Place him alongside Gray's quagga, with his pale stripe- 

 less limbs, underside and hind-quarters, his brown and con- 

 fusedly banded body and fawn-lined neck and head, and 

 you will hardly believe them to be the same species. _ Yet 

 there is no avoidance of the conclusion, since all inter- 

 mediates have been seen either as living specimens or 

 mounted skins. And one of the chief interests centred in 

 the existence of these intermediates lies in the progressive- 

 ness of the change this species undergoes as it passes from 

 north to south over its geographical area. Even in British 

 and German East Africa the pale interspaces on Grant's 

 quagga begin to be washed with brown, and to be filled 

 in with narrower intervening stripes. It will be difficult, 

 perhaps impossible, to distinguish such forms from the 

 quagga of the Mashonaland plateau. The latter, indeed, 

 may be taken as illustrative of the first step in the change 

 above alluded to leading from Grant's to Gray's quagga. 

 From it may be traced a series of gradations represented 

 by the local races named after Chapman, Wahlberg, and 

 Burchell, in which the stripes gradually disappear and thin 

 out upwards from the fetlocks to the shoulders and 

 haunches, while those on the body lose their connection with 

 the mid-ventral band, and, becoming shorter, leave the belly 

 unstriped. Concomitantly the intervening " shadow " 

 stripes increase in number and definition as they extend 

 forwards towards the neck, the normal stripes themselves 

 turn brown, and the ochre-stained ground colour deepens 

 in hue. In the typical form of Burchell's quagga the 

 " shadow " stripes reach the head, and the last of the 

 complete stripes is the one that extends backwards from the 

 stifle -to the root of the tail, the hind-quarters and legs 

 being practically, and the belly actually, stripeless. It is 

 but a step from this to the extinct Gray's quagga, in which 

 the stripes of the body were fused together and blended 

 to a great extent with the brown of the intervening areas, 

 those on the neck being exceedingly broad and broken up 

 by paler tracts of hair. 



The tendency of these modifications is to convert a striped 

 and conspicuously parti-coloured animal into one which, 

 even at a short distance, must have appeared to be 

 an almost uniform brown, paling into cream on the under- 

 side, limbs and back of the haunches. What is the mean- 

 ing of this change? Inferentially we may conclude it was 

 protective in the sense of subserving concealment. 



The testimony of observers in the field has established 

 the truth that the coloration of the coat renders a zebra 

 invisible under three conditions, namely, at a distance on 

 the open plain in midday, at close quarters in the dusk and 

 on moonlit nights, and in the cover afforded by thickets. 

 The procryptic result is achieved by the cooperation of 

 several factors. The white stripes blend with the shafts of 

 light sifted through the foliage and branches and reflected 

 by the leaves of the trees, and in an uncertain light or at 

 long range they mutually counteract each other and fuse 

 to a uniform grey. It is probable, too, that the alternate 

 arrangement of the black and white bars contributes some- 

 thing to the effect produced, by imparting a blurred appear- 



