552 



NA TURE 



{April 16, 1885 



these lines, upwards positive, downwards negative, are 

 the co-ordinates of the straight line MN. So much for 

 the parallel co-ordinates. Take a straight line, Ox, for 

 axis, and on this line a point, O, the pole of the system. 

 A straight line is determined by the angle 6, which it 

 makes with the axis, and by the length X from O of its 

 intersection with Ox. These are the axial co-ordinates. 

 Elementary details of these two systems are given for the 

 former in Chapters I.-V. (pp. 1-33) ; for the latter, in 

 Chapters VI. -VI II. (pp. 36-43). Several applications to 

 examples are discussed. Chapters IX., X. (pp. 52-73) are 

 devoted to a " Methode de transformation ge'ometrique 

 fondle sur la simple comparaison des coordonndes paral- 

 leles aveclescoordonndes rectangulaires." The " proce'de 

 nouveau " is the closing portion of this chapter (pp. 73-82). 

 The illustrations in the pamphlet are mostly taken from 

 curves of the second degree, but these co-ordinates — a 

 kind of tangential co-ordinates — are useful for such ques- 

 tions as the following : — Find a curve such that a portion 

 of a tangent intercepted between the point of contact and 

 the axis has a constant length (the tractrix is such that 

 the area between it and the axis is equal the area of a 

 semi-circle, radius equal distance from origin to cusp of 

 tractrix) ; find a curve such that the portion of a perpen- 

 dicular 77 to the axis Ox drawn through the foot T of 

 the tangent, limited on one side by Ox and on the other 

 by the corresponding normal, has a given length (the 

 curve, of course, is readily seen to be a cycloid). 



The pamphlet is an interesting one, and suggests 

 methods of procedure which in some cases have advant- 

 ages over other methods more familiar. 



LETTERS TO THE EDITOR 



\_The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.'] 



The Colours of Arctic Animals 

 I AM sorry that I cannot agree with my friend Mr. Meldola 

 as to the insufficiency of the explanation of the white coloration 

 of Arctic mammals and birds as due to protective adaptation, 

 since it appears to me that there is no important group of facts 

 in natural history of which the explanation is more complete; 

 while on the other hand I venture (though with some hesitation) 

 to question the basis of his counter explanation, as I am not 

 aware of any sufficient proof that colour, per sc, affects the radia- 

 tion of low grade heat. At all events I feel tolerably certain 

 that this cause, if it exists, has had no perceptible influence in 

 determining the white colours of Arctic animals. 



I am not myself aware of there being ' ' many species " pos- 

 sessing the white coloration as to which there is any difficulty 

 in seeing the advantage they may derive from it, and there is 

 certainly a large body of facts showing that colour is, in almost 

 all animals and in every part of the world, more or less pro- 

 tective or adaptive. If the white coloration of Arctic animals 

 stood alone, it might be thought necessary to supplement the 

 protective theory by any available physical explanation, hut we 

 have to take account of the parallel ca^es of the sand-coloured 

 desert animals and the green- coloured denizens of the ever- 

 verdant tropical forests ; and though in both these regions there 

 are numerous exceptional cases, we can almost always see the 

 reason of these, either in the absence of the need of protection 

 or in the greater importance of conspicuous colouring. In the 

 Arctic regions these exceptions are particularly instructive be- 

 cause in almost every case the reason of them is obvious. Let 

 me call attention to a few which now occur to me. 



In the Arctic zone the wolf does not turn white like the fox, 

 the reason evidently being that he hums in packs, and conceal- 

 ment from his prey is not needed. So the musk-sheep and the 

 yak, though both exposed to the extremest cold, are not white, 



because they are both swift and strong and need no concealment 

 from their enemies. For the same reason neither the moose, 

 the caribou, nor the reindeer are wholly white. Again, the 

 glutton and the sable are dark- coloured, though inhabiting the 

 coldest regions, and this is clearly because they are arboreal, and 

 are better concealed from their prey by a dark than a light 

 colour. If any useful protection from cold were to be obtained 

 by a white coat, we should expect it to appear in such a case as 

 the Esquimaux dogs, exposed for countless generations to the 

 severest climate. But they gained the required warmth by a 

 thickening of the woolly undercoat in winter, as do many other 

 animals ; and this suggests the general proposition that it will 

 be always easier and safer to gain warmth in this way than by a 

 modification of colour, which could certainly have but a very 

 small effect, and might often interfere with adaptations of far 

 greater importance. Exactly analogous cases occur among birds. 

 The raven is, perhaps, the extremest Arctic species, but, feeding 

 on carrion, it has no need of concealment in approaching its 

 prey, and thus it keeps its jet black coat in the depths of the 

 Polar winter. 



The physical explanation of melanism in butterflies and some 

 other insects, on the other hand, seems to me to be probably a 

 sound one ; but even that requires more evidence and a fuller 

 knowledge of the habits of the species before we can admit it 

 as proved. It may be that the dark colouring is protective, 

 assimilating with the surroundings of the insect when at rest, 

 and this can only be decided by observations specially directed 

 to the point in question. 



But even if, in this case, the dark colour has been produced in 

 order to favour the absorption of the direct rays of the northern 

 sun, it affords no support whatever to the totally different case 

 in which the radiation of the obscure heat from an animal body 

 has to be checked. I may, perhaps, be ignorant on the point, 

 as it is rather out of my line, but I am not aware of any good 

 experiments to determine the influence of colour per se, as distinct 

 fi om the structure and surface-texture of coloured substances, on 

 the radiation or absorption of heat of a low grade of temperature, 

 and from a dark source. The only authority I have at hand 

 (Ganot's ■■ Physics," eighth edition) seems rather to imply that 

 colour has no effect in such cases, for I find it stated, at p. 338, 

 that the radiating power of lampblack and whitelead are identical, 

 both being given as 100, while Indian ink is only 88. Again, at 

 p. 352, the absorptive power of these two substances is given as 

 100, the source of heat being copper at 100° C, while that of 

 Indian ink is given as 85. This seems to show that surface- 

 texture or molecular structure is the important point, while 

 colour has no effect whatever. 



In order to determine experimentally whether white fur or 

 feathers are inferior to black as radiators of animal heat, it would 

 not do to employ stained or dyed materials, because the pigments 

 employed might affect the texture of the surface, and produce an 

 effect not at all due to the colour. A fair test would be afforded 

 by two samples of cloth or flannel woven from white and black 

 natural wool respectively, the wool to be obtained from the 

 same breed of sheep, and, if possible, from the same district, 

 while the material must be as nearly as possible identical in 

 weight and texture. I shall be glad to learn from Mr. Meldola, 

 or any other of your readers, whether any experiment of this 

 kind has been made, or whether there is any valid reason for 

 believing that the radiation of animal heat is at all affected by 

 colour alone. Alfred R. Wallace 



Civilisation and Eyesight 

 The statistics of eyesight given by Mr. H. B. Guppy in 

 Nature (p. 503) relating to the inhabitants of the Solomon 

 Islands as tested by the Army test-dots, bring us nearer, I think, 

 to the solution of the question of the relative acuteness of vision 

 of civilised and savage races than any previous communication 

 which has appeared in your columns, as « e are able to compare 

 them with statistics obtained under similar conditions in this 

 country. The Anthropometric Committee of the British Asso- 

 ciation gave a series of tables in their Report for 1881 showing 

 the results of their inquiries into the sight of different classes of 

 the community, carried out by means of the Army test-dots ; and 

 for the purpose of comparison with Mr. Guppy's figures 1 have 

 extracted the returns relating to men employed in agriculture 

 and other out-door occupations as most nearly agreeing with the 

 conditions of life of savage people, and have embodied them, 

 together w ith Mr. Guppy's, in the following table : — 



