April 2, 1885] 



NA TURE 



5°5 



occurs always, I think, at F., the cross-section of the bar being 

 proportionately more reduced than the load. 



Alex. B. \V. Kennedy 

 University College, March 23 



The Colours of Arctic Animals 



The white colour of Arctic mammals and birds has hitherto 

 been generally ascribed by evolutionists to protective resem- 

 blance, the adaptation to a snow-covered country being attri- 

 buted to the preservation of individuals which by assimilating 

 to their environment in colour, either escaped detection by their 

 foes, or, on the other hand, were by this means enabled to 

 secure their prey more advantageously. Although a certain 

 weight may, in the case of some species, be fairly given to these 

 organic factors, it always appeared to me that this explanation 

 was not in itself sufficient, in face of the consideration that many 

 of the species so coloured could hardly be said to require such 

 protection on account of persecution, or to derive any obvious 

 advantage therefrom for predatory purposes. A more satis- 

 factory explanation seemed to be that the mode of coloration in 

 question had, at any rate in the first instance, been brought 

 about by natural selection through physical rather than through 

 organic agencies. It is well known that white, as the worst 

 absorber, is also the worst radiator of all forms of radiant 

 energy, so that warm-blooded creatures thus clad would be 

 better enabled to withstand the severity of an Arctic climate — 

 the loss of heat by radiation might, in fact, be expected to be 

 less rapid than if the hairs or feathers were of a darker colour. 1 

 According to a paper recently published by Lord Walsingham, 2 

 it seems that this view was entertained as far back as 1846 by 

 Craven, 3 the only addition to the theory required by modern 

 evolution being that we must regard the white covering as having 

 been acquired by the ordinary Darwinian process of the survival 

 of the fittest, i.e. by the climatic selection of those individuals 

 best fitted to withstand the extremely low temperatures of their 

 habitat. 



It is perfectly familiar to zoologists that most animals occasion- 

 ally give rise to white varieties, so that the basic variations 

 necessary for the establishment of the required modification in 

 the colour of the hair and feathers would not have been wanting 

 during the gradual approach of the Glacial Epoch. It may be 

 conjectured whether white may not have been the prevailing 

 colour among all warm-blooded animals during this period, with 

 the exception, perhaps, of those species in which the severity of 

 the climate may have been met by an equally effective thickening 

 of the fur. Certain species which, like the stoat and ptarmigan, 

 become white during winter, may, from this point of view, be 

 regarded as reverting seasonally to the mode of coloration which 

 in their ancestors was normal during the Glacial Epoch, the re- 

 version being in these cases brought about by the same influ- 

 ences which formerly fixed white as the most advantageous form 

 of covering. In accordance with this view, it is sometimes 

 asserted that the stoat does not commonly turn white during 

 winter in the south of England, excepting in very severe 

 seasons. 4 Further observations on this point are much needed. 



In striking contrast to the white covering of Arctic and Alpine 

 mammals and birds, it has been found that there is a quite oppo- 

 site tendency for the insects to become darker and more suffused, 

 this melanism being especially noticeable among many of the 

 Lepidoptera. Although numerous speculations as to the cause 

 of this phenomenon have from time to time been advanced, it is 

 in the paper by Lord Walsingham above referred to that what 

 appears to be a true cause has for the first time been suggested. 

 The author has, in fact, most ingeniously extended the very 

 argument which had been adduced to account for the white 

 colour of the mammals and birds to explain the quite opposite 

 melanism of the insect--. According to the present view the 

 the melanic tendency of northern Lepidoptera must be ascribed 

 to the natural selection of the darker forms owing to the ad- 

 vantage which these would possess in being able to absorb more 

 of the solar radiation than their lighter congeners. The same 

 action must be regarded as here bringing about opposite effects : 

 in the case of warm-blooded animals the loss of heat by radia- 

 tion is retarded by the white covering, whilst in insects, which 



1 Trans. Essex Field Club, vol. i. Prcc, March 20. 18S0, p. vi. 



3 "On some probable causes of a tendency to melanic variation in Lepi- 

 doptera of high latitudes ; " the Annual Presidential Addres- to the York- 

 shire Naturalists' Union, Doncaster, March 3, 1885. 



3 " Recreations in Shooting," p. 101. 



4 R. M. Christy in Trans. Essex Field Clnl, vol. i. p. 67. 



develop but little heat by respiration, it is of the utmost import- 

 ance to utilise as much as possible of the solar energy. This 

 will be seen to be all the more necessary when it is considered 

 that, under Arctic conditions, the solar rays have but little power, 

 and that the pairing of the insects has to be effected. with great 

 rapidity. In order to test these views experimentally, the author 

 exposed numerous species of Lepidoptera of various colours to 

 the sun's rays on a surface of snow, and observed the rate at 

 which the insects sank beneath the surface. As might have 

 been anticipated, the darker insects, like Tanagra chtsrophyllata, 

 sank more rapidly than white moths like Acidalia immutata, 

 which made but little impression on the snow. 



The questions raised by these suggestions and observations 

 certainly appear to be well worthy of consideration when dis- 

 cussing the subject of animal coloration. Thus the explanation 

 of the melanism of Arctic insects now advanced may perhaps, 

 when more fully elaborated, throw further light upon the theory 

 of seasonal dimorphism first proposed by Weismann. 1 If, in 

 accordance with the views of this author, we regard the present 

 winter forms of these seasonally dimorphic Lepidoptera as the 

 ancestral Glacial types, it becomes clear why in such white 

 species as Pieris napi, the parent Glacial form Bryonia should 

 be the darker. In the case of Arasehnia lez'ana the theory does 

 not at first sight apply, inasmuch as the winter form is lighter 

 than the summer generation (Prorsa) ; here, however, both 

 forms are coloured, and there would be but little difference in 

 their relative heat-absorbing powers. The same remark may 

 apply in the case of our own seasonally dimorphic species of 

 Seienia and Ephyra. R. Meldgla 



An Error in Ganot's "Physics" 



In your issue of February 19 (p. 361), E. Douglas Archibald 

 calls attention to a typical error in Ganot's "Physics," 10th 

 edition, p. 325, and assumed that it had run through the ten 

 editions. If he had taken the pains to look back to previous 

 editions the formula would have appeared right, viz. : — 

 p _ 0-31 V(H - %F£) 

 (I + at) 760 

 In going over the text of earlier issues of the book some minor 

 errors are discoverable, but do not detract materially from the 

 value of the same to the careful student 



Frank E Emery, 

 1st Asst. Sci. De ■-. 

 Mountainville, Orange Co., New York, March 4 



With reference to the letter of Mr. Frank E. Emery on mine, 

 calling attention to the typical error in Ganot's " Physics," I 

 beg to say that though in some of the earlier editions the error 

 may not exist, it occurs in the 5th and 10th, both of which are 

 in my possession. The inference is very strong that if it occurs 

 in these two it occurs in the editions intervening, and thus in 

 HALF of the editions published. The first five editions are now 

 getting out of date, so it is not of much value to people if the 

 error does not exist in them. 



I would also observe that if Mr. Emery takes the pains of 

 reading my letter over again he will notice it was explicitly 

 stated to be for the benefit of the large class of students who are 

 not eareful. 



My purpose was in no way to run down Ganot, but to warn 

 people of a pitfall in it. E. Douglas Archibald 



Tunbridge Wells, March 23 



Exceptional Whiteness in Tropical Man 

 Singularly' enough, being encamped in the same place as 

 that from which the paper on " The Blackness of Tropical Man " 

 was written to Nature some months ago, the converse, a case 

 of the whiteness of this class of man, presented itself unexpectedly. 

 While entering, to-day, the native village of J eykondasholapurm, 

 that had sunk to nothing from having been the capital of a native 

 dynasty in the south of India, and situated about lat 11° N. and 

 long. 78" E., the writer observed an apparently white woman 

 sitting on a doorstep by the side of the road, with flaxen-coloured 



' I may take the present opportunity of pointing out to those who possess 

 the English edition of the " Studies in the Theory' of Descent " that an error 

 inadvertently occurs in the numbering of the figures in Plate I. Figs. 2, 3, 

 4, and 5 should have been numbered respectively 3, 5, 2, and 4 ; I am 

 indebted to Mr. F.. R. Poulton for kindly callin? my : 

 position. 



