June i i, 190SJ 



NA 7 VRE 



12- 



III ihc early days of ornithology oology played its part in 

 classification, but tliough the eggs of plovers, gulls, &c., 

 characterise their suborders, this is rather exceptional than 

 otherwise, and Huxley has settled the question of avian 

 taxonomy upon a sound morphological basis. Coloration 

 of eggs seems to have no connection with inherent 

 hereditary tendencies, nor is it apparently the result of 

 acquired characters in the birds themselves. In a large 

 number of cases it can be traced to the necessity for a 

 protective resemblance, just as in shells of mollusca. This 

 would serve to ensure escape frcr.ii the jaws or beaks of 

 natural enemies, e.g. hedgehog, snakes, and egg-sucking 

 birds and mammals, or Citi recent times) from the collecting 

 instinct of man. Where eggs exhibit brilliant or con- 

 spicuous marlvings, for no purpose apparently, we may 

 perhaps assume that the nesting-site has been modified, or 

 that, like the colour of the plumage, that of the egg is a 

 source of attraction, and connected with courtship, or, more 

 probably, as a means of identification by the individual of 

 its own nest and eggs, when the process would naturally 

 be hereditary (memory and heredity being intimately allied), 

 the instinct employed in distinguishing similar clutches 

 characterised by merely slight differences being likewise 

 acquired. • 



No two clutches of eggs of the same species are exactly 

 alike, particularly amongst birds nesting in colonies, e.g. 

 guillemots, penguins, &c., but each bird knows its own 

 egg. A few general principles may be recognised in the 

 coloration of birds' eggs. Usually white eggs are laid by 

 birds nesting in holes in trees or in dark situations, where 

 light seldom penetrates, as by the barn owl, woodpeckers, 

 and some pigeons, which build sometimes in the open, 

 though usually in dark woods (wood-pigeon), sometimes in 

 holes in trees, or in rabbit-burrows (stock-dove). Though 

 all owls lay white eggs, not all of them nest in holes in 

 trees, e.g. long-eared owl, snowy owl. This rule, the.i, 

 holds good in a large number of cases, but not invariably. 

 Most birds nesting on or near the ground lay eggs of a 

 uniform olive-green or brown ground-colour, e.g. pheasant, 

 partridge, nightingale, &c., the eggs harmonising with the 

 ground or vegetation. 



The eggs of grouse, ptarmigan, &c., resemble the heather 

 amongst which they are hid. Those of the ringed plover, 

 little tern, and oyster-catcher resemble sand and shingle 

 on the beach. The lapwing's eggs closely simulate bare 

 soil or dried bents. In these eggs secondary markings 

 break up the ground-colour, and further help to render the 

 eggs quite invisible except to an eye trained to detect slight 

 differences. The experienced field naturalist can find his 

 way to the immediate whereabouts of a nest by noticing 

 the existence of some distinctive mark in the surroundings, 

 <'.^. a stick, boulder, bush, mole-heap, &c., indicating to 

 the birds themselves at a distance the vicinity of the nest, 

 and thus enabling them to return quicklv and stealthily 

 without laying themselves open to observation by long 

 searching for the nest. The same protective resemblance 

 occurs amongst the chicks of these birds. Adaptation to 

 external surroundings, now or in the past, seems to explain 

 this matter of coloration in a large number of cases, and 

 exceptions to the rule are usually simply examples of 

 reversions to, or rather survivals of, ancestral trails before 

 protection was called for. In seeking for the causes of 

 variation, &c., the influence of environment or external 

 conditions seems to have been largely overlooked, too great 

 prominence having been given to the influence of the 

 inherent tendency to vary. In the case of the colours of 

 birds' eggs we have an instance in which, I think, external 

 conditions have played the greatest part. 



Whether all birds' eggs were originally white, and the 

 pigmentary layer has since been added to aid in conceal- 

 ment or to counteract the heat of the sun's rays, is not 

 definitely known. The number of eggs ornamented with 

 spots, &c., is very great. The creepers, nut-hatch, &c., 

 lay spotted eggs in holes in trees, &c., possibly after 

 originally having had some other nesting-site. 



Summing up the general conclusions drawn from lai 

 coloration of birds' eggs, we find different species of birds 

 of the same genus in a large number of cases lay eggs of 

 much the same type, e.g. warblers, tits, nut-hatches, 

 creepers, plovers, ducks, pigeons, gulls, terns, &c. In very 

 many cases, however, this is not the case, and an cxccp- 



NO. 2015, VOL. 78] 



tion in any genus may generally be traced to influence of 

 environment. Amongst the Turdidie, the eggs of the 

 missel-thrush, thrush, and blackbird are very dissimilar, 

 though their nesting sites are much alike. \'ariation in the 

 colours of eggs goes, in fact, largely with difference in 

 nesting-site. The starling and jackdaw lay blue eggs like 

 the three last-named birds in holes in trees. Probably these 

 birds have only recently betaken themselves to such nest- 

 ing quarters. The influence of man and his habitations, 

 and the conversion of dark forests into fields simply 

 enclosed with lines of trees into which light readily pene- 

 trates, may have induced alterations in some instances, if 

 not in coloration of the ^ZZ' ^^ least in nesting-sites, of 

 many birds intimately associated with human undertakings. 



A. R. HoRwoOD. 

 Leicester Corporation Museum, May 26. 



Electrical Action of Sodium. 



In a recent letter (X.\tl-re, May 28) I directed attention 

 to the fact that a negatively electrified body lost its charge 

 in air when held near to a clean surface of sodium. 



I have now ascertained that different portions of the 

 same rod may show the effect to a greater or less extent 

 owing to inequalities of temperature. Diminishing the 

 oxidation by cooling the metal produced a more complete 

 diselectrification, and this result seemed, at first sight, to 

 point to a cause other than chemical action. The influence 

 of a current of air, as well as the fact that even a soap 

 film stopped the discharging action, supported the view 

 that an electrified gas was emanating from the metal. 

 A bright surface of potassium gave no appreciable dis- 

 charging effect when cooled with a mixture of ice and 

 salt. In all cases the surfaces could be seen in the dark 

 to be glow-ing strongly. 



Further experiment has shown that no active gas can 

 be driven from sodium by heat, and that the true explana- 

 tion of the action lies in the positive electrification of the 

 air surrounding the freshly cut surface. With warm 

 sodium it is seen that the gold leaf falls rapidly for a 

 very short distance, while after cooling the action is more 

 prolonged. It is clear, therefore, in the first case, that 

 the action, although violent, is so transient, owing to the 

 whole surface being rapidly oxidised, as to appear of small 

 amount. The far larger discharging action was obtained 

 with reduced oxidation owing to the effect being more 

 prolonged. Cphrles E. S. Phillips. 



Castle House, Shooters Hill, Kent. 



Tabular Accuracy. 



I DO not know whether you will consider the following 

 suggestion suitable for publication. Though obvious, I 

 do not remember meeting with it. 



.All are agreed upon the enormous importance of securing 

 accuracy in mathematical tables, and of making known 

 any errors, but I am not aware of any definite centralised 

 method of registering mistakes, and publishing, in an 

 easily accessible form, corrections of them. 



What I venture to suggest is that, in connection, say. 

 with the National Physical Laboratory, there should be a 

 department dealing with mathematical tables. When an 

 error is discovered in any recognised table, the discoverer 

 should at once send a note of the fact to this department, 

 which would duly investigate the matter. Then, at suit- 

 able intervals, the department would publish a list of 

 errors, with their corrections, in a form purchasable by 

 those interested. By some such arrangement he might 

 hope in time to secure the accuracy so essential to the 

 numerical data employed in scientific calculations. 



C. T. Whitmell. 



Invermay, Hyde Park, Leeds, June 2. 



The "Sky-coloured Clojds." 



i'liERE w'as a very feeble display of " sky-coloured 

 clouds " here on May 27 from 10 to 11.15 P-'"- This 

 is the first time I have seen this phenomenon since July 19, 

 1906. Since Mav 27 the sky has not been clear enough 

 for them to be visible. ' T. W. Backhouse. 



West Hendon House, Sunderland, June 4. 



