April 24, 1902] 



NA TURE 



597 



The Law which underlies Protective Coloration. 



I desire at the outset to point out tnat my demon- 

 stration of the principle of Protective Coloration is not the 

 demonstration of a theory, but of an indisputable fact, namely, 

 that if an object be coloured so that its tones constitute a grada- 

 tion of shading and of colouring counter to the gradation of 

 shading and of colouring which light thrown upon it would 

 produce, such object will appear perfectly flat, retaining its 

 length and breadth, but having lost its appearance of thickness, 

 and when seen against a background of colour and pattern 

 similar to its own will be essentially indistinguishable at a 

 short distance. All persons who have seen the models which 

 illustrate this fact know that they prove it. 



Now, if this stands proved, the fact that a vast majority of 

 the whoJe Animal Kingdom wear this gradation, developed to 

 an exquisitely minute degree, and are famous for being hard to 

 see in their homes speaks for itself It is plain that their colour- 

 gradation can no more escape effacing their look of solidity 

 than the Law of Gravitation can escape drawing a projectile to 

 the earth. 



This is so obvious that one hears on all sides expressions of 

 wonder that it was so long unnoticed. I may add that all per- 

 sons of trained sight, such as artists, perceive it everywhere 

 among wild creatures. Other people supplement their un- 

 developed sight sense by their other senses, and if they know 

 the animal is solid think he looks solid. But the time will come 

 when even at zoological gardens, where animals are more or 

 less abnormally environed, people will find a new charm in 

 recognising everywhere this wonderful adjustment of their 

 colouring, and in perceiving its eflfect. 



Let anyone look at a ball, or egg-shaped object, placed any- 

 where out of doors, and when he has recognised its shading 

 from its light side to its dark, try to so colour it, where it 

 stands, as to obliterate both its shading and its colour-gradation. 

 (The sky-lit side is commonly the bluer). If he succeed, he will 

 find that Nature has swiftly guided him through the same pro- 

 cess which has taken her so long on the coats of animals, and 

 that he has given the object the counter-gradation I speak of, 

 and it will have dawned on him that so long as light makes its 

 i7«^ gradation on objects, there is only the one way to neutralise 

 it. In short, I simply prove that this arrangement of animals' 

 colours is what so marvellously effaces them, and leave it to 

 others to discuss the question whether concealment be a benefit 

 to an animal and whether the fact that it is a benefit be the 

 cause of his being concealed. 



All who believe in Natural Selection will, ol course, feel that 

 this colour law is its work, and since it is so almost universally 

 in use, and accounts, apparently, so almost exhaustively, for 

 all the attributes of graded animal colouring, I believe it will 

 ultimately be recognised as the most wonderful form of Darwin's 

 great Law. 



It stands alone in the startling attribute of being the only 

 known or so far conceivable device for making objects in full 

 light not appear to exist. This is a distinct plane above even 

 the great beauty of Protective Resemblance, where the deception 

 is of a more material nature, one thing passing itself off for 

 another thing. The beautiful sequence of this law, which 

 causes the grading colours to become a picture of the back- 

 ground, I will not force upon those who have not yet digested 

 the first part. 



It might be worth pointing out that the old theory that the 

 bellies of fish and tree birds were white to match the sky when 

 seen from below finds itself essentially done away with, since 

 the fishes' or birds' opacity causes even their white to look very 

 dark against an ordinary sky, while this same white proves to 

 work so brilliant a success for the purpose I have shown. All 

 people know the ghostly transparent look of fish in the water. 

 The white bellies of birds do help them to match the translucent 

 foliage overhead when seen from below, but the cold sky-holes 

 between the leaves are far too bright. Natural Selection has, 

 of course, surely modified all attributes to suit, not merely main 

 ends, but all minor ones, according to the rank of their im- 

 portance. 



Since publishing my papers in The Auk for April and October, 

 1896, I find that Prof. Poulton perceived years before their 

 appearance the power of a countergrading of light to make the 

 round surface of a pupa appear flat, and in another case the 

 power of light colour in a depression to make the concavity 

 disappear. In both of these cases he perceived the very Law of 

 Light and Shade on which the Fact of Protective Coloration 



NO. 1695, VOL- ^5l 



rests, and recognised the Fact itself in these instances. In his 

 " Notes in 1886 upon Lepidopterous Larvse, &c. ," read April 6, 

 18S7, he says(7Va?«. Ent. Soc. Lond., iSS7,p. 294), "Although 

 the cleft [between the posterior part of the body ol the larva of 

 Rumia crataegata and the branch] is largely filled up, ... a 

 considerable furrow remains, but this is not apparent because of 

 the light colour of the fleshy processes, which prevent the 

 attention from being directed to the shadow which would other- 

 wise indicate the position of the groove. The processes, there- 

 fore, attain the object of softening the contact between the larva 

 and its food-plant in a two-fold manner, by partially filling up 

 the cleft and by neutralising the shadow in the groove which 

 remains. I have also noticed the processes in the larva of 

 A. bctularia, and I believe that they are of very general 

 occurrence in Gcometrae." 



His other case is to be found in his "Notes in 18S7 upon 

 Lepidopterous Larvas, &c.," read October 3, 1888. He says 

 (Trans. Ent. Soc. Lond., pp. 595-6), "The most extra- 

 ordinary thing about this resemblance [of the pupa of Apatiira 

 iris to a sallow-leaf] was the leaf-like impression of flatness 

 conveyed by a pupa which was in reality very far from flat. 

 Thus the length of the pupa was 30 '5 mm. ; the greatest 

 breadth (dorso-ventral diameter), 11 "5 mm.; the greatest 

 thickness (from side to side), 8'5 mm. . . . But exactly in 

 these places, where the obvious thickness would destroy the 

 resemblance to a leaf, the whole effect of the roundness is 

 neutralised by the increasing lightness of these parts — a light- 

 ness which is so disposed as to just compensate for the shadow 

 by which alone we judge of the roundness of small objects. 

 (Much larger objects can be judged of by the change of focus, 

 which becomes necessary as their near or distant parts are 

 observed.) In shading the drawing of an object so as to 

 represent roundness, the shade is made to become gradually 

 less and less deep as the tangential planes represented come 

 nearer and nearer to a right angle with the axis of vision. So 

 here, the converse of shading — the whiteness neutralising the 

 shadow which shading is intended to represent — dies off 

 gradually as the [representation of the] mid-rib is approached. 



" The whiteness is produced by the relative abundance of 

 white dots and a fine white marking of the surface which is 

 present everywhere, mingled with the green. The effect is, in 

 fact, produced by a process exactly analogous to stippling. 



" By this beautiful and simple method a pupa, which is 8'5 mm. 

 from side to side in its thickest part, appears flat and offers the 

 most remarkable resemblance to a leaf which is a small fraction 

 of I mm. in thickness." Abbott H. Thayer. 



Scarbro, New York. 



REPORT OF THE SMITHSONIAN 

 INSTITUTION. 



■p)R. S. P. LANGLEY'S report upon the operations of the 

 ■'-^ Smithsonian Institution for the year ending last June has 

 just reached this country. Many subjects of interest are referred 

 to in the report, but we are only able to mention a few, which 

 will, however, be sufficient to show that the Institution is taking 

 a foremost part in the advancement and diffusion of knowledge 

 among men of all civilised nations. 



Hodgkins Fund. — Among the many applications for grants 

 from the Hodgkins fund, it has been found practicable to ap- 

 prove several which conform to the conditions of the bequest. 

 Prof. Wallace C. Sabine, of Harvard University, has received a 

 grantfor the aid of his investigations on sound, the particular phase 

 of the problem under investigation being the subject of loudness 

 and interference. This research requires apparatus of special 

 design, part of which is now complete and is satisfactory. Prof. 

 Sabine, who had charge of the design of the new symphony hall 

 in Boston, has for several years given much attention to the 

 problem of architectural acoustics, or the science of sound as 

 applied to buildings. It is expected that his complete report 

 will be of much practical interest in connection with this 

 subject. 



Details of the progress of the research mentioned in the last 

 report as conducted by Dr. Victor .Schumann, of Leipzig, 

 have been received. The most noteworthy points in the 

 results so far ' refer, perhaps, to the relation of light and 

 electricity and to the probable insight into the nature of the 

 Rcintgen rays to be gained in the course of this investigation. 



The investigations of Dr. von Lendenfeld, of the University 



