44 



KNOWLEDGE 



[Nov. 18, 1881. 



resistance to flow, or what wc have agroed to call viscosity. 

 Hut wut«r also oHVrs such resistance to flow or viscosity, 

 therefore the (liflereiiee Letween iron or c<>l>pur-wire and 

 licjuid water as re^anls their fluidity is only a difterence of 

 dejirce, and not of kind ; tlie demarcation liutween solids 

 and liquids is not a liroad, cleiirlydetined line, hut a hand 

 of hlendiu),' shade, the dejitiis of tint representing varying 

 degrees of viscosity. 



THE 



PHILOSOPHY OF ANIMAL 

 COLOURS. 



Bv Dr. Andhew Wilson, F.R.S.E. 

 PART II. 



BUT, descending to still lower gi-atles of life, we may 

 discover examples of this " mimicry," not only of 

 surroundings, l)ut also of lifeless or inorganic objects, and 

 of, it may he, plant structures as well, on the part of 

 animals. The so-called " stick insects," or " walking 

 twigs," as they arc often called— the Phaumkla; of the 

 naturalist— present us with the most perfect reproductions 

 of bits of dried twigs. A figure of one of these insects is 

 before me as I write. It is represented climbing on the 

 delicate branch of a shrub, and but for the expectation 

 of what one is looking for, there would be considerable 

 difficulty in detei-mining which is insect and which plant 

 The bodies of these " tw ig insects "—which belong, by tlie 

 way, to the Orthoptcm, or that order which harbours the 

 familiar crickets and grasshoppers— are represented by 

 mere lines. The wings liave disappeared, and it has been 

 remarked that in their gait these insects exhibit a peculiar 

 habit of using their legs in a singularly awkward fashion, 

 and thus ujiparently aid the illusion of the .spectator that 

 he is regarding a" dried twig, moved erratically by tlie 

 wind. 



More extraordinary still are the " Leaf insects ; " near 

 allies, indeed, of the walking-sticks." Here "mimicry" 

 of the plant proceeds so far as to fully justify- the eminent 

 naturalist's remarks, that it is strange to tind the a.nimal 

 assuming a mimetic disguise and apeing the actor's art. 

 The wings in the " leaf in.sects " exactly imitate leaves. 

 The venation, or arrangement of the veins in the leaf, is 

 clearly seen, and in one form (Plii/llium) even the chest 

 and legs of the animal assume leaf-like characters. When 

 such an insect rests amid foliage, the value of such a close 

 resemblance to its plant suiTOundings as a means of pro- 

 tection can be readily understood. In some "leaf insects" 

 —all of which are tropical species— the wings resemble 

 leaves that are dried and withered. In others, the minute 

 fungi that attack leaves are imitated. Mr. A. R. Wallace 

 tells us that one of the " walking-sticks " obtained by him 

 in Borneo, " was covered over with foliaceous excrescences 

 of a clear olive green colour, so as exactly to resemble a 

 stick grown over by a creeping moss or Jungennannia. The 

 Dyak who brought it me assured me it was grown over 

 with moss, though alive, and it was only after a most minute 

 examination that I could convince myself it was not so." 



Lastly, there may be noticed in connection with these 

 curious traits of animal life, the fact that certain 

 animals, themselves liarndcss and inoffensive, may assume 

 tlie exact appearand! of offensive neighbours. In this 

 respect, certain butterflies are /'dci/e principe. Certain 

 South American butterflies, known collectively under their 

 *iinily name of J/rlironultr, exhibit a brilliant colouration, 

 ••'se possess a very strong odour ; and, it may be 

 but liken- i.„ sequel, a highly disagreeable taste as 



presumed from w- "^n'ipicuous' insects, and the under 



well. They arc highly w. J 



sides of their wings are as brilliantly coloured as the upper 

 surfaces ; so that, even in rr^pose, and when resting with the 

 wings apposed over tlie back, they are readily enough .seen. 

 Th(;ir colours are prominent, not to say gaudy. Yellow.s, reds, 

 and whit<-.s, coiniuingle with blacks, blues, and other tints 

 in a striking fashion. They are, further, by no means 

 rapid flyers, and, putting the foregoing circumstances of 

 their gaudy colour and their slow movements together, no 

 group of animals would seem more liable to tlie attacks 

 of bird-enemies than these Helicon butterflies. Yet the 

 reverse is the case. So far from being decimated, their 

 race flourishes apace, and this result is clearly duo to the 

 strong odour and nauseous taste they possess. The mere 

 touch of a Helicon is in itself a pungent matter, which 

 reminds one of nothing so much as the persistence of the 

 musk-rat's secretion, or the still more awful effluvium of 

 the American skunk. Their neighbour butterflies may fall 

 victims by the score to the rapacity of their feathered 

 enemies, but the Helicons are spared from even the sem- 

 blance of attack. 



So far there seems nothing unusual or striking in a 

 gioup of butterfles being protected, through strong odour 

 and w'orse taste, from their natural enemies, the birds. But 

 now comes the most curious phase of this history. Another 

 and distinct family of butterflies, known as the LfptaUd<p, 

 allied to the common white cabbage butterfly, and removed 

 from the Helicons, also possesses representatives in South 

 -\merica. There are no points of agreement between the 

 Lc'ptalides and the Helicons, save, indeed, that both are 

 butterflies. Furthermore, the Leptalides are entirely de- 

 stitute of the nauseous odour and of the strong taste of the 

 Helicons, and in respect of their more agreeable presence, 

 should become a prominent article — as do other butterflies 

 — in the bill of fare of the birds. Yet, strangely enough, 

 the Leptalides escape persecution ; and the reason is not 

 far to seek or difficult to tind. When they are carefully 

 examined, certain species of the Leptalides are seen to be 

 exact facsimiles, in colour and appearance, of the stinking 

 Helicons ! Xaturalists at first classed both as Helicons, 

 until a closer examination showed the difference between- 

 these butterflies, and likewise proved that the Leptalides 

 had thus " mimicked " in the plainest possible manner the 

 colours of their strong-smelling neighbours. Nor are the 

 colours alone imitated. The ^■ery shape of the Helicon's 

 wings is reproduced in those of the Leptalides, and the 

 " feelers " likewise mimic those of the former group. Again, 

 special fonus of Leptalides " mimic " special forms of 

 Helicons. The flight has become of similar character in 

 both species, and the habits have been also slavishly copied. 

 Such instances as these certainly present " food for 

 thought ' to the reflective mind. It is the business of 

 philosophy to account for facts by placing the facts in 

 scientific juxtaposition — pihilosophy, in this light, is the 

 thread lipon which the piearls of knowledge are strung. 

 What, then, it may be asked, is the philosophy which can 

 explain the curious resemblances seen in the animal world, 

 ranging from, say, a mere likeness in tint to the surround- 

 ings (as in the flounder or woodcock), through more in- 

 t<!nsified likenesses, to the exact " mimicry " and to the 

 slavish copy of colour and form, a.s in the butterflies ? 



A first and highly important feature in the consideration 

 of the case is found in the fact that there is a gi-adation in 

 the degi'ee of "mimicry." From tlie mere sand or ground 

 tinting of the flounder to the exact colouring of the 

 butterflies is, of course, a wide st<»p, but it is one which is 

 bridged over by intermediate examples and stages. Then, 

 secondly, we discover a purpose or use in the disguises : that 

 purpose, apart from any considerations of its origin, being 

 the protection of the animid from its enemies, and the con- 



