2lS 



HARD WICKE' S S CIE NCE - G SSIP. 



suggestion that since dark colours absorb and part 

 with radiant heat more readily than light ones, it is 

 advantageous for birds and mammals living in cold 

 regions to be white, so as to retain as much heat as 

 possible in their bodies, while insects are dark 

 coloured in northern latitudes so that they may 

 readily take advantage of transient gleams of sun- 

 shine. But as Mr. Wallace has shown in his book on 

 "Darwinism," the principle of protective resem- 

 blance explains the white colour of most Arctic 

 animals, and also the exceptions to this general rule, 

 while the radiation theory cannot explain the excep- 

 tions. Still, the checking of radiation may well come 

 in as an additional advantage for the white animals. 



Mr. Poulton next discusses protective resem- 

 blances, especially in the case of lepidoptera. He 

 of course describes the " stick caterpillars " of many 

 geometers, and one very curious case which is worth 

 quoting. The description is headed " A caterpillar 

 which makes its surroundings resemble itself." "It 

 gnaws the leaf in such a manner as to leave a 

 number of rough models of itself attached to the 

 midrib, and then sits down beside them. The 

 caterpillar is green above and dark beneath, although 

 the former colour interrupts the latter at certain points 

 and comes into contact with the midrib on which the 

 insect is resting. The dark colour is not distinguish- 

 able from the deep shadow behind the leaf, and 

 therefore the appearance is that of an elongated patch 

 of green connected with the midrib by two narrow 

 stalks. The larva, in eating, leaves several pieces of 

 leaf attached to the midrib by one or two stalks, 

 which, therefore, present a very similar appearance to 

 that of the larva itself" (pp. 36-37). The protective 

 resemblances of the pupae and perfect insects are 

 next discussed, and a brief notice of similar devices in 

 the higher animals follows. Under " Alluring 

 Colouration " Mr. H. O. Forbes' curious case of a 

 Javan spider which exactly resembles a bird's 

 dropping on a leaf (even to the dried-up fluid part 

 which has run towards the edge of the leaf !) in order 

 to attract butterflies to their destruction, is described. 



The author discusses those most wonderful 

 mechanisms — variable protective resemblances — at 

 considerable length. It is obvious that it is much 

 more advantageous for a creature to be able to adapt 

 itself to the colour of any environment in which it 

 happens to be, than merely to have its colour fixed to 

 correspond with the usual environment. To take one 

 example from among the vertebrata. Trout will 

 quickly change their colour in passing from a part of 

 a stream with a gravelly or sandy to one with a 

 muddy bottom. Such changes take place through 

 the action of the coloured light upon the nervous 

 system, and the stimulus is given through the eye. 

 Thus blind trout cannot in this way adapt themselves. 

 In the case of lepidopterous larvae and chrysalides, 

 Mr. Poulton has established by a long series of care- 

 ful experiments that these changes also take place 



through the nervous system, but that the coloured 

 light affects equally all the nerve-ends of the skin, 

 and not especially the eye as in the higher 

 animals. He also finds that when a chrysalis of the 

 small tortoiseshell is to be suited in colour to any 

 particular environment, the period of maximum 

 susceptibility of the larva to the colour is the period 

 of about eighteen hours during which it rests motion- 

 less on the surface from which it will subsequently 

 suspend itself. The meaning of this period, then, is 

 obviously that the larva may receive the influence 

 which shall determine the colour of the chrysalis. 

 The larva is, however, still slightly sensitive during 

 the next stage, i.e. while it is suspended and before 

 the larval coat splits. Many larvae can also adapt 

 their own colour to the environment in which they 

 find themselves. Mr. Poulton demonstrates very 

 forcibly that these wonderful adaptations could only 

 have arisen through natural selection, and are not the 

 accumulated result of the colours of the environment 

 directly acting on the larva for generations. 



Our author next discusses " Warning Colours," i.e. 

 bright, conspicuous colours known by the animal's 

 enemies to connote some disagreeable quality, and 

 thus securing impunity to the animal exhibiting them. 

 Among insects especially these colours are well- 

 marked. The original suggestion as to their 

 significance is due to Mr. Wallace, whose attention 

 was especially called by Mr. Darwin to the fact of 

 many caterpillars possessing bright and startling 

 colours, which could certainly not be classed as pro- 

 tective resemblances to the environment, nor as the 

 result of sexual selection. Darwin was delighted 

 with the suggestion, which was very soon put upon 

 an experimental basis. Poulton's discussion of the 

 whole subject is extremely able and very interest- 

 ing. He points out that warning colours can only 

 be adopted by a small proportion of insects in any 

 given country, as, if anything like a large number 

 of them adopted this means of defence, their enemies 

 would be compelled to eat them, and would gradually 

 get used to the nauseous taste, or even to the 

 poisonous properties. Similarly, when insect life 

 is scarce, species with warning colouration disappear. 

 Thus the common ladybird hides itself carefully when 

 the autumn comes on. Mr. Poulton points out that 

 only a limited number of colours (black and white, 

 black and yellow, orange or red) and of patterns, 

 (alternating rings of different colours, or alternating 

 longitudinal stripes for caterpillars, etc., and spots 

 upon a background of a contrasted colour for the 

 wings of moths and butterflies), are the most efficient 

 for attracting the attention of the enemy, and 

 impressing the pattern on his mind. Again the 

 fact of there being only a limited number of warning 

 colours and patterns facilitates the education of 

 enemies, so that each individual insect-eating 

 vertebrate only has to make trial of a very small 

 number of nauseous insects, and this is of course 



