342 OUTLINES OF EVOLUTIONARY BIOLOGY 



system, whose function it is to bring about a varying adaptation 

 for concealment under varying conditions of the environment. 

 How perfect the adaptation may be will be realized by all who 

 have ever observed with what marvellous accuracy the colour 

 markings of a turbot in an aquarium are made to match the 

 sand or gravel upon which it is lying. 



In striking contrast with the cryptic colouration by which an 

 animal seeks, as it were, to avoid observation, are those numerous 

 cases in which self-advertisement appears to be the main object 

 in view. The British army, which only in recent years has 

 learnt the advantages of khaki clothing when in the field, still 

 exhibits some of the most startling instances of conspicuous 

 colouration met with anywhere in the animal kingdom, though 

 whether these examples should be classed under the head of 

 warning colours, or regarded as belonging to the epigamic 

 category, is perhaps an open question. We must, however, con- 

 fine our attention in this place to a few examples of warning 

 colours met with amongst the lower animals. 



We have seen that both warning and signalling colours, or 

 recognition marks, are spoken of as sematic. The former are 

 further distinguished as aposematic and the latter as episematic. 

 Aposematic colours are exhibited by many animals which possess 

 some special means of defence and find it advantageous to 

 advertize the fact. Wasps and hornets, with their conspicuous 

 orange- and black-banded bodies, are excellent examples. Such 

 animals do not seek to conceal themselves but rely upon their 

 warning colours to remind their enemies that they had better 

 leave them alone. It is not enough that they should possess the 

 power of making themselves disagreeable; the fact must be 

 clearly recognized, otherwise they would be constantly exposed 

 to experimental attack, and suffer injuries for which any damage 

 which they might inflict upon their pursuers would be but a 

 poor consolation. Orange, red and black, owing to their great 

 conspicuousness, especially when associated with one another, 

 are the colours most frequently met with in this connection, and 

 we find these colours, not only in noxious insects, but in various 

 vertebrate animals, such as poisonous reptiles, toads and sala- 

 manders. The Gila monster (Heloderma suspectum), of Mexico 

 and Arizona, is the only known poisonous lizard, and is con- 

 spicuously coloured in tints of blackish brown, yellow and 

 orange, while other members of the same group are usually 



