EVOLUTION 369 



Almost any character useful to its possessors may be 

 regarded as naturally selected in the struggle for existence ; 

 many characters have without doubt been so selected. It 

 must be admitted that to a very great degree natural selection 

 helps us to understand adaptations, and in recent discussion 

 it has been suggested that the Darwinian theory explains 

 adaptation rather than the origin of species. The justifica- 

 tion for this view lies in the fact that most of the characters 

 of an insect which fit it to the conditions of its life are 

 common to groups more comprehensive than the species — 

 to genera, families, orders, while definitely specific characters 

 seem often of no obvious value in the struggle for existence. 

 Darwin pointed out the value to insects of possessing such 

 colours, markings, and patterns as harmonise with their 

 surroundings, and examples of " protective resemblance " 

 have become almost hackneyed in discussions on the subject, 

 though the keen field-naturalist can never lose his admiration 

 at the wonder of many of them, for example, the close 

 likeness of a large group of the Orthoptera the " stick- 

 insects " (Plate I, A) and of the " looper " caterpillars of 

 Geometrid moths (Plate X, A) to twigs the colours of 

 many of the latter simulating the lichens that encrust 

 the bark. We have previously mentioned (p. 273) the 

 resemblance of the upper forewing surface of many 

 moths to dead leaves, tree-bark, and similar objects on 

 which the insects rest. Such resemblance is the more 

 striking in cases where the hindwings are brightly 

 coloured as in the *' yellow underwing " and '' crimson 

 underwing " motjis, the brilHant areas of the wings being 

 entirely hidden when the moths are at rest. These pro- 

 tective colours and patterns are in the main common to 

 groups of species ; for a conspicuous discriminative character 

 between two species we must often depend on some 

 feature of no value at all as an adaptation to environment. 

 Of our two large, common British yellow underwing moths, 

 for example, Triphaena pronuha is recognised by the narrow 

 and T. orbona by the broad, dark border to the flaring 

 orange-yellow of the hindwings. Turning to larval stages 



2 B 



