4i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



Praying Mantis on the green herbage, and the brown variety among 

 the withered leaves. For in this case the protective value has been 

 proved up to the hilt and statistically measured, which is certainly 

 not yet true of the white winter dress of the mountain-hare among 

 the snow. The value of colour in supplying a cloak of invisibility 

 is increased when the markings are also like those of the surround- 

 ings, as in the brooding woodcock, or when there are added 

 possibilities of colour-adjustment, as in the flat-fish among the 

 gravel. The climax is reached in cases of true mimicry, as when a 

 palatable butterfly is the "double" of an unpalatable species with 

 which it consorts. 



Somewhat difficult, we think, are many of the cases of alleged 

 "warning colours", where an unpalatable animal is very con- 

 spicuous, like a yellow and black salamander or a red and green 

 bumet moth. The theory is that the conspicuousness serves as a 

 noli-me-tangere advertisement, impressing itself on the memory of 

 a forgetful enemy, saying, as it were: This gaily coloured creature 

 is of no use, leave it alone. Of some such warning colours there is 

 considerable experimental evidence. 



On a firmer footing are the numerous cases where the bright 

 colouring of the male animal contributes to the totit ensemble that 

 interests or excites his desired mate. Among the bower-birds there 

 seems to be a delight in brightly coloured objects, such as shells 

 and pods, for their own sake — the dawning of an esthetic sense. 



Sometimes the colours serve as advertisements of palatability, 

 as in some insect-attracting flowers or bird-attracting fruits, but 

 here again, in many cases, there is need for replacing surmise by 

 precise demonstration. Thus, without denying that colour may 

 have some attractive value to hive-bees, we are sure that the 

 fragrance of the flowers is much more important. 



The mention of flowers recalls another use of colour, what may 

 be called a way-post function. As Sprengel indicated long before 

 Darwin, there are often conspicuous spots on the petals of flowers 

 which certainly look as if they might be of use in guiding the 

 insect visitors to the nectar. It is not necessary to suppose that 

 the insects would not find their way without the "honey-guides", 

 but anything that prevents fumbling will tend to have survival 

 value. The whitewashed stones by the footpath are not indis- 

 pensable to the belated traveller, but they enable him to walk 

 more rapidly and confidently. 



Many nestlings have very bright colours inside their mouth, 

 and Pycraft has pointed out that the conspicuousness of these, 

 when the young birds gape, may enable the parents to supply the 

 food with greater rapidity and precision. This may seem a trivial 

 matter, but survival in the struggle for existence often depends on 

 the difference between shibboleth and sibboleth. 



