204 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



markings such as the red heads of woodpeckers, distinct white or black 

 bars on the wings of other birds, may serve for recognition purposes 

 within the species. 



Confusing coloration. — Many butterflies and moths, and not a 

 few birds have rather conspicuous markings when in flight, which may 

 serve as specific recognition marks; but when they alight after a 

 zigzag course through the air, they cover up the conspicuous markings 

 and blend with the background in various ways. They are supposed 

 to alight when in danger of capture and they apparently disappear, 

 much to the confusion of the pursuer. Thus Kallima, the dead-leaf 

 butterfly, is quite conspicuous from above while in flight, but when it 

 alights, it cannot be distinguished from a dead leaf. 



Sexual coloration. — A great many groups of animals exhibit a 

 pronounced sexual dimorphism in color and pattern. The most 

 conspicuous instance of this is that of birds among which the female 

 is usually protectively colored so as to be inconspicuous when on the 

 nest or when sheltering the young, while the male of the same species 

 is frequently conspicuously colored. Similar situations are found 

 among butterflies and moths in which sometimes one sex and some- 

 times the other is the more elaborately colored. Sexual coloration is 

 also common among teleost fishes, lizards, spiders, and many other 

 groups. Charles Darwin devised a special "sexual selection" theory 

 to account for just this type of adaptation. 



One more kind of coloration that is not specifically dealt with by 

 Metcalf is what has now come to be known as "camouflage. " Many 

 animals when viewed out of their environment appear to be very 

 conspicuous owing to the juxtaposition of patches of irregular colors. 

 In their natural surroundings, however, they become practically 

 invisible. Thus the nighthawk with its strongly contrasted patterns 

 almost fades from view against the bark of a tree. 



For excellent illustrations of animal colorations the reader is 

 referred to Professor Metcalf's book Organic Evolution, where he has 

 gathered together in color plates many of the finest examples of the 

 phenomena under discussion. 



General considerations. — Adaptations are characteristic of all 

 living organisms and must be accounted for by any evolutionary theory 

 that is to be acceptable. Any theory that claims to account for new 

 species but does not account for adaptations is at best only a partial 

 explanation. All of the phenomena which have been briefly men- 

 tioned in this chapter, together with the more intricate phases of 



