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 
