394 BIOLOGY AND ITS MAKERS 



resembling the mid-rib of a leaf, so that the whole butterfly 

 in the resting position becomes inconspicuous, being pro- 

 tected by mimicry. 



One can readily see how natural selection would be evoked 

 in order to explain this condition of affairs. Those forms 

 that varied in the direction of looking like a leaf would be 

 the most perfectly protected, and this feature being fostered 

 by natural selection, would, in the course of time, produce a 

 race of butterflies the resemblance of whose folded wings to 

 a leaf would serve as a protection from enemies. 



It may not be out of place to remind the reader that the 

 illustrations cited are introduced merely to elucidate Dar- 

 win's theory and the writer is not committed to accepting 

 them as explanations of the phenomena involved. He is 

 not unmindful of the force of the criticisms against the ade- 

 quacy of natural selection to explain the evolution of all 

 kinds of organic structures. 



Many other instances of the action of color might be 

 added, such as the wearing of warning colors, those colors 

 which belong to butterflies, grubs, and other animals that 

 have a noxious taste. These warning colors have taught 

 birds to leave alone the forms possessing those colors. Some- 

 times forms which do not possess a disagreeable taste 

 secure protection by mimicking the colors of the noxious 

 varieties. 



Sexual Selection. There is an entirely different set of 

 cases which at first sight would seem difficult to explain on 

 the principle of selection. How, for instance, could we 

 explain the feathers in the tails of the birds of paradise, or 

 that peculiar arrangement of feathers in the tail of the lyre- 

 bird, or the gorgeous display of tail-feathers of the male 

 peacock? Here Mr. Darwin seized upon a selective prin- 

 ciple arising from the influence of mating. The male birds 

 in becoming suitors for a particular female have been accus- 



