388 BIOLOGY AND ITS MAKERS 
resembling the mid-rib of a leaf, so that the whole buttertly 
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- 
