Darwin's Theory of Sexual Selection 187 
be serviceable, and might have been gained by variation and 
the survival of the most easily recognized individuals.” 
It need scarcely be pointed out that an occasional peck 
can scarcely be supposed to have led to the splendid develop- 
ment of color shown by some caterpillars, and Darwin con- 
fesses that at first sight this hypothesis appears bold, but 
nevertheless he believes that it will be found to be true. He 
adds, “We cannot, however, at present thus explain the 
elegant diversity in the colors of many caterpillars.” 
A most important fact in this connection should not be over- 
looked, namely, that in the caterpillar stage the sexual organs 
are so little developed that it is generally impossible at this 
time to distinguish between the sexes, unless a microscopic 
examination is made. This gives us, perhaps, a clew as to the 
difference: between the mature sexual forms. These differ- 
ences are connected with difference of sex itself, This con- 
clusion also fits in well with the fact that during the period 
when the sexual organs are at the height of their develop- 
ment the individuals are most brilliantly colored. The pri- 
mary cause of the brilliant color of many animals concerns 
us here only secondarily, for, since it is known that many of 
the lower forms are no less brilliantly and elaborately colored 
than are the sexes of the higher forms, it is not surprising 
that the sexes themselves sometimes differ in this respect. 
Organs for producing sounds of different sorts are present 
in some insects, and these organs Darwin includes under the 
head of secondary sexual organs. In the group of Hemiptera, 
or bugs, the cicadas are the most familiar species that pro- 
duce sounds. The noise is made by the males; the females 
are quite mute. 
“ With respect to the object of the music, Dr. Hartman, in 
speaking of the Cicada septemdecim of the United States, 
says, ‘the drums are now (June 6th and 7th, 1851) heard in 
all directions. This I believe to be the marital summons 
from the males. Standing in thick chestnut sprouts about 
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