Darwin's Artificial and Natural Selection 133 
Darwin also refers to the phosphorescent, or luminous, 
organs as a supposed case of difficulty for his theory. 
“The luminous organs which occur in a few insects, be- 
longing to widely different families, and which are situated in 
different parts of the body, offer, under our present state of 
ignorance, a difficulty almost exactly parallel with that of the 
electric organs.” 
In this case also, as in that of the electric organs, the 
structures appear in entirely different parts of the body of 
the insect in different species, so that their occurrence in this 
group cannot be accounted for on a common descent. In 
whatever way they have arisen, they must have evolved in- 
dependently in different species. Darwin advances no ex- 
planation of the origin of the luminous organs, but states 
that they “‘offer under our present state of ignorance a diffi- 
culty almost exactly parallel with that of the electric organs.” 
It will be noticed that the difficulty referred to rests on the 
assumption that since the organs are well developed they 
must have some important use! 
We may next consider “organs of little apparent impor- 
tance as affected by natural selection.” Darwin says :— 
“ As natural selection acts by life and death, — by the sur- 
vival of the fittest, and by the destruction of the less well- 
fitted individuals, —I have sometimes felt great difficulty in 
understanding the origin or formation of parts of little impor- 
tance; almost as great, though of a very different kind, as in 
the case of the most perfect and complex organs.” 
His answers to this difficulty are: (1) we are too ignorant 
‘“‘in regard to the whole economy of any one organic being to 
say what slight modifications would be of importance or not,” 
—thus such apparently trifling characters as the down on fruit, 
or the colors of the skin and hair of quadrupeds, which from 
being correlated with constitutional differences or from de- 
termining the attacks of insects might be acted on by nat- 
ural selection; (2) organs now of trifling importance have in 
