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, belonging 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 independently in different species. Darwin advances no explanation of the origin of the luminous organs, but states that they “offer under our present state of ignorance a difficulty 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 importance as affected by natural selection.” Darwin says:— “As natural selection acts by life and death,—by the survival 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 importance; 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 determining the attacks of insects might be acted on by natural selection; (2) organs now of trifling importance have in some cases been of high importance to an early progenitor; (3) the changed conditions of life may account for some of the useless organs; (4) reversion accounts for others; (5) the complex laws of growth account for still others, such as correlation, compensation of the pressure of one part on another, etc.; (6) the action of sexual selection is responsible for many characters not to be explained by natural selection. Admitting that there may be cases that can be accounted for on one or the other of these six possibilities, yet there can be no doubt that there are still a considerable number of specific characters that cannot be explained in any of these ways. I do not think that Darwin has by any means met this objection, even if all these six possibilities be admitted as generally valid.