Metchnikoff[305] has recently investigated the cause of “natural” death in the butterfly of the silkworm. The butterfly in this species lacks the organs necessary for taking up food, like the male rotifer or the ephemeridæ and hence is already, by this fact, condemned to a short life. Metchnikoff observed that these butterflies could live twenty-three days, but the average duration of life was 15.6 for the males and 16.6 days for the females; and that seventy-five per cent. of them contained no parasitic fauna or flora in their intestine. They lose considerably in weight during their lives, but the males still contain the fat body at the time of death. None of the changes accompanying “old age” in man are found in the tissues of these butterflies before death. Metchnikoff is inclined to believe that the animal is poisoned by some excretion retained in the body; namely, the urine, and that this poison also causes the symptoms of weakness which characterize the animal. He could prove the toxic character of their urine on other animals. This combined with starvation could sufficiently account for the short duration of life. The facts of the case show that it is due to an imperfection in the construction of the organism such as one would expect to find more or less in each animal if one discards the idea of purposefulness and divine wisdom in nature. Only a slight, perhaps an infinitesimal, fraction, of those species which are theoretically possible and which at one time or another arise can survive. Those which are durable show all transitions from the grossest disharmonies to an apparent lack of such shortcomings. [305] Metchnikoff, E., _Ann. d. l’Inst. Pasteur_, 1915, xxix., 477. 5. Minot had tried to prove that the death of metazoa is due to the greater differentiation and specialization of their tissues. Admitting the immortality of the unicellular organisms he argues that death is the price metazoa pay for the higher differentiation of their cells. This is of course purely metaphorical, but we may put it into a form in which it is capable of discussion in physicochemical terms, by assuming that death is a necessary stage in the development of a species. We are inclined, however, to follow Metchnikoff and suspect some poison accidentally or unavoidably formed in the body or some structural shortcoming as the cause of “natural” death.