In the Zoölogical Station at Naples in 1906, an actinian, Actinia equina, was alive after having been in captivity fifteen years, and another one, Cerianthus, had been observed for twenty-four years. Korschelt kept earthworms for as long as ten years. The fresh-water mussel may reach the age of sixty years or more and crayfish may live for over twenty years. The differences in the dura­tion of life of mammals are too well known to need discussion. If the cells and tissues are immortal, how does it happen that the dura­tion of life is so characteristic for each species?

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.

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.

An unusually favourable object for the study of natural death is the animal egg. The egg of the starfish Asterias forbesii when taken out of the body is usually immature, but in the spawning season it ripens in sea water. The writer[306] observed that eggs which ripen disintegrate very rapidly when not fertilized. This disintegra­tion may be due to a process of autolysis, which sets in only after the egg has extruded the two polar bodies. The writer found that by preventing the matura­tion of the egg either by withdrawing the oxygen or by replacing the alkaline sea water by a neutral solu­tion or by exposing the eggs for some time to acidulated sea water, the disintegra­tion could also be prevented.

Further experi­ments showed that even in the mature egg rapid disintegra­tion could be prevented by lack of oxygen, and similar results were obtained by Mathews. When the egg is fertilized it does not disintegrate in the presence of oxygen but it gradually dies in the absence of oxygen. One is almost tempted to say that while the fertilized egg is a strict aërobe the mature unfertilized egg is an anaërobe. This latter statement, however, becomes doubtful since the presence of oxygen may help the disintegra­tion only indirectly by allowing certain changes to go on in the egg. The important points for us are that dura­tion of life in the mature unfertilized egg is comparatively short and that the entrance of a spermato­zoön or the process of artificial parthenogenesis saves the life of the egg. Loeb and Lewis found that the life of the unfertilized sea-urchin egg (which is usually mature when removed from the ovaries) can also be prolonged when its oxida­tions are suppressed. The decay of the unfertilized egg seems to be due to the fact that those altera­tions in the cortical layer which underlie the membrane forma­tion and which are responsible for the starting of development gradually take place. In such a condi­tion the egg will die quickly unless deprived of oxygen. This view is supported by the observa­tion of Wasteneys that unfertilized eggs of Arbacia show an increased rate of oxida­tions when allowed to remain for some time in sea water; we have seen in Chapter V that such an increase also accompanies artificial membrane forma­tion.

6. If the limited dura­tion of life of an organism is determined by one or more definite harmful chemical processes, we should expect to find a temperature coefficient for the dura­tion of life or at least be able to show that if all other condi­tions are the same the dura­tion of life is for a given organism a func­tion of temperature. The writer[307] investigated the dura­tion of life of fertilized and unfertilized eggs of Strongylo­centrotus purpuratus for the upper temperature limits.

TABLE XX