possibility of the death having to be interpreted as violent, it would be necessary to know that some very rapid infectious disease does not attack these insects as soon as they emerge from the water. This possibility, although remote, must be examined. Instances are known of large numbers of insects dying very rapidly as the result of attack by a species of mould which causes an epidemic. Every one has seen, especially in autumn, dead flies anchored to the window pane by a little tuft of white fluff. As so many individuals die about the same time, we might be disposed to assign the fact to natural death. The actual cause, however, is an infectious and fatal disease caused by a parasitic mould.

The occurrence of some terrible epidemic may be excluded from consideration in the case of ephemerids. I have made investigations which show that such an epidemic does not occur. The bodies of the dying ephemerids contain no microbe which could be the cause of death. Their death must be regarded as natural, as the result of their organisation, as essentially a part of the nature of the insects. Among the cells of their body there are many active phagocytes. Is it possible to attribute death to ravages that these cells may cause among the higher cells and tissues? Microscopic examination, so far from supporting such a possibility, shows that the organs are quite normal in their intimate structure. The brain and central nervous system, the muscles and other organs, show no signs of that invasion by phagocytes found in cases of senile degeneration. In this example of natural death there is certainly no possibility of phagocytic intervention

Some biologists have suggested that the rapid death of ephemerids and of some other insects is due to debility

caused by the great effort of depositing the male and female sexual cells. On this supposition, the case would be analogous to the shock which is sometimes the consequence of a surgical operation. This hypothesis, however, may be excluded, for among the dead ephemerids there are many males that have not united with females. Among ephemerids males are much more numerous than females; many males have no opportunity of undergoing the sexual shock and of emptying the reproductive organs, and these, none the less, die as rapidly as the others.

As yet we do not know if all the tissues of the ephemerids die simultaneously in natural death. Most probably the cells of the nervous centres perish first, and so bring death on the others. The investigation ought to be made.

Death comes to the ephemerids in the midst of love, at the moment when their sexual instincts are satisfied. It would be very interesting to know the sensations of these creatures as they feel death come on them in the act of reproduction. Naturally it would be impossible to obtain a full answer to the question, but many interesting facts regarding it may be ascertained. All the ephemerids, not only those the life of which is so brief, but those that live for several days (Chloe, for instance), are extremely easy to capture. It is unnecessary to take them unawares or to use a net as in the case of flies, wasps, and many other insects. Ephemerids may be taken with the fingers in the simplest way, because they offer no resistance and show no desire to escape, although they have six legs and two or four wings. This is not an isolated case, for some other insects (as, for example, winged ants and aphides) allow themselves to be captured with the same carelessness.

Although the adult ephemerids are careless, the wingless larvæ are timid. When a tube is brought near them, among

the water plants, with the object of capturing them, they rapidly move off. It often requires much patience and quickness to capture these larvæ (Fig. 20). The instinct of preservation of life displays itself by rapid flight.

FIG. 20.-Larva of

an ephemerid

It is remarkable that the adult insect has lost the instinct of self-preservation. If it be touched it may move a short distance off, but it does not take to flight although its wings are very large, and its body, which of itself weighs little, is still lighter because the digestive tube is filled with air and not with food. As a rule, an ephemerid that has been touched does not even move off, but allows itself to be captured without any resist(Chloë rufulum). ance. It would not be accurate to say that the larva's instinct of self-preservation has been replaced in the adult by an instinct for death; but it must be admitted that the instinct of preservation has been totally lost. The lack of resistance cannot be explained by any defect in the organs of sense. Not only are the eyes of the larval stage fully preserved in the adult, but the adult males have enormous eyes to enable them to recognise the female in the turbulent flight which takes place at the close of the day. Ephemerids of all ages possess well developed tactile organs, and it is thus in spite of a highly organised sensory system that the adults offer no resistance to enemies.

It is no mere accident that the most striking examples of natural death occur among insects, for these creatures display an unusual stability in their cellular structure with a corresponding lack of the power of regeneration, in these particulars resembling man and the higher animals. The cells of the nervous system are very complex, and are well adapted for the highest function, that is to say, the psychical function. These highly endowed cells, however, are devoid

of the power of reproduction. Many experiments have been made in relation to this, and it has been proved clearly that in cold-blooded vertebrates the brain and spinal cord with the nerve cells contained in them are capable of regeneration, whilst among mammals only extremely rare cases are known in which there has been any regeneration of the nervous elements. It is to be expected, then, that cases of natural death occur in the higher animals and especially in man. However, no case is known so plain as that presented by the ephemerids. I have already stated that of deaths apparently due to senile debility in man, a large proportion are certainly due to various infectious diseases that affect the old, such as pneumonia and nephritis. Close examination of the tissues confirms this conclusion, for the destruction of the higher elements by phagocytes produces what is really violent death and not a natural death like that of the ephemerids.

Natural death in man is probably a possibility rather than an actual occurrence. Old age is not a true physiological process but exhibits many morbid characters. That being the case, it is not surprising that it seldom ends in natural death. It is probable, however, that natural death occasionally occurs in very old men.

Attempts have been made to estimate the natural limits of human life. Flourens * based a calculation on the duration of the period of growth. If the latter be taken as one fifth the natural life, then human life ought to last a century. As centenarians are rare, the vast majority of deaths, which happen before that age has been reached, must be regarded as violent or accidental. The rule of Flourens, however, is arbitrary, and there is no evidence to show that it is exact. Probably in the human race, as * "De la longévité humaine," Second Edition. Paris, 1885.