no 



SCIENCE. 



[N. S. Vol. XX. No. 499. 



digestion and absorption. The loss of a large 

 part of it could easily be endured by man. 

 On the other hand, it is a source, through 

 autointoxication, and diseases peculiar to it, 

 of much danger to man's health and life. 



There are disharmonies in our organs of 

 sense, and, particularly, in the organs and 

 functions of reproduction; and there are dis- 

 iiarmonies in our instincts as well. 



Senility should be a natural, physiological 

 j)hase of man's existence, but as it exists, it is 

 to a large extent a pathological condition. 

 The fundamental organic change in senility is 

 the atrophy of the higher and specific cells of 

 our tissues and their replacement by hyper- 

 trophied connective tissue. A very important 

 destructive role in the process is played by the 

 cells known as macrophags. The degenera- 

 tion of the higher cellular elements and facili- 

 tation of the work of the macrophags is aided 

 by all agencies that weaken the organism. 

 Among such stand foremost the poison of 

 syphilis, alcohol and the products of intestinal 

 fermentations. 



As to death, we are so accustomed to look 

 upon it as something natural and inevitable, 

 that it has long since come to be regarded as 

 inherent in organisms. This has been dis- 

 proved by biology. Low, particularly unicel- 

 lular, organisms, are not subject to the natural 

 death that comes inevitably to man and 

 higher animals. There are even somewhat 

 higher organisms, such as many polyps and 

 some worms, to which natural death does not 

 come; these animals divide indefinitely into 

 new individuals. Hence, death is not neces- 

 sarily inherent in living organisms. Even our 

 own bodies contain elements practically im- 

 mortal, the spermatozoa and ova. Natural 

 death in man is probably a possibility rather 

 than an actual occurrence. Old age, as it 

 exists, is not a true physiological process, but 

 exhibits many morbid characters. That be- 

 ing the case, it is not surprising that it seldom 

 ends in natural death. 



Would the appearance of natural death in 

 man be accompanied by the disappearance of 

 the instinct of self-preservation and the ap- 

 pearance of another instinct — that of death? 

 To this the author has no exact answer ; yet he 



adduces some testimony favoring that view. 

 It is well known that the instincts of hunger, 

 thirst, movement, etc., allied to those of desire 

 of life and fear of death, often change with age. 

 The instinct of death seems to lie, in some po- 

 tential form, deep in man's nature. If human 

 life followed its ideal course, fulfilling all its 

 physiological functions, then the instinct of 

 death would appear in its time, after a normal 

 life and an old age healthy and prolonged. 

 As it is, old men die in morbid old age and 

 in the fear of death, without having known 

 the instinct of death, and this constitutes the 

 greatest disharmony of human nature. The 

 goal of existence is the accomplishment of a 

 complete and physiological cycle, in which oc- 

 curs a normal old age, ending in the loss of 

 the instinct of life and the appearance of the 

 instinct of death. The normal end, coming 

 after the appearance of the instinct of death, 

 may truly be regarded as the ultimate goal of 

 human existence. But before attaining it a 

 normal life must be lived: a life filled all 

 through with the feeling that comes from 

 proper accomplishment of function. 



What is to be done? Before all things, 

 it is necessary to try to amend the evolu- 

 tion of human life; that is to say, to trans- 

 form its disharmonies into harmonies. This 

 can be undertaken only by science, and to 

 science the opportunity of accomplishing it 

 must be given. We must know the human 

 constitution thoroughly, must understand the 

 most intimate details of its mechanism. In 

 the problem of his own fate, man must not 

 be content with the gifts of nature; he must 

 direct them by his own efForts. Just as he 

 has been able to modify the nature of animals 

 and plants, man must attempt to modify his 

 own constitution, so as to readjust its dishar- 

 monies. 



As will be plain from this brief reference, 

 Metchnikofi's ' Nature of Man,' notwithstand- 

 ing its slightly misleading title, is a disserta- 

 tion on some of the most important phases of 

 man's natural history. It is a work remark- 

 able for its simple language and clear style. 

 It is not, as the author admits, a finished 

 work, but it is a well advanced one. 



There are a few points which are open to 



