310 



SCIENCE 



[N. S. Vol. XXXVI. No. 923 



are employed to supplement the action of 

 those which our own cells produce. It is 

 not too much to assert that the knowledge 

 of the parasitic origin of so many diseases 

 and of the chemical agents which on the 

 one hand cause, and on the other combat, 

 their symptoms, has transformed medicine 

 from a mere art practised empirically, into 

 a real science based upon experiment. The 

 transformation has opened out an illimit- 

 able vista sf possibilities in the direction 

 not only of cure, but, more important still, 

 of prevention. It has taken place within 

 the memory of most of us who are here 

 present. And only last February the world 

 was mourning the death of one of the 

 greatest of its benefactors — a former presi- 

 dent of this association — who, by applying 

 this knowledge to the practise of surgery, 

 was instrumental, even in his own lifetime, 

 in saving more lives than were destroyed 

 in all the bloody wars of the nineteenth 

 century ! 



The question has been debated whether, 

 if all accidental modes of destruction of 

 the life of the cell could be eliminated, 

 there would remain a possibility of indi- 

 vidual cell-life, and even of aggregate cell- 

 life, continuing indefinitely; in other 

 words, Are the phenomena of senescence 

 and death a natural and necessary se- 

 quence to the existence of life? To most 

 of my audience it will appear that the sub- 

 ject is not open to debate. But some 

 physiologists (e. g., MetchnikofE) hold that 

 the condition of senescence is itself ab- 

 normal; that old age is a form of disease 

 or is due to disease, and, theoretically at 

 least, is capable of being eliminated. We 

 have already seen that individual cell-life, 

 such as that of the white blood-corpuscles 

 and of the cells of many tissues, can under 

 suitable conditions be prolonged for days 

 or weeks or months after general death. 

 Unicellular organisms kept under suitable 



conditions of nutrition have been observed 

 to cai'ry on their functions normally for 

 prolonged periods and to show no degen- 

 eration such as would accompany senes- 

 cence. They give rise by division to others 

 of the same kind, which also, under favor- 

 able conditions, continue to live, to all ap- 

 pearance indefinitely. But these in- 

 stances, although they indicate that in the 

 simplest forms of organization existence 

 may be greatly extended without signs of 

 decay, do not furnish conclusive evidence 

 of indefinite prolongation of life. Most of 

 the cells which constitute the body, after a 

 period of growth and activity, sometimes 

 more, sometimes less prolonged, eventually 

 undergo atrophy and cease to perform 

 satisfactorily the functions which are 

 allotted to them. And when we consider 

 the body, as a whole, we find that in every 

 case the life of the aggregate consists of a 

 definite cycle of changes which, after pass- 

 ing through the stages of growth and ma- 

 turity, always leads to senescence, and fin- 

 ally terminates in death. The only excep- 

 tion is in the reproductive cells, in which 

 the processes of maturation and fertiliza- 

 tion result in rejuvenescence, so that in- 

 stead of the usual downward change 

 towards senescence, the fertilized ovum ob- 

 tains a new lease of life, which is carried 

 on into the new-formed organism. The 

 latter again itself ultimately forms repro- 

 ductive cells, and thus the life of the 

 species is continued. It is only in the 

 sense of its propagation in this way from 

 one generation to another that we can 

 speak of the indefinite continuance of life: 

 we can only be immortal through our de- 

 scendants ! 



The individuals of every species of ani- 

 mal appear to have an average duration of 

 existence. Some species are known the 

 individuals of which live only for a few 

 hours, whilst others survive for a hundred 



