SOME CURRENT THEORIES 435 



aroused great popular interest, largely because of his scheme for 

 prolonging life by preventing the intestinal intoxications, they 

 have received little support among scientists. The evidence for 

 the universal or almost universal occurrence of chronic intoxica- 

 tion in man, and of arteriosclerosis as a result of it, is far from 

 convincing, and the hypothesis of the action of the phagocytes 

 under such conditions has proved even less acceptable. At best 

 Metchnikofl's hypothesis is not widely applicable, for many animals 

 which possess no large intestine grow old and die. But, as is evi- 

 dent from his statement that natural death occurs very rarely, 

 Metchnikoff is really concerned with certain pathological aspects 

 of advanced hfe in man and not at all with the problem of physio- 

 logical senescence. While his ideas may or may not be of practical 

 value, they have no general theoretical significance. 



According to JickeU ('02) metabolism is an incomplete process 

 and injurious substances accumulate in the cell because of this 

 incompleteness of metabolism. The secretions of cuticular sub- 

 stances, cysts, cellulose membranes, etc., the formation of hair, 

 feathers, and various other products of cellular activity represent 

 these injurious substances of which the cell attempts to rid itself 

 by excretion, or the body by giving rise to parts which are sooner 

 or later cast off. In other cases the cells react to the accumulation 

 of injurious substances by increased rate of division, which results 

 in increase of surface and so in greater possibility of excretion. 

 The accumulation of the injurious substances brings about senes- 

 cence and death, and excretion by the cell, or the casting off of 

 parts by the organism, is a process of rejuvenescence. This 

 hypothesis is based entirely on a teleological conception of the cell 

 and the organism and cannot be regarded as in any real sense 

 physiological, although in his fundamental idea that senescence 

 results from accumulation of substances in the cell and rejuvenes- 

 cence from their ehmination Jickeli approaches my own position. 

 But for him the substances concerned are not the protoplasmic 

 substratum of the cell, but something "injurious" which remains 

 in the cell only because metaboKsm is an incomplete process, and 

 the cell and the organism are all the time struggling, apparently with 

 superhuman intelligence, to rid themselves of their burdens. 



