514 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



out that the total calorific output of a variety of animals from birth 

 to old age is approximately the same, a striking exception, being, 

 however, afforded by man: 

 TOTAL CONSUMPTION OF CALORIES PER KILOGRAM OF BODY^-WEIGHT. 



Man 725,770 



Horse 169,900 



Cow 141,090 



Dog 163,900 



Cat 223,800 



Guinea-pig 265,500 



The instances are, however, not very numerous and if one marked 

 exception to the "rule" occurs among such a small number of cases, 

 other exceptions will doubtless be encountered. Indeed we may 

 with more probability attribute the exceptional position of man in 

 this small group to the much larger proportion of Nervous Tissues; 

 tissues, that is, of high metabolic rate, which his body contains in 

 comparison with the other animals enumerated. His duration of life 

 is also, and possibly for the same reason, exceptionally great. 



Quite a different type of hypothesis to the foregoing is that pro- 

 posed by Metchnikoff, who attributes senescence in part to the aber- 

 rant activities of Phacocytes and in part to the absorption of toxic 

 substances which are products of bacterial decomposition in the lower 

 intestine. While there can be little doubt that some of the tissue- 

 changes which are characteristic of old age, such as sclerosis, vascular 

 lesions and so forth may be hastened or even brought about by repeated 

 administrations of basic substances, such as Adrenaline or Tyramine 

 which may be derived from amino-acids by Decarboxylation, yet as 

 a general hypothesis of senescence this is too specific, too limited in its 

 scope and applicability, to account for the phenomenon in the multi- 

 tude of the forms of life which exhibit it. In fact, Metchnikoff did 

 not advance his hypothesis as an explanation of "natural" old age, 

 although he is commonly accredited with having done so, but as an 

 explanation of what he considered to be the "premature" senescence 

 of human beings, and, as such, it is a hypothesis which deserves very 

 serious consideration. The effects produced by basic Nitrogenous 

 Poisons related to the amino-acids are, however, confined to certain 

 tissues and especially the circulatory and renal systems, while the 

 effects of senescent atrophy modify in greater or less degree every 

 tissue in the body. Organisms in which the structural changes pro- 

 ducible by poisons of this character could not constitute an irreparable 

 injury nevertheless display senescence and its necessary outcome, 

 "natural death." 



The unicellular animals and certain unorganized types of living 

 tissue, such as cancer-tissue, are, as Wiessmann and Loeb have espe- 

 cially emphasized, actually or potentially immortal. 1 The Unicellular 



Those forms which undergo periodical conjugation may also exhibit senescence, 

 which, however, may very possibly be due to causes analogous to those described below 

 which lead to senescence in the metazoa. Cf., G. N. Calkins: Proc. Soc. Exper. Biol. 

 and Med.,'1919, 16, p. 57, 



