306 SENESCENCE AND REJUVENESCENCE 



The totals for all except man show a rather close agreement in 

 each table, and while Rubner admits that the data on which these 

 figures are based are not in all cases satisfactory, he concludes 

 from the figures that the amounts of energy required, first, for the 

 doubling of weight in growth and, secondly, for the maintenance of 

 each kilogram of body-weight during adult life, are the same in 

 all species in the tables except man. Man uses a much greater 



TABLE VI 



Horse 4,512 Pig 3,754 



Cow 4,243 Dog 4,304 



Sheep 3,936 Cat 4,554 



Man 28,864 Rabbit 5,o66 



TABLE VII 



Man 725,770 Dog 163,900 



Horse 163,900 Cat 223,800 



Cow 141,090 Guinea-pig 265,500 



amount of energy in both cases, i.e., a much smaller percentage of 

 the energy of food is concerned in growth and maintenance of body- 

 weight in man than in the other mammals. These results of his cal- 

 culations lead Rubner to suggest that the living substance can 

 undergo only a certain number of atomic rearrangements before 

 becoming exhausted and breaking down. According to this view, 

 life is terminated by the completion of a complex chemical 

 reaction. 



While I do not regard myself as qualified to criticize the methods 

 of calculation, or the data on which these are based, though they 

 may be open to criticism at certain points, Rubner's general conclu- 

 sion demands consideration on general biological grounds. Assum- 

 ing the validity of the data and methods of treatment, considerable 

 uniformity in energy requirement in the mammals is to be expected, 

 for they are closely related to each other, the rate of metabolism is 

 not widely different in different species, and progressive develop- 

 ment is not to any great extent interrupted by regression and 

 rejuvenescence. The facts scarcely warrant us in going beyond the 

 conclusion that development is a similar process in all these mam- 

 malian species. If life is terminated, not by the completion of a 



