182 PHIL RAU AND NELLIE RAU 
by incubation in the widest sense, and so on. Further it has the func- 
tion of strengthening the power of endurance of the species by ‘he inher- 
itance of acquired characters.”’ “Reproduction is unending growth. 
Not reproduction is the essential cause of death. . . . . the 
soma is not really an end in itself, but rather its principal function is 
to ensure the maintanence of organic life by tavoring reproduction. 
Minot’ seems to conclude that ‘‘the duration of life depends 
upon the rate of cytomorphosis. If that cytomorphosis is rapid 
the fatal condition is reached soon, if it is slow the fatal condition 
is postponed.” 
Flourens® thinks that the length of life of an animal is equival- 
ent to five times its period of growth, while Nageli® thinks that 
“natural death does not exist in nature, for trees more than a 
thousand years old perish not by natural death, that is to say 
natural decay of their vitality, but by some catastrophe.”’ 
Metchnikoff® says: “‘It is impossible to regard natural death, 
if indeed it exist, as the product of natural selection for the bene- 
fit of the species. In the press of the world natural death could 
hardly come into operation because maladies or the voracity of 
animals so frequently cause natural death.” 
While Morgan" says that “in some cases the length of life 
and the coming to maturity of the germ-cell may be, in some way, 
physiologieally connected seems not improbable, but that this 
relation has been regulated by the competition of species with 
each other can scarcely be seriously maintained,” he will ‘not 
pretend to say whether the mutation theory can or cannot be 
made to appear to give the semblance of an explanation of the 
length of life in each species.”’ 
Other theories of less importance are reviewed by Metchnikoff 
in ‘The Prolongation of Life” and “The Nature of Man.” 
7 The Problem of Age, Growth and Death, p. 228, 1908. 
§ Quoted by Metchnikoff, Prolongation of Life, p. 40, 1908. 
® Quoted by Metchnikoff, The Nature of Man, p. 265. 
1OTOC, Cite, Da Love 
11 Evolution and Adaptation, p. 371, 1908. 
