In the protozoa and some other lowly forms of animal life multiplication by binary fission occurs indefinitely, so that one organism is succeeded by two, and as there is no vestige of a corpse, as Weissmann expressed it, the organism is immortal. Maupas’s[9] investigations indicated that without occasional conjugation of two individual protozoa and the resulting rejuvenation the organisms undergo senile degeneration and die; but Woodruff[10] has shown by observations extending over 13½ years that _Paramoecium_, and so presumably infusoria in general, can multiply indefinitely in favourable conditions without conjugation, though periodically an internal reorganization (endomixis[11]) of the individual takes place. It seems not improbable that endomixis provides the rejuvenation otherwise resulting from conjugation. C. M. Child[12] brings forward reasons for modification of the explanation that death is escaped simply by division; he shows that in certain circumstances senescence occurs in the protozoa as it does in the higher animals, but that death is avoided by the rejuvenescence in each process of reproduction, reconstitution occurring and new organs being formed in the place of the old. In other words, all organisms from the protozoa to man undergo senescence and die unless rejuvenescence intervenes; this compensation holds good in the lower forms and death may never occur; but in the higher forms rejuvenescence is so much restricted by the evolutionary increase in the physiological stability of the protoplasmic substratum and the resulting higher degree of individuation that senescence is much more continuous and death is inevitable. It is the penalty for high individuation, and is the result of conditions and processes that have produced the complicated mechanism of the higher animals and man. While holding out the faint hope that the advance of knowledge and of experimental technique may make it possible at some future time to bring about a greater rejuvenescence and retardation of senescence in man and the higher animals, Child points out that the present condition of their protoplasmic substratum is the result of millions of years of evolutionary equilibration, and that this task must therefore be one of extreme difficulty.