I have already shown, in the “Nature of Man,” the difference which exists between senile degeneration in our own bodies and the phenomena of senescence amongst Infusoria which, as M. Maupas described, are followed by a process of rejuvenescence. According to the more recent results of several investigators, the difference is still greater than I had supposed. Enriquez[4] has been able to propagate Infusoria to the 700th generation without any sign of senility being displayed. Here we are far from the condition in the human race. R. Hertwig,[5] one of the best observers of the lower animals, has recently attempted to show that the very simple animalculæ of the genus _Actinosphærium_ are subject to true physiological degeneration. He has several times seen cultures of this Rhizopod degenerate, until all the individuals had died, notwithstanding the presence of abundant food. Prof. Hertwig attributed this to the “constitution of the _Actinosphærium_ having been weakened by too great vital activity at an earlier stage.” I should have thought that it was a much more natural explanation to suppose that the culture had undergone infection by one of the contagious diseases which so often destroy cultures of different kinds of lower animals and plants. As this idea had not occurred to the observer, he had not searched for parasitic microbes amongst the granulations which are always present in the body of an _Actinosphærium_. However this may be, I cannot accept the facts brought forward by this distinguished German as a valid proof of the existence of senile degeneration in these lowly creatures. The facts that I have brought together in this chapter justify the conclusion that human beings who reach extreme old age may preserve their mental qualities notwithstanding serious physical decay. Moreover, it is equally plain that the organism of some vertebrates is able to resist the influence of time much longer than is the case with man under present conditions. II THEORIES OF CAUSATION OF SENILITY Hypothesis of the causation of senility—Senility cannot be attributed to the cessation of the power of reproduction of the cells of the body—Growth of the hair and the nails in old age—Inner mechanism of the senescence of the tissues—Notwithstanding the criticisms of M. Marinesco, the neuronophags are true phagocytes—The whitening of hair and the destruction of nerve cells, as arguments against a theory of old age based on the failure of the reproductive powers of the cells Although it has not been proved that living matter must inevitably undergo senile decrepitude, it is none the less true that man and his nearest allies generally exhibit such degeneration. It is therefore extremely important to recognise the real causes of our senescence. There have been many hypotheses on the subject, but there are comparatively few definite facts known.