The periodical debility, which precedes conjugation is, according to Maupas, an instance of senile degeneration among infusoria. He has recognised its existence in the case of many species of the higher infusoria (_Ciliata_), but while this phenomenon has been observed in the case of many other simple organisms, it cannot be set down as universal among microscopic beings. Among bacteria, a group that includes the greater number of pathogenic organisms, conjugation has been very rarely observed. Even the largest kinds, such as, for instance, the _anthrax_ bacillus, may be propagated for a long series of generations without the occurrence of conjugation. [Illustration: FIG. 13. Conjugation of two Paramecia (after Bütschli). ] Even in the case of the infusoria which by means of the process of conjugation can reproduce indefinitely, the preconjugal debility cannot be identified with the senile degeneration of human beings, the higher animals and trees. In all these debility is the antecedent, not of conjugation and rejuvenescence, but of the end of life. Another important difference is that in the case of infusoria the preconjugal debility does not occur in every individual, as is the case with the animals and plants which display real old age. In the infusoria an indefinite number of generations occur between the individuals that display debility and those that are ready for conjugation. If, in spite of these differences, we were to insist on the existence of an essential resemblance between senile degeneration in man and preconjugal debility in infusoria, it would be enough to reflect on the result of applying to the case of man what is an infallible remedy in the case of the infusoria. For conjugation brings about a real rejuvenescence of the infusoria and a similar event in the case of man would only increase the debility. Moreover, according to recent investigations of Calkins,[288] infusoria, weakened by degeneration, may become young again not only by conjugating with their kind, but by the addition of bouillon or extract of brain to the medium in which they live. Real old age is a phase of existence in which the natural forces abate never to be renewed. In animals, the life cycle of which is very definite, the signs of senile degeneration are not visible. Insects, in the adult condition, very often live only a short time, and die without displaying the slightest mark of age. In the case of lower vertebrates, old age is little known, and has few signs. On the other hand, mammals and birds display senile atrophy in a marked fashion.