_The Two Modes of Multiplication._—Infusoria are, in fact, capable of multiplication by simple division. It is true to say that in addition to this resource, the only one which interests us here, because it is the only one which confers immortality, they possess another. They present and exercise under certain circumstances a second mode of reproduction, caryogamic conjugation. It is a rather complicated process in its detail, but it is definitively summed up as the temporary pairing of two individuals, which are otherwise very much alike, and which cannot be distinguished as male and female. They become closely united on one of their faces; they reciprocally exchange a semi-nucleus which passes into the conjoint individual; and then they separate. But infusoria can be prevented from this conjunction by regularly isolating them immediately after their birth. Then they grow, and are constrained after a lapse of time to divide according to the first method. Maupas has shown that the infusoria could not accommodate themselves to this régime indefinitely; they could not go on dividing for ever. After a certain number of divisions they show signs of degeneration and of evident decay. The size diminishes, the nuclear organs become atrophied, all the activities fail, and the infusorian perishes. It succumbs to this kind of senile atrophy unless it is given an opportunity of conjugation with another infusorian in the same plight. In this act it then derives new strength, it grows larger, attains its proper size, and builds up its organs once more. Conjugation gives it life, youth, and immortality. _Alimentary Rejuvenescence._—Recent observations due to Mr. G. N. Calkins, an American biologist, and confirmed by other investigators, have shown that this method of rejuvenescence is not the only one, and is not even the most efficacious. Conjugation has no mysterious, specific virtue. The infusoria need not be married in order to be rejuvenated. It is sufficient to improve their food. In the case of the “tailed” paramecium we may substitute beef broth and phosphates for conjugation. Calkins observed 665 consecutive generations without blemish, without exhaustion, and without any sign of old age. Plenty of food and simple drugs have successfully resisted senility and the train of atrophic degenerations which it involves. _Causes of Senescence._—As for the causes of senescence which have been remedied with such success, they are not exactly known. Calkins thinks that senescence results from the progressive losses to the organism of some substance essential to life. Conjugation or intensive alimentation would act by building up again this necessary compound. G. Loisel believes on the contrary that it is a matter of the progressive accumulation of toxic products due to a kind of alimentary auto-intoxication. CHAPTER VI. LETHALITY OF THE METAZOA AND OF DIFFERENTIATED CELLS.