Not only is no remedy for old age known to science, but little or nothing is known with regard to that period in the lives of men and animals. It was no easy task to compress an account of the present position of medicine within a few pages, the subject matter being overwhelming in quantity. With regard to old age it is quite the contrary, our knowledge being so limited that the subject may be dealt with in a few lines. With the advance of years, man and the higher animals undergo important modifications. They become weaker, the body shrinks, the hair whitens, and the teeth decay; in fact, all the phenomena connected with senile decay manifest themselves. At this period of life which overtakes various species of animals at different ages, the body becomes an easy prey to pernicious influences and diseases. The direct cause of death cannot always be determined, and is attributed to the general breaking up of the system which we call natural death. The first question which presents itself to the scientific mind is whether this degeneration or senile decay is proper only to man and the higher animals, or is common to all forms of life. We have all seen very old trees, the appearance of which proclaims their age. The trunk is decayed, the bark gnarled, the branches shrivelled, and the leaves scanty. Some kinds of trees live for hundreds—possibly thousands—of years, while others age with comparative rapidity. Senile decay is not unknown in the vegetable kingdom, and its presence is suspected even among creatures of very simple organisation belonging to the group of infusoria. These creatures may be reared with ease in vessels containing macerations of chopped hay or leaves. They multiply by means of division (Fig. 12), an operation which takes place at very short intervals, some of them dividing nearly every hour. Owing to this rapid multiplication the vessels soon become full of a mass of infusoria. M. Maupas,[287] a very distinguished zoologist, observed that the infusoria became smaller and smaller after a number of generations, exhausting themselves, as it were, and perishing unless two individuals succeed in uniting. This process of “conjugation” (Fig. 13), involves an exchange of portions of the bodies of the two creatures and brings about a complete rejuvenescence of the two individuals. After conjugation, a process essentially similar to the details of sexual fertilisation, the infusoria resume the normal appearance and again become capable of reproduction by simple division for many generations. [Illustration: FIG. 12. Paramecium about to divide in two. ]