Johannes Muller had, as early as 1840, rejected the prevailing hypothesis which held the death of animals to be due to “the influences of the organic environment, which gradually wear away the life of the individual.” Muller argued that if this were so “the organic energy of an individual would steadily decrease from the beginning.” Everybody knows, however, that in spite of the wear and tear caused by the “environment,” be it organic or inorganic, the volume of life increases, until a certain stage is reached in all animals. But Muller had failed to fill the gap his criticism had created. This problem Weismann solved by analysing the methods of reproduction among animals. These generally speaking are two; sexual, and non-sexual or, as it is sometimes termed, a-sexual. This latter form is the mode that prevails at the bottom of the organic scale--among the protozoa, animals consisting of a single cell. This method has a variety of forms which are classified by Haeckel as (1) self-division; (2) formation of buds; (3) the formation of germ-cells or spores. We shall here deal only with the first, self-division, or fission, which is the most universal of all methods of propagation, being the progress by which the individual cells which compose all the higher animals multiply themselves. This is the method vital to Weismann’s theory and the other two are no more than distinct modifications of fission. When a Moneron or an Amoeba reaches a certain size, it begins to pinch in the middle like a tightly-laced corset. This increases until the creature divides into two equal halves. Each of these halves becomes a complete individual which continues to thrive until the next division takes place. What Weismann observed as the most significant thing about this was that in this process and among these unicellular (single celled) organisms there is no such thing as natural death. Accidental death is wholesale in its proportions, but no Moneron ever dies of old age. Astounding as it may seem to the layman, the race-old, world-wide idea that death is “essential to the very nature of life itself” is here totally and indisputably overthrown. “I pointed out,” says Weismann, in the second lecture and referring to the first “that we could not speak of natural death among unicellular animals, for their growth has no termination which is comparable with death. The origin of new individuals is not connected with the death of the old; but increase by division takes place in such a way that the two parts into which an organism separates are exactly equivalent to one another, and neither of them is older or younger than the other. In this way countless numbers of individuals arise, each of which is as old as the species itself, while each possesses the capability of living on indefinitely, by means of divisions.”