Biologists teach us that the phenomenon of old age is not evident in the case of the unicellular organisms which reproduce by fission. The cell can be killed, but it need neither grow old nor die. Death appears to be a prerogative of the higher organisms. But even among these Professor Weismann adopts and defends the view that "death is not a primary necessity, but that it has been secondarily acquired by adaptation." The cell is not inherently limited in its number of cell-generations. The low unicellular organism is potentially immortal; the higher multicellular form, with well-differentiated organs, contains the germ of death within its _soma_. Death seems to supervene by reason of its utility to the species: continued life of an individual after a certain stage being comparatively useless. From the point of view of the race the soma or main body is "a secondary appendage of the real bearer of life--the reproductive cells." The somatic cells probably lost their immortal qualities on this immortality becoming useless to the species. Their mortality may have been a mere consequence of their differentiation. "Natural death was not introduced from absolute intrinsic necessity, inherent in the nature of living matter," says Weismann, "but on grounds of utility; that is from necessities which sprang up, not from the general conditions of life, but from those special conditions which dominate the life of multicellular organisms." It is not the germ cell itself, but the bodily accretion or appendage, which is abandoned by life, and which accordingly dies and decays. [Footnote 32: See Explanatory Note A at end of chapter.] [Footnote 33: See Explanatory Note B.] CHAPTER II THE MEANING OF THE TERM DEATH "And Life, still wreathing flowers for Death to wear."--ROSSETTI Whatever Life may really be, it is to us an abstraction: for the word is a generalised term to signify that which is common to all animals and plants, and which is not directly operative in the inorganic world. To understand life we must study living things, to see what is common to them all. An organism is alive when it moulds matter to a characteristic form, and utilises energy for its own purposes--the purposes especially of growth and reproduction. A living organism, so far as it is alive, preserves its complicated structure from deterioration and decay.[34] Death is the cessation of that controlling influence over matter and energy, so that thereafter the uncontrolled activity of physical and chemical forces supervene. Death is not the absence of life merely, the term signifies its departure or separation, the severance of the abstract principle from the concrete residue. The term only truly applies to that which has been living.