The consequences to fitness of several types of small age-specific effects on mortality are
formulated mathematically. An effect of given form always has a larger consequence, or at
least one as large, when it occurs earlier. By reference to a model in which mortality is
constant it is shown that this implication cannot be avoided by any conceivable organism. A
basis for the theory that senescence is an inevitable outcome of evolution is thus
established. The simple theory cannot explain specially high infant mortalities. Fisher's …