The efforts to verify Maupas' result and test Weismann's conjecture occupied the first thirty years of the century, before petering out towards the end of the 1930s. I have listed the most important of these investigations in Table 1. They represent the product of enormous labour. Must of this was ill-directed; some of the most extensive investigations, such as those Galadjieff and Metalnikov, are only sketchily described; and of course the raw data for most of the experiments is now irretrievably lost. What remains, however, is still an impressive number of long-term records in which the average fission rates of isolate lines have been reported. I have identified 75 cases in which a fresh and uniform quantitative analysis is feasible, and these are described more fully in Table 2. The million or so daily isolations analysed in this table represent the great bulk of what we know about clonal longevity from direct observation.

Unfortunately, it is difficult to analyse the fission-rate date straightforwardly. Indeed, the proposition that a clone of protozoans can be propagated indefinitely, given the right conditions, might almost serve as the model of a badly-formulated hypothesis, since it could not be affirmed in a finite experiment, and if falsified might merely demonstrate the failure of the experimenter to maintain constant favourable conditions. The survival or extinction of a line is therefore of little direct interest; what is crucial is the trend in fission rate, since a universal decline in fission rate through the history of isolate cultures would provide a conclusive demonstration of senescence. However, even the hypothesis that fission rates decline through time is not easily tested, since the pattern of decline is not specified.