The main topic of the symposium" Biological Clocks" says nothing about the types of clocks we will be concerned with. It is an open question whether the clocks run continuously or stop after one revolution and have to be started anew. Also the term" clock" does not imply that one revolution is finished in 24 hours. Watches are instruments to measure time. Organisms have to measure time-spans of quite different lengths and for different purposes. Each measurement of speed, for instance, needs timing and mostly within the limits of milliseconds. One can expect, therefore, that organisms possess several clocks with perhaps extremely different periods [I]. There is no need for these clocks to run continuously; for some purposes it would be sufficient if the clock were started always at the beginning of timing (principle of a sandglass, stop watch)." Biological clocks" therefore is a more general concept than what will be mainly discussed at the symposium. On the other hand, the term" circadian," as introduced by Halberg [2] and used in 45 percent of all titles concerning the special clockproblem, comprises two different things: a) the average period of the clock is about 24 hours; b) the clock runs continuously.

At present the circadian clock can be studied only by measuring the periodic course of one or more functions in an organism. Still, nobody can say how these observable functions are related to the clock, how well respectively" the clock" is represented by one function. To answer this question, it may be in the future still more necessary than now to measure several functions simultaneously in one organism, and to observe how they are synchronized or become desynchronized during certain experimental conditions. Nevertheless, many circadian problems can be solved if only one function is followed. It may be that to answer some special questions one or the other function is more useful. But with respect to the general mechanism of the clock, all functions are nearly equivalent. Scientists who work on eosinophiles or on locomotor activity are surely not nearer to, but may be also not so much further from, the clock than those who train animals to feed at definite times of day or to run always toward the same quarter. It will be more important--at least with regard to the problem of synchronization--if we can demonstrate