Summary. 1. In the first part of the paper, the model of non-parametric entrainment of circadian pacemakers is tested for the case of nocturnal rodents. The model makes use of the available data on freerunning period (z) in constant darkness (Pittendrigh and Daan, 1976a) and on phase response curves (PRC) for short light pulses (Daan and Pittendrigh, 1976 a). It is tested in experiments using 1 or 2 light pulses per cycle. Mesocricetus auratus andPeromyscus leucopus entrain to zeitgebers involving 1 pulse (15' or 60') per cycle. The phase angle differences between rhythm and light cycle depends on the periods (z and T) as predicted by the model. Entrainment of P. leucopus is unstable due to the after effects on created by the light pulse.3. The limiting values of zeitgeber period to which the animals entrain are much closer to 24 h than in Drosophila pseudoobscura, as the model predicts.However, frequent failures to entrain to T= 23 and T= 25 h are only explained if we take considerable interindividual variation in both z and PRC into account. 4. With 2 pulses per cycle, the model predicts that entrainment will be more stable when activity is in the longer interval between the pulses than when it is in the shorter interval. This is true in the experimental data, where the phase relationships match predictions for skeleton photoperiods up to ca. 14 h. Increasing asymmetry forces animals into a "phase jump", so that activity shifts from the shorter to the longer interval. These 0-jumps are accurately predicted in the hamster, but they occur at much longer photoperiods than predicted in P. leucopus.5. Thus, the unqualified model, using a rigidly fixed species ~ and PRC, is surely inadequate to explain entrainment. The extent to which variations in and PRC-shape, both "spontaneous" and induced by the entrainment process, can be known or inferred restricts the validity of the predictions. Yet * Present address: Zoologisch Laboratorium, Rijksuniversiteit Groningen, Haren (Gr.), The Netherlands 292 c.s. Pittendrigh and S. Daan we conclude, from a good deal of agreement between experiment and prediction (i), from the close correspondence between complete and skeleton photoperiods (ii), and on behavioural grounds (iii), that non-parametric entrainment by short light signals has a major share in the entrainment of nocturnal rodent rhythms in nature.6. With these restrictions in mind, we analyse in the second part of the paper how the empirical regularities concerning ~ and PRC, and reported earlier (Pittendrigh and Daan, 1976; Daan and Pittendrigh, 1976a, b), contribute to the stabilization of the phase angle difference (0) between the pacemaker and the external world. Use is made of computer simulations of artificial pacemakers with variable ~ and PRC.7. 0 is most sensitive to instabilities in ~ when ~ is close to 24 h. Thus the very circadian nature of these pacemakers helps to conserve 0. Selection pressure for homeostasis of -c has been large in a species (M. auratus) where = 24 h. The effect of 0-instability is furth...

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DARWINIAN SPECTACLES A Broken Window Leads to Darwin This essay was prompted by the Editor's invitation to illustrate the excitement and adventure inherent in scientific work while reflecting on my own preoccupation, as an evolutionary biologist, with biological clocks. In considering the challenge, the first adventure that came to mind occurred one evening 30 years ago when a drunken graduate student, frustrated by an unwanted experimental result, attempted to throw me out of a second story window in Princeton. He didn't succeed. That seemed a good Indiana Jones start, but nothing as exciting occurred in later years, and all the adventures I can recount are less spectacular-the excitement of experiment and the hazardous fate of observation and ideas in the pursuit of understanding. While circadian periodicities have been the immediate object of my research for 40 years, that "view of life," which Darwin so eloquently summarized in the last paragraph of "The Origin of Species," has so dominated and guided my approach that it gets substantial attention in these reflections and hopefully ties together much of what I have to say about biological clocks. This Darwinian approach to behavioral and physiological interests traces to the accidental way I became a biologist. At 15 I kicked a soccer ball through a very large window in the Tow n Hall where I lived in the north of England. The only foreseeable source of the 13 shillings needed to replace it was a prize offered to local Boy Scouts for the best wild flower collection. In winning that prize, I got more money than was needed for the window and was seduced into a lasting love affair with plant 17

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