How do organisms respond to orbitally-forced climate change? Chapter 4 demonstrated that climates have been changing globally with frequencies on Milankovitch time-scales (10–100 kyr) throughout Earth history. However, it is only during the most recent part of the geological record that sediments have been sampled finely enough, and there is a time-scale precise enough, to make possible useful statements about the response of organisms to climatic change. The climatic changes of the Quaternary were, of course, exaggerated locally by the development of continental ice-sheets with consequent impacts on neighbouring regions. Nevertheless, many parts of the world experienced smaller degrees of change that have been similar to the magnitude of changes experienced during parts of Earth history when continental glaciation was minimal or absent. The modelling experiment of Glancy et al. (1986) suggests that Cretaceous geography (apparently with no glaciation (Hambrey & Harland 1981)), at least, could produce temperature and precipitation changes as severe as Quaternary changes in response to varying orbital parameters. There is, in any case, little choice; it so happens that the portion of the geological record most accessible to us (the Quaternary) was a period of fluctuating glaciation, and the response of organisms to climatic change cannot be studied with anything like the same detail in other parts of the geological record. Even within the Quaternary, the evidence is, unfortunately, patchy, with particularly awkward gaps in tropical regions, which may be the best modern parallel for conditions in much of the pre-Quaternary record.