Finally, mechanists have found it necessary to distinguish between stable mechanisms, which rely fundamentally upon the more or less fixed arrangement of parts and activities, and ephemeral mechanisms, which involve a process evolving through time without fixed spatial and temporal arrangement (Glennan 2009). The time-keeping mechanism in a clock, for example, is a relatively stable assemblage of components in relatively fixed locations that work the same way, with the same organizational features, each time it works. Ephemeral mechanisms, in contrast, involve a much looser kind of organization: items still interact in space and time, but they do not do so in virtue of robust, stable structures. Many chemical mechanisms in a cell are like that (Richardson and Stephan 2007). Ephemeral mechanisms are surely a primary focus of historical sciences, such as archaeology, history, and evolutionary biology (Glennan 2009).