 There is always an element of contingency in the choice of model organisms; no biological problem determines the choice of the best organism to study it. Even though considerations of convenience, easy breeding and maintenance, as well as suitability for the study of specific phenomena play a role (Clarke and Fujimura 1992; Burian 1992, 1993; Lederman and Burian 1993; Clause 1993; Creager 2002; Geison and Creager 1999; Lederman and Tolin 1993), there are always many different organisms that would satisfy these criteria. Thus, which model organism is chosen sometimes is a matter of chance.