The concept of ritualization, as used in the study of the signal movements of lower vertebrates, refers primarily to the evolutionary changes which such movements have undergone in adaptation to their function in communication. In this context, the term is thus used in reference only to movements which have such a function, and only when there is evidence that the resultant signal has undergone changes which make it more effective in that role. Many movements which influence the behaviour of others (e.g. penile erection, eating and drinking in rhesus monkeys, according to Altman 1962) have apparently not been ritualized, though homologous movements in other species may have been (e.g. penile erection in squirrel monkeys (Ploog & Maclean 1963)). The changes involved have almost invariably been evolutionary ones, and thus reference to ritualization implies evidence that the properties of the signal have changed on an evolutionary time scale. This usually comes from the comparative study of contemporary closely related species. Just as the comparison, between related species, of morphological structures may suggest not only homologies but also views as to the evolutionary origins of the homologous structures, so also does comparison of patterns of behaviour. In addition, just as comparison within a species of related structures, such as the segmental limbs of a crustacean, or of different developmental stages of the same structure, can provide evidence of the course of evolution, so also can comparison of related movement patterns (e.g. Lorenz 1935, 1941; Tinbergen 1952, 1959, 1962).