The rust fungi are fully parasitic and arose from parasitic ancestors. Heteroecism was evidently universal during their major evolutionary outburst. Thus, the ecology of hosts and parasites was complexly interrelated and profundly influenced the evolution of the rusts. Divergence with their hosts probably has been the chief means of speciation in the rusts. Part of the process, especially in the more advanced groups, has been the initiation of autoecious species on the aecial hosts of parental heteroecious ones. This change may have triggered an evolutionary outburst through partial release from the ecological limits of the heteroecious ancestor. Jumps to hosts of varying taxonomic separation from the ancestral host must have given rise to many species and to some lineages; but few are easily documented. A single example suggests that a hybrid rust may be adapted to a hybrid between its parental hosts; and, if the hybrid host speciates, the rust may do so also.