The view that interspecific competition with balanoid barnacles has caused the evolutionary decline of chthamaloids (Stanley and Newman 1980), is challenged on 3 grounds. First, as a phyletic stock, chthamaloid size, as indicated by the mean adult basal diameter of extant taxa, has been decreasing, probably since the end of the Cretaceous. Small organisms will lose in interspecific competition to larger ones. However small body size reduces the susceptibility to predation and various forms of disturbance, and therefore allows chthamaloids to inhabit intertidal areas denied to balanoids. Second, although many species of Chthamalus occupy a high intertidal “refuge,” there is little evidence that competition is the primary ecological force maintaining them there. Predation is the most significant influence in tropical areas. In many temperate zone communities characterized by intense invertebrate predation or physical disturbance, chthamaloids achieve their maximum density lower in the intertidal than do balanoids or coronuloids. Last, the fossil evidence supporting a decline in chthamaloids is missing. The difficulty of seeking a single mechanism to explain global, multispecific phenomena is discussed.