Preston Cloud argued in 1948 and 1968 that the process was "explosive",[43] and in the early 1970s Niles Eldredge and Stephen Jay Gould developed their theory of punctuated equilibrium, which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.[44] On the other hand, around the same time Wyatt Durham and Martin Glaessner both argued that the animal kingdom had a long Proterozoic history that was hidden by the lack of fossils.[45][46] Whittington (1975) concluded that Opabinia, and other taxa such as Marrella and Yohoia, cannot be accommodated in modern groups.