 The notion of a passive trend is closely related to other themes of macroevolutionary theory. Gould, in particular, was enthusiastic about the possibility that Cope’s rule might be a passive trend. If that were the case, then it would mean that evolutionary size increase is not attributable to natural selection. What’s more, if we explain size increase by supposing it to be a passive trend, then the explanation need not reference any micro-level causes. The fixed boundary in the state space could be maintained by developmental constraints—another favorite theme of Gould’s. And finally, a trend resulting from a random walk away from a fixed boundary would not seem to be progressive at all. To the extent that larger-scale evolutionary trends are passive, chance looks like a more significant factor in evolutionary history, and selection looks like a less significant one.