 As critics focused their attacks on the claim that such micro-differences between individuals could be accumulated over time without natural limits, Darwin began a series of modifications and revisions of the theory through a back and forth dialogue with his critics that can be followed by revisions to the text of the Origin. In the fourth edition of 1866, for example, Darwin inserted the claim that the continuous gradualism illustrated by his branching diagram was misleading, and that transformative change does not necessarily go on continuously. “It is far more probable that each form remains for long periods unaltered, and then again undergoes modification” (Origin 1866: 132 [Peckham 1959 [2006: 213]). This change-stasis-change model presumably allowed variation to stabilize for a period of time around a mean value from which additional change could then resume. Such a model would, however, presumably require even more time for its working than the multi-millions of years assumed in the original presentation of the theory.