Ah. Now there you’re asking. The complexities of cause and effect defy analysis. Not only is the continuum like a human body, it is also very like a piece of badly put up wallpaper. Push down a bubble somewhere, another one pops up somewhere else. There are no more dodos because of my interference. In the end I imposed the rule on myself because I simply couldn’t bear it any more. The only thing that really gets hurt when you try and change time is yourself.” He smiled bleakly and looked away.