Directing is exhausting, but not for the actual directing part, when you say "Action!" and give creative notes. As a director, the exhausting part is that you are a professional answer-machine.
We have now the remarkable spectacle that just when many scientific men are agreed that there is no part of the Darwinian system that is of any great influence, and that, as a whole, the theory is not only unproved, but impossible, the ignorant, half-educated masses have acquired the idea that it is to be accepted as a fundamental fact.