The theory of evolution is outstandingly the most important theory in biology, and it is always a pleasure to be a member, facing in either direction, of the class that is fortunate enough to be studying it. No other idea in biology is so powerful scientifically, or so stimulating intellectually. Evolution can add an extra dimension of interest to the most appealing sides of natural history a we shall see, for example, how modern evolutionary biologists tend to argue that the existence of sex is the profoundest puzzle of all, and quite possibly a mistake that half the living creatures of this planet would be better off without. Evolution gives meaning to the drier facts of life too, and it is one of the delights of the subject to see how there are ideas as well as facts within the disorienting technicalities of the genetics laboratory, and how deep theories about the history of life can hinge on measurements of the width of a region called “prodissoconch II” in the larval shell of a snail, or the number of ribs in a trilobite’s tail. So great is the depth and range of evolutionary biology that every other classroom on campus must feel (as you can sort of tell) locked in with more superficial and ephemeral materials.