Lichtenberg wrote of a man who “was working on a system of natural history in which he had classified the animals according to the shape of their excrement.” He singled out three categories: cylindrical, spherical, and cake-shaped.
~ Aldo Buzzi, Journey to the Land of the Flies
Rather than deciding once and for all what separates one species from another, Darwin emptied the word ‘species’ of much sense at all. A dividing-line ratio of genetic variation could be settled upon, I suppose, but if the boundaries between species are going to be arbitrary, then we might as well keep the taxonomy simple. Why burden ourselves with eight million species when we can have three instead? I can think of at least two recommendations for Lichtenberg’s friend’s scheme: by placing them in the same category it accounts for the similarities (speed, long legs, ear shape) between jackrabbits and horses; it also explains why people look so much like their dogs.