Sexual aberrations are of two sorts. The cause of the error is internal, or external. The flower of the arum muscivorum (fly-catching arum) by its cadaverous odour attracts flies in search of rotting flesh in which to lay their eggs. Schopenhauer has supported by this, or analogous, fact a theory just, but somewhat summary, of aberration from external cause. Aberration from internal cause is sometimes explained by the statement that the same arteries irrigate and the same nerves animate the region of the sacrum, anterior and posterior; the excretal canals being always near each other, and sometimes common, at least for part of their length. One has spoken seriously of the drake's sodomy, but anatomy refuses to understand it. Whether a drake frequents another drake or a duck, he addresses himself in both cases to the single door of a vestibule into which all excretions are poured. Doubtless the drake is aberrated, and his accomplice still more so, but nature deserves part of the blame. In general, animal aberrations require very simple explanations. There is a keen desire, and very urgent need, which if unsatisfied produces an inquietude, which may augment until a sort of momentary madness takes hold of the animal, and throws it blindly upon all sorts of illusions. This may go, doubtless, to the point of hallucination. There is also a need, purely muscular, of at least sketching in the sexual act, either passive or active; one sees, by singular inversion, cows in heat mounting each other, perhaps with the idea of exciting the male, or perhaps the visual representation which they make themselves of the desired act, forces them to try an imitation: it is a marvellous example, because it is absurd, of the motor force of images.