Man, even in the state of semi-civilization, is burdened with too much knowledge for it all to be transmitted hereditarily; but there is no doubt that the oldest and most useful reaches us in this manner. When we walk in the forest there are berries that tempt us, whortleberries, for example, but never alderberries. Who has taught us (I am supposing a real ignorance), that they are purgative and even dangerous? Instinct? What is instinct? The hereditary transmission of knowledge. This transmission can, without doubt, occur in the case of abstract ideas, as well as of practical ideas--that is to say, useful for the conservation of life. Some, besides, are really useful, and even primordial. It is as reasonable to believe that they are inherited as to suppose them personally acquired. It might be possible to rehabilitate the theory of innate ideas, by revising it carefully and eliminating from its catalogue all sorts of Platonic or Christian inventions, too recent to have entered our blood. As to the direct knowledge of ideas, this is gained in a form sensibly analogous to the knowledge of matter by absorption. Once they have entered us, ideas either remain inert, unknown, or else are disintegrated. In the first case, it is not long before they are expelled from the brain, much like an indigestible morsel which has entered the intestines. Their stay may produce a certain irritation, even lesions. That is to say, it may provoke absurd acts, manifestly without logical relation to the normal physiology of the patient. This effect may be observed in all countries, but especially in France, at the time of great political or moral crises. We see people tormented by the presence of a parasitic idea in their brain, like sheep by the residence of a trumpet-fly's egg in their frontal sinus. Man, like the sheep, has the "itch." That ends badly for the sheep--for the man also, very often.