§ 5. The True Significance of Instinct A third class of facts commonly cited as evidence of bestial intelligence are the remarkable phenomena of _instinct_.[14] The beaver acts as though it were acquainted with the principles of hydraulics and engineering, when it maintains the water at the height requisite to submerge the entrance to its dwelling by building a dam of mud, logs, and sticks across the stream at a point below the site of its habitation. The predatory wasp _Pompilius_ is endowed with surgical art, that suggests a knowledge of anatomy, inasmuch as it first disarms and afterwards paralyzes its formidable prey, the _Lycosa_ or black Tarantula. Another predatory wasp, the _Stizus ruficornis_, disables Mantids in a similar fashion. One of the American Pompilids, the black wasp _Priocnemis flavicornis_, is an adept in the art of navigation, since it adopts the principle of the French hydroglissia (an air-driven boat which skims the water under the propulsion of an aeroplane propeller). This insect tows a huge black spider several times its own size and too heavy to be carried, propelling its prey with buzzing wings along the open waterway, and leaving behind a miniature wake like that of a steamer. It thus avoids the obstacles of the dense vegetation, and saves time and energy in transporting the huge carcass of its paralyzed quarry to the haven of its distant burrow. Spiders like the _Epeira_, for example, are endowed with the mathematical ability of constructing their webs on the patterns of the logarithmic spiral of Jacques Bernouilli (1654-1705), a curve which it took _man_ centuries to discover. The dog infested with parasitic tapeworms (_Taenia_) evinces a seeming knowledge of pharmaceutics, seeing that it will avidly devour Common Wormwood (_Artemisia absynthium_), an herb which it never touches otherwise. [14] J. Henri Fabre and Erich Wasmann, S.J., have formulated very sound and critical views on the subject of instinct. The works of these authors are now available in English. (_Cf._ de Mattos’ translation of the _Souvenirs etymologiques_: “The Mason Bees,” Ch. VII; “The Bramble Bees,” Ch. VI; “The Hunting Wasps,” Chs. IX, X, XX; _cf._ also Wasmann’s _Instinct and Intelligence_, and _Psychology of Ants and of Higher Animals_, Engl. translation by Gummersbach.)