Having thus viewed the matter broadly, we may consider in detail a few examples of the process of tumescence among the lower animals and man, for, as will be seen, the process in both is identical.  As regards animal courtship, the best treasury of facts is Brehm’s Thierleben, while Buechner’s Liebe und Liebes-Leben in der Thierwelt is a useful summary; the admirable discussion of bird-dancing and other forms of courtship in Haecker’s Gesang der Voegel, chapter iv, may also be consulted.  As regards man, Wallaschek’s Primitive Music, chapter vii, brings together much scattered material, and is all the more valuable since the author rejects any form of sexual selection; Hirn’s Origins of Art, chapter xvii, is well worth reading, and Finck’s Primitive Love and Love-stories contains a large amount of miscellaneous information.  I have preferred not to draw on any of these easily accessible sources (except that in one or two cases I have utilized references they supplied), but here simply furnish illustrations met with in the course of my own reading.

Even in the hermaphroditic slugs (Limax maximus) the process of courtship is slow and elaborate.  It has been described by James Bladon ("The Loves of the Slug [Limax cinereus],” Zooelogist, vol. xv, 1857, p. 6272).  It begins toward midnight on sultry summer nights, one slug slowly following another, resting its mouth on what may be called the tail of the first, and following its every movement.  Finally they stop and begin crawling around each other, emitting large quantities of mucus.  When this has constituted a mass of sufficient size and consistence they suspend themselves from it by a cord of mucus from nine to fifteen inches in length, continuing to turn round each other till their bodies form a cone.  Then the organs of generation are protruded from their orifice near the mouth and, hanging down a short distance, touch each other.  They also then begin again the same spiral motion, twisting around each other, like a two-strand cord, assuming various and beautiful forms, sometimes like an inverted agaric, or a foliated murex, or a leaf of curled parsley, the light falling on the ever-varying surface of the generative organs sometimes producing iridescence.  It is not until after a considerable time that the organs untwist and are withdrawn and the bodies separate, to crawl up the suspending cord and depart.

Some snails have a special organ for creating sexual excitement.  A remarkable part of the reproductive system in many of the true Helicidae is the so-called dart, Liebespfeil, or telum Veneris.  It consists of a straight or curved, sometimes slightly twisted, tubular shaft of carbonate of lime, tapering to a fine point above, and enlarging gradually, more often somewhat abruptly, to the base.  The sides of the shaft are sometimes furnished with two or more blades; these are apparently not for cutting