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 orga