Darwin demonstrated, to look upon a flower for the first time and from
its structure foretell the method of its intended cross-fertilization;
nay, more, possibly the kind, or even the species, of insect to which
this cross-fertilization is intrusted.

Let us look at our Arethusa. The writer has never happened to observe an
insect at work upon this flower, but the intention of its structure is
so plain that by a mere examination we may safely prophesy not only what
must happen when the insect seeks its nectar, but with equal assurance
the kind of insect thus invited and expected. I have indicated a group
of the orchids in their usual marshy haunt, and in Fig. 4, separately, a
series of diagrams presents sections of the flower, natural size and
duly indexed, which renders detailed description hardly necessary. The
column is here quite elongated, forked at the tip, the space between the
forks occupied by the anther, which is hinged to the upper division.
This anther lid is closed tightly, with the sticky mass of pollen hidden
behind it in the cavity. The stigma is on the external inner side of the
lower division, and thus distinctly separated from the pollen. The "lip"
is extended forward as a hospitable threshold to the insect. And to what
insect might we assume this invitation of color, fragrance, nectar, and
threshold to be extended?

Let us consider the flower simply as a device to insure its own
cross-fertilization. The insect is welcomed; it must alight and sip the
nectar; in departing it must bear away this pollen upon its body, and
convey it to the _next_ Arethusa blossom which it visits, and leave it
upon its stigma. These are the conditions expressed; and how admirably
they are fulfilled we may observe when we examine flower after flower of
a group, and find their nectaries drained, their anther cells empty, and
pollen upon all their stigmas. The nectar is here secreted in a
well--not very deep--and the depth of this nectar from the entrance is
of great significance among all the flowers, having distinct reference
to the length of the tongue which is expected to sip it. In the
Arethusa, it is true, the butterfly or moth might sip at the throat of
the flower, but the long tongues of these insects might permit the
nectary to be drained without bringing their bodies in contact with the
stigma. Smaller insects might creep into the nectary and sip without the
intended fulfilment. It is clear that to neither of such vi