I cannot conclude this account of flower-adaptations without considering the orchids somewhat more in detail, for it is among them that we find the most far-reaching adaptations to the visits of insects. Among them, too, great diversity prevails, as is evident from the fact that Darwin devoted a whole book to the arrangements for fertilization in orchids, but the main features are very much the same in the majority. Figure 46 gives a representation of one of our commonest species (_Orchis mascula_), A shows the flower in side view, _B_ as it appears from in front. The flower seems as it were to float on the end of the stalk (_st_), stretching out horizontally the spur (_sp_) which contains the nectar. Between the large, broad under lip (_U_), marked with a honey-guide (_sm_), and offering a convenient alighting surface, and the broad, cushion-like stigma (_n_) lies the entrance to the spur. Fertilization occurs in the following way:--The fly or bee, when it is in the act of pushing its proboscis into the nectar-containing spur, knocks with its head against the so-called rostellum (_r_), a little beak-like process at the base of the stamens (_p_). The pollen masses are of very peculiar construction, not falling to dust, but forming little stalked clubs, with the pollen grains glued together, and so arranged that they spring off when the rostellum is touched and attach themselves to the head of the insect, as at _D_ on the pencil (Fig. 46). When the bee has sucked up the nectar out of the spur, and then proceeds to penetrate into another flower of the same species, the pollinia have bent downwards on its forehead (_E_), and must unfailingly come in contact with the stigma of the second flower, to which they now remain attached, and effect its fertilization. What a long chain of purposeful arrangements in a single flower, and no interpretation of them is available except through natural selection! [Illustration: FIG. 46. Common Orchis (_Orchis mascula_). _A_, flower in side view. _st_, stalk. _sp_, spur with the nectary (_n_). _ei_, entrance to the spur. _U_, lower lip. _B_, flower from in front. _p_, pollinia. _Sm_, honey-guide. _ei_, entrance to the nectar. _na_, stigma. _r_, rostellum. _U_, lower lip. _C_, vertical section through the rostellum (_r_), pollinium (_p_). _ei_, entrance. _D_, the pollinia removed and standing erect on the tip of a lead-pencil. _E_, the same, somewhat later, curved downwards.]