Although it is only under exceptional circumstances that a flower is likely to mimic another blossom closely, vague general resemblances are not uncommon, such as that between the rock-rose and the buttercup, between the milkwort and the vetch, and between Veronica and Valerianella. A more decided likeness is that of the garden annual Collinsia to the butterfly blossoms of the pea tribe. This case is peculiarly instructive since the homomorphism can be traced to its cause. The butterfly-like corolla of Leguminosæ seems to have afforded the pattern after which a number of flowers have been fashioned. The Papilionaceæ are adapted to bees rather than to butterflies or moths, and the pollen is applied to the ventral surface of the insect, the essential organs being lodged in the carina or pouch formed by the two lower petals. Among the Scrophulariaceæ to which Collinsia belongs, the pollen is commonly sprinkled on the back of the insect and the stamens are contained in the upper lip of the corolla; Collinsia is, however, exceptional; the stamens are lodged within the lower lip of the flower and the pollen is applied to the ventral surface of the bee. Here the resemblance is evidently an indirect result brought about by the flowers of Collinsia having become adapted to the same class of visitors as the Papilionaceæ, viz., bees which have their brushes or baskets of hair for collecting pollen attached to the abdomen. Where two flowers are very like insects are apt to mistake the one species for the other, but this will not involve any loss if there is an interval between their periods of blossoming. Homomorphic likenesses are not confined to homologous organs; an organ of one plant sometimes exhibits a perfect resemblance to a different organ on some other plant. Thus Aristolochia sipho, the Dutchman’s pipe, so-called from the appearance of its flowers, has a perianth singularly like the leaf-pitchers of Nepenthes, and the curious little nectaries of Nigella might almost be compared with the pitchers of the Australian insectivorous plant Cephalotus. As the Aristolochias imprison small dipterous insects in their flowers these instances favor to some extent Henslow’s idea that both flowers and pitchers have arisen by hypertrophy caused through the irritation set up by insects.