A class of facts on which it will be desirable for us here to dwell a moment, before proceeding to deal with the matter deductively, is presented by the _Cactaceæ_. In this remarkable group of plants, deviating in such varied ways from the ordinary phænogamic type, we find many highly instructive modifications of form and structure. By contemplating the changes here displayed within the limits of a single order, we shall greatly widen our conception of the possibilities of metamorphosis in the vegetal kingdom, taken as a whole. Two different, but similarly-significant, truths are illustrated. First, we are shown how, of these two components of a flowering plant, commonly regarded as primordially distinguished, one may assume, throughout numerous species, the functions, and to a great degree the appearance, of the other. Second, we are shown how, in the same individual, there may occur a re-metamorphosis: the usurped function and appearance being maintained in one part of the plant, while in another part there is a return to the ordinary appearance and function. We will consider these two truths separately. Some of the _Euphorbiaceæ_, which simulate Cactuses, show us the stages through which such abnormal structures are arrived at. In _Euphorbia splendens_, the lateral axes are considerably swollen at their distal ends, so as often to be club-shaped: still, however, being covered with bark of the ordinary colour, and still bearing leaves. But in kindred plants, as _Euphorbia neriifolia_, this swelling of the lateral axes is carried to a far greater extent; and, at the same time, a green colour and a fleshy consistence have been acquired: the typical relations nevertheless being still shown by the few leaves that grow out of these soft and swollen axes. In the _Cactaceæ_, which are thus resembled by plants not otherwise allied to them, we have indications of a parallel transformation. Some kinds, not commonly brought to England, bear leaves; but in the species most familiar to us, the leaves are undeveloped and the axes assume their functions. Passing over the many varieties of form and combination which these green succulent growths display, we have to note that in some genera, as in _Phyllocactus_, they become flattened out into foliaceous shapes, having mid-ribs and something approaching to veins. So that here, and in the genus _Epiphyllum_, which has this character still more marked, the plant appears to be composed of fleshy leaves growing one upon another. And then, in _Rhipsalis_, the same parts are so leaf-like, that an uncritical observer would regard them as leaves. These which are axial organs in their homologies, have become foliar organs in their analogies. When, instead of comparing these strangely-modified axes in different genera of Cactuses, we compare them in the same individual, we meet with transformations no less striking. Where a tree-like form is produced by the growth of