from the plant, and grow into new plants, as in _Mnium androgynum_ among the Mosses.” Now in the way above explained, these propagative cells and proliferous buds, may continue developing in connexion with the parent to various degrees before separating; or the buds which are about to become fructifying organs may similarly, under increased nutrition, develop into young fronds. As Sir W. Hooker says of the male fructification in _Metzgeria furcata_,--“It has the appearance of being a young shoot or innovation (for in colour and texture I can perceive no difference) rolled up into a spherical figure.” On finding in this same plant, that sometimes the proliferously-produced frond buds out from itself another frond before separating from the parent, as shown in Fig. 46, it becomes clear that this long-continued connexion may readily pass into permanent connexion. And when we see how, even among Phænogams, buds may either detach themselves as bulbils, or remain attached and become shoots; we can scarcely doubt that among inferior plants, less definite in their modes of organization, such transitions must continually occur. [Illustration: Figs. 73–76.] Let us suppose, then, that Fig. 73 is the frond of some primitive Archegoniate, similar in general characters to _Pellia epiphylla_, Fig. 43; bearing, like it, the fructifying buds on its upper surface, and having a slightly-marked mid-rib and rootlets. And suppose that, as shown, a secondary frond is proliferously produced from the mid-rib, and continues attached to it. Evidently the ordinary discontinuous development, can thus become a continuous development, only on condition that there is an adequate supply, to the secondary frond, of such materials as are furnished by the rootlets: the remaining materials being obtainable by itself from the air. Hence, that portion of the mid-rib lying between the secondary frond and the chief rootlets, having its function increased, will increase in bulk. An additional consequence will be a greater concentration of the rootlets--there will be extra growth of those which are most serviceably placed. Observe, next, that the structure so arising is likely to be maintained. Such a variation implying, as it does, circumstances especially favourable to the growth of the plant, will give to the plant extra chances of leaving descendants; since the area of frond supported by a given area of the soil, being greater than in other individuals, there may be a greater production of spores. And then, among the more numerous descendants thus secured by it, the variation will give advantages to those in which it recurs. Such a mode of growth having, in this manner, become established, let us ask what is next likely to result. If it becomes the habit of the primary frond to bear a secondary frond from its mid-rib, this secondary frond, composed of physiological units of the same kind, will inherit the habit; and supposing that the supply of mineral matters obtained by