We see from this that the two capacities are not co-extensive, that germ-plasm may be contained in numerous cells of the body in a latent state, and yet that regeneration of each and every detailed defect may not be possible. This is the case in the higher plants in regard to most of their parts. A leaf in which a hole has been cut does not close the hole with new cell-material; a fern frond from which some of the pinnules have been cut off does not grow new ones, but remains mutilated. Even leaves which, if laid on damp earth, readily give off buds which grow to new plants, as the Begonias do, do not replace a piece cut out of the leaf; they are not at all adapted to regeneration. From the standpoint of utility this is readily intelligible. It was, so to speak, not worth Nature's while to make such adaptations in the case of leaves or blossoms, partly because these are very transient structures, and partly because they are rapidly and easily replaceable by the development of others of the same kind. Moreover, the leaf in which we have cut a hole continues to function, but the polyp whose mouth and tentacles we have cut off could no longer take nourishment unless it were adapted for regeneration. But that this adaptation _could_ have been made in the case of plants is proved by the root-tips which are formed anew when they are injured, and the closing of wounds on the stem by a 'callus.' I shall return to plants when we are dealing with the mechanism of regeneration, but I must now direct more attention to animals, inquiring further into the question as to whether the faculty of regeneration is correlated with the degree of liability to injury to which the animal is exposed, and with the biological importance of the injured part, for this must be the case if regeneration be really regulated by adaptation.