In two of these general types of restitution, in regeneration proper and in the production of adventitious organs, the potencies which underlie these processes may be said to be “complex.” It is a complicated series of events, a proper morphogenesis in itself, for which the potency has to account, if, for instance, a worm newly forms its head by regeneration, or if a plant restores a whole branch in the form of an adventitious bud. Such generalisations as are possible about the distribution of complex potencies are reserved for a special part of our future discussion. Secondary restitution is always, like ontogeny, a process of morphogenesis, and therefore all the questions about single formative stimuli, and about internal and external conditions or means, occur again. But of course we cannot enter into these problems a second time, and may only say that, especially in regeneration proper, the specific type of the regenerative formation of any part may differ very much from the ontogenetic type of its origin: the end of both is the same, but the way can be even fundamentally different in every respect. *The Stimuli of Restitutions*[49] [49] For a fuller analysis compare my opening address delivered before the section of “Experimental Zoology” at the Seventh Zoological Congress, Boston, 1907: “The Stimuli of Restitutions” (see Proceedings of that Congress). But now we turn to the important question: what is the precise stimulus[50] that calls forth processes of restitution; or, in other words, what must have happened in order that restitution may occur? [50] The problem of the stimulus of a secondary restitution as a whole must not be confused with the very different question, what the single “formative stimuli” concerned in the performance of a certain restitutive act may be. With regard to restitution as a *whole* these single “formative stimuli” might properly be said to belong to its “internal means”--in the widest sense of the word. That the operation in itself, by its removing of mechanical obstacles, cannot be the true stimulus of any restitutions, is simply shown by all those restitutions that do not happen at the place of the wound. If we took a narrower point of view, and if we only considered regeneration proper from the wound itself, we might probably at first be inclined to advocate the doctrine that the removing of some obstacles might in fact be the stimulus to the process of restoration; but, even then, why is it that just what is wanted grows out? Why is there not only growth, but specific growth, growth followed by specification? The removing of an obstacle could hardly account for that. But, of course, taking account of all the adventitious restitutions--that is, all restorations not beginning at the wound itself--the theory that the removing of obstacles is the stimulus to restoration becomes, as we have said, quite impossible.[51]