where several kinds of tissue make up an organ. For instance, if the leg of the mammal is cut off, the old cells may give rise to new ones, but the processes that would bring about the formation of the new leg are not present, or, rather, if present, cannot act. Thus, although the production of new cells from each of the different parts of the leg of a mammal may take place, yet the conditions are unfavorable to the subsequent formation of a new leg out of the proliferated cells. We should not infer that this power does not exist, but that under the conditions it cannot be carried out. The assumption that physiological regeneration is the forerunner of restorative regeneration, in the sense that historically the former preceded the latter and furnished the basis for the development of the latter, cannot be shown, I think to be even probable. This way of looking at the two processes puts them, I believe, in a wrong relation to each other. We find both processes taking place in the simplest forms as in the unicellular protozoa, and present throughout the entire animal kingdom without any connection, excepting so far as they both depend on the general processes of growth characteristic of each organ and of each animal. This leads us to consider the general question of regeneration in its relation to the phenomena of growth. _REGENERATION AND GROWTH_ It has been pointed out in several cases in which external factors influence the growth of a plant, or of an animal, that the same factors play a similar part in the regeneration. The action of gravity on the growth of plants has been long known, and that it is a factor in the regeneration of a piece of a plant has also been shown. The only animal in which gravity has been definitely shown to be an important factor during growth is antennularia, and it has been found that gravity is also a factor in the regeneration of the same form. Not only is this influence shown in the growth of the new part that has developed, but the same influence seems to be one of the factors that determines where the new growth takes place. This latter relation is known in only a few cases, for instance in plants, according to Vöchting, and in antennularia, according to Loeb, so that, until further evidence is forthcoming, it is best not to extend this generalization too far; but it seems not impossible that it may be generally true. How an external factor may determine the location of new growth, as well as the subsequent development of the new part, we do not know at present.