In addition to the theory of "Natural Selection," by which it has been attempted to account for the origin of species, Mr. Darwin has also put forward what he modestly terms "a provisional hypothesis" (that of _Pangenesis_), by which to account for the origin of each and every individual form. Now, though the hypothesis of Pangenesis is no necessary part of "Natural Selection," still any treatise on specific origination would be incomplete if it did not take into consideration this last speculation of Mr. Darwin. The hypothesis in question may be stated as follows: That each living organism is ultimately made up of an almost infinite number of minute particles, or organic atoms, termed "gemmules," each of which has the power of reproducing its kind. Moreover, that these particles circulate freely about the organism which is made up of them, and are derived from all the parts of all the organs of the less remote ancestors of each such {209} organism during all the states and stages of such several ancestors' existence; and therefore of the several states of each of such ancestors' organs. That such a complete collection of gemmules is aggregated in each ovum and spermatozoon in most animals, and in each part capable of reproducing by gemmation (budding) in the lowest animals and in plants. Therefore in many of such lower organisms such a congeries of ancestral gemmules must exist in every part of their bodies, since in them every part is capable of reproducing by gemmation. Mr. Darwin must evidently admit this, since he says: "It has often been said by naturalists that each cell of a plant has the actual or potential capacity of reproducing the whole plant; but it has this power only in virtue of containing gemmules _derived from every part_."[218] Moreover, these gemmules are supposed to tend to aggregate themselves, and to reproduce in certain definite relations to other gemmules. Thus, when the foot of an eft is cut off, its reproduction is explained by Mr. Darwin as resulting from the aggregation of those floating gemmules which come next in order to those of the cut surface, and the successive aggregations of the other kinds of gemmules which come after in regular order. Also, the most ordinary processes of repair are similarly accounted for, and the successive development of similar parts and organs in creatures in which such complex evolutions occur is explained in the same way, by the independent action of separate gemmules.