Phenomena of this kind, evoked by mutilation or injury, together with the cognate observations on regeneration throw very curious lights on the nature of living things. To an understanding of the nature of the mechanics of living matter and its relation to matter at large they offer the most hopeful line of approach. I allude especially to the examples in which it has been established that the part which is produced after mutilation is a structure different from that which was removed. The term "regeneration" was introduced before such phenomena were discovered, and though every one recognizes its inapplicability to these remarkable cases, the word still misleads us by presenting a wrong picture to the mind. The expression "heteromorphosis" (Loeb) has been appropriately applied to various phenomena of this kind, and Morgan has given the name "morphallaxis" to another group of cases in which the renewal occurs by the transformation of a previously existing part.[10] But we must continually remember that all these occurrences which we know only as abnormalities and curiosities must in reality be exemplifications of the normal mechanics of division and growth. The conditions needed to call them forth are abnormal, but the responses which the system makes are evidences of its normal constitution. When therefore, for example, the posterior end of a worm produces a reversed tail from its cut end we have a proof that there must be in the normal body forces ready to cause this outgrowth. The new structure is not an ill-shaped head-end, for, as Morgan shows, the nephridial ducts have their funnels perforating the segments in a reversed direction. The "tension" of growth is actually reversed.[11] So also when in a Planarian amputation of the body immediately behind the head leads to the formation of a new reversed head at the back of the normal head, while amputation further back leads to the regeneration of a new tail, these responses give indications of forces normally present in the body of the Planarian. Such facts open up a great field of speculation and research. Especially important it would be to determine where the critical region may be at which the one response is replaced by the other. I suppose it is even possible that there is some neutral zone in which neither kind of response is made.