it is rather a purely mechanical process, which takes its course quite independently of what is useful or disadvantageous, and that it must take this course according to the given regeneration-mechanism and the stimulus supplied in the special case. It cannot be supposed that these supernumerary heads and tails are purposeful, but who would expect an adaptive reaction from the animal in a case like this, since cuts of the kind which we make artificially, and _must keep open artificially_ if the deformities are to develop, hardly occur in nature, and, if they did occur, would very quickly close up again? Adaptations can only develop in response to conditions which occur and recur in a majority of cases, and when they have a useful, that is, species-preserving result. The adaptiveness of the organism is blind, it does not see the individual case, it only takes into account the cases in the mass, and acts as it must after the mechanism has once been evolved. The case is the same as that of 'aberrant' or mistaken instincts, whose origin by means of selection is the more clearly proved, since we must recognize such an instinct as a pure mechanism and not as the outcome of purposeful forces. In the regeneration of Planarians we must think of the regeneration-idioplasm as containing the full complex of the collective determinants of the three germinal layers, and possibly we must add to this cells with the complete germ-plasm for giving rise to the reproductive cells. But when the amputated tail of the newt is regenerated, or its leg, or the arm of a starfish, or the bill of a bird, we have no ground for assuming that the cells, from which regeneration starts, contain the whole germ-plasm, since the determinants of the replaceable parts suffice to explain the facts. We must even dispute the possibility of the presence of the whole germ-plasm in this case, because the faculty of regeneration of the relevant cells is really no longer a general one, but is limited to the reproduction of a particular part. This is seen in the fact that, in the starfish, whose high regenerative capacity is well known, the central disk of the body may indeed give rise to new arms[2]; but an excised arm, to which no part of the disk adheres, is in most starfishes unable to give rise to the body. Thus the arm does not contain in its cells the determinants of the disk, but the latter contains those of the arm. We are not surprised that the amputated tail of the salamander does not reproduce the whole animal, but this can only be because the impelling forces to the regeneration of the whole animal are wanting, that is, that the cut surface only contains the determinants of the tail and not the complete germ-plasm. It might be objected here that the tail-piece is too small to give rise to the whole body, but in _Planaria_ it is only very diminutive heads and tails which grow from the artificial incisions, and the same is true