Weismann in his book on _The Germ Plasm_ has adopted the principle of a connection between regeneration and liability to injury and has carried it much farther than other writers. We can, therefore, most profitably make a careful examination of Weismann’s position. His general idea may be gathered from the following quotation:[38] “The dissimilarity, moreover, as regards the power of regeneration _in various members of the same species_, also indicates that adaptation is an important factor in the process. In proteus, which in other respects possesses so slight a capacity for regeneration, the gills grow again rapidly when they have been cut off. In lizards again this power is confined to the tail, and the limbs cannot become restored. In these animals, however, the tail is obviously far more likely to become mutilated than are the limbs, which, as a matter of fact, are seldom lost, although individuals with stumps of legs are occasionally met with. The physiological importance of the tail of a lizard consists in the fact that it preserves the animal from total destruction, for pursuers will generally aim at the long trailing tail,[39] and thus the animal often escapes, as the tail breaks off when it is firmly seized. It is, in fact, as Leydig was the first to point out, specially adapted for breaking off, the bodies of the caudal vertebræ from the seventh onward being provided with a special plane of fracture so that they easily break into two transversely. Now if this capability of fracture is provided for by a special arrangement and modification of the parts of the tail, we shall not be making too daring an inference if we regard the regenerative power of the tail as _a special adaptation, produced by selection, of this particular part of the body, the frequent loss of which is in a certain measure provided for_, and not as the outcome of an unknown ‘regenerative power’ possessed by the entire animal. This arrangement would not have been provided if the part had been of no, or of only slight, physiological importance, as is the case in snakes and chelonians, although these animals are as highly organized as lizards. The reason that the limbs of lizards are not replaced is, I believe, due to the fact that these animals are seldom seized by the leg, owing to their extremely rapid movements.” Overlooking the numerous cases of the regeneration of internal organs that have been known for several years, and basing his conclusion on a small, unconvincing experiment of his own on the lungs of a few salamanders, Weismann concludes: “Hence there is no such thing as a general power of regeneration; in each kind of animal this power is graduated according to the need of regeneration in the part under consideration; that is to say, the degree in which it is present is mainly in proportion to the liability of the part to injury.”