Since the degeneration of disused parts and capacities goes on so slowly it need not surprise us that we meet many parts which still possess regenerative capacity, although they are protected from injury. Thus Morgan found that, in the hermit-crab, the limbs which are protected within the mollusc shell were quite as ready to regrow as those which are actually used for walking, and thus are exposed to possibility of attack, but this proves nothing against the conclusion we drew from the facts cited above, according to which the faculty of regeneration comes under the law of adaptation. For the disappearance of this faculty must take place very much more _slowly than its growth_. For instance, the development of the tail-fin of the whale has long been an accomplished fact, while the hind-legs of this colossal mammal, which were rendered useless by the development of the tail-fin, still lie concealed in a rudimentary state within the muscles of the trunk. Yet these limbs must have lost their significance for the animal exactly at the time that the tail-fin became more powerful. Thus the retrogression must have taken place more slowly than the progressive transformation. It is clear, then, that the faculty of regeneration is not a primary character of living beings occurring uniformly in all species of equally high organization and in all parts of an animal in the same degree; it is a power which occurs in animals of equal complexity in as varying degrees as in their parts, and which is manifestly regulated by adaptation. Between parts with the faculty of regeneration and parts without it there must be an essential difference; there must be present in the former something that is wanting in the latter, and, according to our theory, this is the equipment with regeneration-determinants, that is, with the determinants of the parts which are to be reconstructed.