In lizards, a leg which has been cut off does not grow again, but an amputated tail does, and this has quite a definite biological reason, since the active little animal will seldom be caught by the foot by any pursuer, but may easily be caught by the tail, which is far behind. Thus the tail is adapted not only for regeneration, but also for 'autotomy' that is, for breaking off easily when it is caught hold of.
We have already seen that some segmented worms have a very high regenerative capacity; yet every part cannot produce every other, and while, in Lumbriculus, any piece of from five to nine segments is able to grow a new head or tail, neither ten nor twenty nor all the segments together, if they are halved longitudinally, can reproduce the other half, and the cause of this inability does not lie in the fact that the animal is thereby hindered from taking food, for even the transversely cut pieces do not feed until they have grown a new head and tail. The reason must lie in the fact that the primary constituents for this kind of regeneration are wanting, and they are so because a longitudinal splitting of this cylindrical and relatively thin animal never occurs under natural conditions, and thus could not be provided against by Nature[1].
[1] Morgan maintains that this statement is incorrect, and that Lumbriculus is capable of lateral regeneration. But if we look into the matter more closely we find that all he says is, that small gaps made by cutting a piece out of one side are filled up again, while the cut pieces perish. If the whole animal be halved, according to Morgan, both halves die, or if a 'very long piece' be cut out of one side, not only this piece dies, but also 'the remaining piece.' There is thus, as I have said, an essential difference between the regenerative capacity of Lumbriculus and that of Planaria.
That regeneration of this kind could have been arranged for if it had been useful we learn from the Planarians among the flat worms, in which every piece cut out of the body, large or very small, from the middle, from the left side, or from the right side of the animal, grows into a complete Planarian. The animal can be halved longitudinally, as in Fig. 97, and each half will grow to a whole. This again is quite intelligible from the biological point of view, for these flat, soft, and easily torn animals are exposed to all sorts of injuries, and are, in point of fact, frequently mutilated by enemies which are unable to swallow them whole. Von Graaf not infrequently found examples of marine Planarians (Macrostomum) which lacked 'a part of the posterior end or the whole tail region as far as the food-canal,' and of species of Monotus he found 'very often' in May specimens with the posterior end split or broken off. Probably the persecutors of these flat-worms are some species of Crustacean, but, at any rate, so much is proved, that the Planarians have abundant opportunities of making use of their faculty of regeneration, and that the species gains an advantage from it in respect to its preservation.
Fig. 97. A, a Planarian,
which has been divided into
two by a longitudinal cut.
Each half can grow into
an entire animal. B, the
left half at the beginning
of the regenerative process.
C, the same completed. After
Morgan.
In contrast to this, worms which live within other animals, and are thus secure from mutilation, such as the familiar round-worms (Nematoda), have no power of regeneration at all, and do not survive either longitudinal or transverse division.
Until recently birds were regarded as possessing a very low degree of regenerative capacity, and, as a matter of fact, they cannot replace a leg or a wing wholly or in part; but, what is otherwise unheard of among higher vertebrates, they can renew the whole anterior portion of the skeleton of the face, the bill, and can indeed almost reconstruct it with new bones and horny parts. Von Kennel communicated a case of this kind in regard to a stork, and for a long time this remained an isolated case, but a few years ago Bordage showed that, in the cocks which are used in the Island of Bourbon for the favourite sport of cock-fighting, the bill is regularly renewed when it has been broken off or shattered. Quite recently Barfurth gave an account of a case of complete renewal of a broken bill in a parrot. Yet it should not astonish us that the bill in birds has such a high regenerative power, for of all parts in a bird it is the one that is most readily injured; with it the bird defends itself against its enemies and its rivals, masters its prey, and tears it to pieces, pecks holes in trees (woodpecker), or climbs (parrot), or digs and burrows in the ground, or builds its nest, and so on. That the faculty of regeneration could be developed to so high a degree in relation to this particular part of the body, while the rest of the very important but rarely injured parts do not possess it at all, again points to the conclusion that the faculty of regeneration has an adaptive character.
It does not affect matters to discover cases in which we cannot recognize this relation between the regenerative capacity of a part and its importance or its liability to injury. Such instances do not lessen the convincingness of the positive cases, since we do not know the exact conditions which may lead to the increase of regenerative capacity in a part, and, above all, since we do not know the rate at which such an increase may take place. If adaptation in general depends upon processes of selection, these processes must also be able to give rise to an increase in the power of regeneration. On the other hand, it by no means follows that the disappearance of a faculty of regeneration which was once present in a part, but which has become superfluous in the course of time, must take place immediately through natural selection. For it is the very essence of natural selection that it only furthers what is useful, and only removes what is injurious; over what is indifferent it has no power at all. Thus it follows that the faculty of regeneration, when it has once been present in a part, cannot be set aside by natural selection (personal selection), for it is in no way injurious to its possessor. If it gradually decreases and becomes extinct notwithstanding this, when it is of no further use, as seems to be to some extent the case in regard to the legs and tail of the blind Proteus, that must depend on other processes, on those which generally bring about the gradual disappearance of disused parts or capacities. We shall attempt to probe to the roots of these processes later on; for the present let it suffice us to know that, according to our experience, they go on with exceeding slowness, and that it has taken whole geological periods to eliminate the legs of the snake-ancestors so completely as has been done from the structure of most of our modern snakes, while the Proteus which migrated into the caves of Krain as far back as the Cretaceous period is indeed blind, but still retains its eyes under the skin, though in a degenerate condition.
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.