The Evolution Theory, Vol. 2 of 2

by August Weismann

The same holds true of all the lower Metazoa that have highly developed regenerative capacity, not only of polyps, but of worms such as the Planarians. Through the experiments of Loeb, Morgan, Voigt, Bickford, and others, we know that these animals respond to almost every mutilation by complete reconstruction, that they may, for instance, as is indicated in Fig. 96, be cut transversely into nine or ten pieces with the result that each of these pieces grows again to a whole animal, unless external influences are unfavourable and prevent it.

Something similar happens if the head be cut off a Tubularia-polyp, it forms a new head with proboscis and tentacles. It does so, at least, if the stalk of the polyp be left in the normal position; but if it be stuck into the sand in the reverse position a head arises at the end which is uppermost, where the roots arose previously, and the previous head-end now sends out roots. By suspending a beheaded stalk horizontally in the water a head can be caused to develop at each end of the stalk, so that we must assume that every part of the polyp is, under some circumstances, capable of developing a head, and that it must be 'circumstances'—in this case gravity, contact with earth or with water, and the mutual influence of the parts of the animal upon each other—which decide what is to be produced. Loeb, who was the first to observe this form of regeneration, called it heteromorphosis, to express the fact that particular parts of the animal might be produced at quite different places from those originally intended for them.

It would certainly be erroneous to range these cases of heteromorphosis against the determinant theory, but they certainly do not afford any special evidence of its validity as an interpretation, for all that we can say here again is that all, or at least many, cells of the animal must contain the full determinant-complex of the ectoderm, and others those of the endoderm, and that particular groups of determinants become active when they are affected by certain external or internal liberating stimuli. In regard to such animals the theory is hardly more convincing than the rival theory, that the faculty of regeneration is a general property of living substance, which does not attain to equally full expression everywhere, because it is met by ever-increasing difficulties involved in the increasing complexity of structure. The validity of the theory only begins to be seen when we deal with cases where it is demonstrable that every part cannot bring forth every other, where the power of regeneration is limited, and occurs only in definite parts in a definite degree, and can only start from particular parts. Here the assumption of a general and primary regenerative capacity fails. Any one who insists, as O. Hertwig does, that the idioplasm in all cells of the body is the same, can always plead that, in the cases in which regeneration does not occur, the fault lies, not in the regenerative capacity, but in the absence of the adequate liberating stimuli, and at first sight it does seem as if this position were unassailable. We shall find, however, that there are facts which make Hertwig's interpretation quite untenable.

My own view is that the regenerative capacity is not something primary, but rather an adaptation to the organism's susceptibility to injury, that is, a power which occurs in organisms in varying degrees, proportionate to the degree and frequency of their liability to injury. Regeneration prevents the injured animal from perishing, or from living on in a mutilated state, and in this lies an advantage for the maintenance of the species, which is the greater the more frequently injuries occur in the species, and the more they menace its life directly or indirectly. A certain degree of regenerative capacity is thus indispensable to all multicellular animals, even to the highest among them. We ourselves, for instance, could not escape the numerous dangers of infection by bacilli and other micro-organisms if our protective outer skin did not possess the faculty of regeneration, at least so far that it can close up a wound and fill up with cicatrice-tissue a place where a piece of skin has been excised. Obviously, then, the mechanism which evokes regeneration must have been preserved in some degree and in some parts at every stage of the phyletic development, and must have been strengthened or weakened according to the needs of the relevant organism, being concentrated in certain parts which were much exposed to injury and withdrawn from other rarely threatened parts. Thus the great diversity which we can now observe in the strength and localization of the regenerative capacity has been brought about. But all this can only be regarded as adaptation.

I should like to submit a few examples to show that the regenerative capacity is by no means uniformly distributed, and that, as far as we can see, it is greater or less in correspondence with the needs of the animal, both in regard to the whole and to particular parts.

It must first be pointed out that those lower Metazoa, like the Hydroid polyps in particular, which are endowed with such a high and general power of regeneration, do actually require this for their safety; they are not only soft, easily injured and torn, but they are most severely decimated by many enemies. In the beginning of May I found on the walls of the harbour at Marseilles whole forests of polyp-stocks of the genera Campanularia, Gonothyræa, and Obelia, all large and splendidly developed, with thousands of individual polyps and medusoids, but in a very short time the great majority of the polyps were eaten up by little spectre-shrimps (Caprellids) and other crustaceans, worms, and numerous other enemies, and towards the end of May it was no longer possible to find a fine well-grown colony. It must therefore be of decisive importance for these species if the stems and branches, which are spared because protected by horny tubes, possess the faculty of transforming their simple soft parts into polyp-heads, or of giving off buds which become polyps, or even of growing a new stock from the twigs which have been half-eaten and bitten loose from the stock and have fallen to the ground. If, finally, a torn-off polyp-stalk (of Tubularia) falls to the ground with the wrong side up, the end which is now the lower will send out roots, and the end now uppermost will give off a new head. This also appears to us adaptive, and does not surprise us, since we have been long accustomed to recognize that what is adapted to an end will realize this if it be possible at all. Think again of the innumerable adaptations in colour and form which we discussed in the earlier lectures. I hope later to be able to show in more detail how it comes to pass that necessity gives rise to adaptation. In regard to the case of the polyps, we can understand that, as far as a high degree of regeneration and budding was possible in these animals at all, it could not but be developed. Regeneration and budding complete each other in this case, for the former brings about in the individual 'person' what the latter does in the colony, namely, a Restitutio in integrum. It is readily intelligible that the former was not difficult to establish where the latter—the capacity of budding—was already in existence.

It seems at first sight very striking that the higher plants, which all depend upon budding, and which form plant-colonies (corms) in the same sense as the polyps form animal-colonies, only possess the faculty of true regeneration in a very low degree, although they are extremely liable to injury.

We see from this that the two capacities are not co-extensive, that germ-plasm may be contained in numerous cells of the body in a latent state, and yet that regeneration of each and every detailed defect may not be possible. This is the case in the higher plants in regard to most of their parts. A leaf in which a hole has been cut does not close the hole with new cell-material; a fern frond from which some of the pinnules have been cut off does not grow new ones, but remains mutilated. Even leaves which, if laid on damp earth, readily give off buds which grow to new plants, as the Begonias do, do not replace a piece cut out of the leaf; they are not at all adapted to regeneration.

From the standpoint of utility this is readily intelligible. It was, so to speak, not worth Nature's while to make such adaptations in the case of leaves or blossoms, partly because these are very transient structures, and partly because they are rapidly and easily replaceable by the development of others of the same kind. Moreover, the leaf in which we have cut a hole continues to function, but the polyp whose mouth and tentacles we have cut off could no longer take nourishment unless it were adapted for regeneration. But that this adaptation could have been made in the case of plants is proved by the root-tips which are formed anew when they are injured, and the closing of wounds on the stem by a 'callus.'

I shall return to plants when we are dealing with the mechanism of regeneration, but I must now direct more attention to animals, inquiring further into the question as to whether the faculty of regeneration is correlated with the degree of liability to injury to which the animal is exposed, and with the biological importance of the injured part, for this must be the case if regeneration be really regulated by adaptation.