The Evolution Theory, Vol. 2 of 2

by August Weismann

This last point, in particular, indicates that the determinants of the regeneration-rudiment may pursue an independent phylogenetic path of their own, for this scale arrangement of the regenerated tail is an atavistic one, that is, it corresponds to a more primitive mode of scale arrangement in these Saurians. We know quite a number of cases similar to this. It not infrequently happens that cut-off parts regenerate, but that they do so not in the modern form, but in one that is in all probability phyletically older. Thus the legs of various Orthoptera, as of the cockroaches and grasshoppers, regenerate readily, but with a tarsus composed of four joints instead of five[3], and the long-fingered claws of a shrimp (Atyoida potimirim) is replaced by the older short-fingered type of claw, while in the Axolotl an atavistic five-fingered hand grows instead of the amputated four-fingered one.

[3] New investigations, specially directed to this point, by R. Godelmann, have shown that 'in the great majority of cases' the regenerated legs of a Phasmid (Bacillus rossii) exhibit a four-jointed tarsus; but the regeneration of five joints also occurs, though only after autotomy, and only in seven out of fifty cases (Archiv für Entwicklungsmechanik, Bd. xii, Heft 2, July 1901). The regeneration-rudiment in this species seems to be in process of advancing slowly to the five-jointed type.

This last case shows that it is not merely a lesser power of growth that accounts for the difference between the regenerated part and the original, for here more is regenerated than was previously present. There remains nothing for it but the assumption that the regeneration-determinants have remained at a lower phyletic level, while the determinants which direct embryogenesis have varied, and either developed further or retrogressed. It is easy to understand that the regeneration-rudiment must vary phyletically much more slowly than the parts which evolved in the ordinary way and much more slowly than the determinants of these parts, for natural selection means a selection of the fittest, and the speed with which the establishment of a variation is attained depends, ceteris paribus, on the number of individuals that are exposed to selection with respect to the varying part. If in a species of a million living at the same time nine-tenths perish by accident, there will remain only 100,000 from which to select the 1,000 which we will assume constitute the normal number of the species. The more of these 100,000 which possess the useful variation the higher will be the percentage of the normally surviving 1,000 possessing it, and the more rapidly will the useful variation increase. But when it is a question of the variation of the regeneration-primordium, the selection will take place not among all the 100,000 individuals which chance has spared, but only among those of them which have lost a limb by accident, and thus are in a position to regenerate it more or less completely. If we assume that this takes place in 10 per cent. of cases, then selection for the improvement of the regeneration-apparatus will only take place among 1,000 individuals, and thus the process of modification of the regeneration-primordium must go on very much more slowly than that of the limb itself.

I do not see how the opponents of the germ-plasm theory can explain these facts at all, for the appeal to external influences is here entirely futile, and that to internal liberating stimuli does not suffice, since these must be different after a part has been cut off from what they were when the limb developed normally, and also different from those which prevailed at the normal origin of the limb in ancestral forms. The four-jointed tarsus of the ancestors of our cockroaches did not arise as a result of amputation. We cannot therefore avoid referring the processes of regeneration to particular 'regeneration-determinants,' which are contained in the germ-plasm and are handed on in ontogeny with the other determinants from cell-division to cell-division, till ultimately they reach the cells which are to respond, or may have to respond, to the stimulus of injury by some expression of their regenerative capacity. As these determinants, as has been shown, can often only be very slightly subject to the influence of selection processes, they will, in many respects, lag behind in the phyletic development, and will tend to belong to an ancestral type of the relevant part. They will often remain for a long time at this ancestral level, and they will always adapt themselves to new requirements more slowly than the parts which arise in the normal way, and the determinants representing these in the germ. But the regeneration-determinants are variable, and, indeed, are so hereditarily, and independently of the structure of the normal parts. They thus follow their own path of phyletic development, and this one fact is enough to secure a preference for the germ-plasm theory above others that have hitherto been suggested. None of these has even attempted an explanation of this fact; the tendency has rather been to call it in question. This, however, can be done at most only in regard to the explanation of the regenerations as atavistic, certainly not in regard to the progressive variations of the regenerated part, such as have been established by Leydig and Fraisse in regard to the lizard's tail. It may be doubted whether the most primitive insects had only four tarsal joints, but there is no disputing the kainogenetic deviation of the lizard's-tail.

I have interpreted the regenerative capacity as secondary and acquired, not as a primary power of all living substance, and I should like to substantiate this in another way.

Let us go back to the simplest organism conceivable, which must have represented the beginning of life on our earth, and we see that this need not have possessed any special power of regeneration, because, for an organism without differentiation of parts, growth is equivalent to regeneration. But growth is the direct outcome of one of the primary characters of the living substance, the capacity of assimilation. This cannot be an adaptive phenomenon, nor can it have arisen through selection, because selection presupposes reproduction, and reproduction is only a periodic form of growth; but growth follows directly from assimilation. The fundamental characters of the living substance, above all the dissimilation and assimilation which condition metabolism, must have been in existence from the first when living substance arose, and must depend on its unique chemico-physical composition. But the faculty of regeneration could only be acquired when organisms became qualitatively differentiated, so that each part was no longer like every other part or like the whole. As soon as this stage was reached the faculty of regeneration would necessarily be developed, if further multiplication was to take place. For when each fragment could no longer become a whole by simply growing, some arrangement had to be made by which each fragment should receive, in the form of primary constituents, what it lacked to make up the whole. We do not know the first beginning of this adaptation, but, in its further development, it appears in the form of 'nuclear substance,' enclosed in the nucleus of the cell, and, as is well known, it is now to be found in all unicellular organisms. That the nucleus there precedes regeneration in the sense that without a piece of it the cell-soma is not able to complete itself alone, we have already seen, and the explanation of this fact has always seemed to me to be that invisibly minute vital units relating to the regeneration of the injured part leave the nucleus and evoke the development of the missing parts by laws and forces still unknown to us. Loeb has recently claimed that the nucleus is the cell's organ of oxidation; but if that be true it would still not exclude the possibility that the nucleus is also and primarily a storehouse of the material bearers of the primary constituents of a species. It must be regarded as such when we call to mind the phenomena of amphimixis in its twofold aspects as conjugation and as fertilization, and its obvious outcome among higher organisms where it implies the mingling of the parental qualities.

Thus the 'nuclear substance' of unicellular organisms is for us the first demonstrable organ of regeneration, and first of all for normal regeneration, which takes place at every reproduction, for instance, of an Infusorian. For we have already seen that, in the transverse division of a trumpet animalcule (Stentor), the anterior part must develop the posterior half anew, while the posterior half must develop the much more complex anterior half, with mouth region and spiral bands of cilia. But as soon as the arrangement for normal reproduction was elaborated, as soon as the nucleus was present, as a depôt of 'primary constituents,' this implied the possibility of regeneration in exceptional cases, that is, after injury. The mechanism was already there, and it came into operation as soon as a part of the animal was missing.

It is in the first nucleus, therefore, that we have to look for the source of all regenerative capacity, both in unicellular and multicellular organisms. But with the origin of the latter a limitation took place, either quite at the beginning or a little later, for each nucleus of the cell-colony no longer contained the whole complex of 'primary constituents' or determinants of the species, but, in many cases, only the reproductive cell possessed them. As soon as this began to develop into a whole by cell-division the determinant-complex was segregated. Thus the first cell-colonies with two kinds of cells arose, as we have seen in the case of Volvox—the reproductive cells with a complete equipment for regeneration in their nucleus, and the somatic cells with a limited equipment for regeneration in their nuclei. The somatic cell could no longer give rise anew to the whole organism, but could only reproduce itself or its like.

But as many of the lower Metazoa and Metaphyta possess the power of budding, that is, are able not only to produce a new individual from definite cells—the reproductive cells—with or without sexual differentiation, but from other cell-groups also, these must contain the whole complex of determinants appertaining to the reconstruction of the organism, and we have to ask how this is reconcilable with the differentiation of a multicellular organism, whose different kinds of cells depend, according to our interpretation, on the fact that they are controlled by different determinants.

Obviously, there is only one way out of this difficulty, and it is the one we have already indicated, that although the diffuse regenerative capacity which we have just alluded to occurs in species which exhibit gemmation, this does not exclude the control of a cell by a specific determinant; other determinants may be contained in the cell, in a state, however, in which they do not affect it, that is, in an inactive or latent state.