The Evolution Theory, Vol. 2 of 2

by August Weismann

From all these facts, which I have summarized as briefly as possible, we see that the older phyletic characters are gradually crowded by the newer into ever-younger stages in the ontogeny, until ultimately they disappear altogether. We have now to ask to what this phenomenon is due; is it a simple crowding out of the old and less advantageous by the new and better characters as a result of natural selection, or is there some other factor at work? It is clear in regard to these forms of marking that they can have been developed at first only in the almost full-grown larva by natural selection, because they are of use only there, and that, at the same time, the old marking must have been set aside through the influence of the same factor, in as far as it prejudiced the effect of the new adaptation. This seems to be indicated by the persistence of the sub-dorsal line on those segments which are drawn in when Chærocampa assumes a terrifying attitude, or which do not bear oblique stripes in the leaf-like caterpillars, e.g. the three anterior segments in the species of Sphinx and Smerinthus. When newly acquired schemes of marking like the eye-spots of Chærocampa are transmitted from the last stage to the stage before, this can be explained by following the same train of thought, for the caterpillar is already of sufficient size to be able to inspire terror with its eyes; but in still younger stages the spots would not be likely to have that effect, and yet they occur in quite small animals (20 mm.). More obvious still is the uselessness of the oblique striping in the young stages of the Sphinx and Smerinthus caterpillars, for in the earliest stages of life the caterpillars are much too small to look like a leaf, and the oblique stripes stand much closer together than the lateral ribs of any leaf. Moreover, the little green caterpillars require no further protection when they sit on the under side of a leaf; they might then very easily be mistaken in toto for a leaf-rib. Thus it is certainly not natural selection which effects the shunting back of the new characters. Nor can this be caused by the fact that the new character can only be developed gradually and in several stages, for the oblique striping at any rate arises in the ontogeny all at once. There must therefore be some mechanical factor in development to which is due the fact that characters acquired in the later stages are gradually transferred to the younger stages. But this shifting backwards can be checked by the agency of natural selection as soon as it becomes disadvantageous for the stage concerned.

It is in this way that I explain the fact that the majority of the caterpillars of the Sphingidæ are absolutely without markings when they emerge from the egg. Thus, for instance, the caterpillars of Chærocampa (Fig. 116, A), of Macroglossa (Fig. 115), and of Deilephila (Fig. 118, A), as well as those of the Smerinthus species, are at first without stripe or mark of any kind; they are of a pale green colour, almost transparent, and very difficult to recognize when they sit upon a leaf. How very greatly the different stages can be independently adapted to the different conditions of their life, when that is necessary for the preservation of the species, is shown in the most striking manner by many species. Thus the little green caterpillar of Aglia tau, when it leaves the egg, bears five remarkable reddish rod-like thorns, which in form and colour resemble the bud-scales of the young beech-buds among which they live, and which disappear later on; the full-grown caterpillar shows nothing of these, but is leaf-green, marked with oblique stripes. Even if the use of these reddish thorns be other than I have indicated, we have in any case to deal with a special adaptation of one, and that the first caterpillar-stage, and what can happen at this stage is possible also at every other. Nor is it only animals which undergo metamorphosis that can exhibit independent phyletic variation at every stage, but those also with direct development, and indeed, in the case of these, we may assume adaptation of this kind at almost every stage in the history of the organs, as we have already seen, because the great abridgement of the phylogeny into the ontogeny necessitates a very precise mutual adaptation of the organ-rudiments and of the diverse rates of development.

We have thus been led by the facts discussed—and numerous others from other groups in the animal kingdom might be ranked along with them—to two main propositions, which express the relation of phylogeny to ontogeny. The first and fundamental proposition is the one already formulated. The ontogeny arises from the phylogeny by a condensation of its stages, which may be varied, shortened, thrown out, or compressed by the interpolation of new stages. The second proposition refers to individual parts, and may run as follows: As each stage can undergo new adaptations by itself, so can every part, every organ; such new adaptations very often show a tendency to be transferred to the immediately antecedent stage in ontogeny.

It is not my intention to formulate the laws of ontogeny just now, otherwise many others might be added to these, such as that of the regular transference of characters acquired at one end of a segmented animal to the other segments: I must confine myself here to bringing the two main propositions into harmony with the principles of our theory of heredity.

How phylogeny is condensed in ontogeny can be understood readily enough in a general way, although we cannot profess to have any insight into the detailed processes. The continuity of the germ-plasm brings about inheritance, in that it is continually handing over to the germ-plasm of the next generation the determinant-complex of the preceding one. Every new adaptation at any stage whatever depends on the variation of particular determinants within the germ-plasm, and this in its turn depends on germinal selection, that is, on the struggle of the different determinant-variants among themselves, and on the variation in a definite direction which arises from this, as we have already shown. A new kind of determinant can never arise of itself, but always only from already existing determinants, and through variation of these. But as spontaneous variation never causes all the homologous determinants of a germ-plasm to vary in quite the same way, but only a majority of them, there always remains a minority of the old determinants, which may, under certain circumstances, predominate again, as is proved by the aberrations in Vanessa species due to cold, and by many other kinds of reversion.

But it is not this variation which leads to the prolongation of ontogeny, and the repetition of the phyletic stages within it. In this case it is rather that a new character takes the place of an old one, not that it is added to it. A black spot may arise instead of a red one, but not first a black spot and then a red one. Of course we still know far too little in regard to the intimate succession of events in the stages of ontogeny to be able to say definitely that, in such apparently simple transformations, the older stage does not, in every ontogeny, precede the more recent one as a preparation for it, though it may be only for a brief and transient period.

It is certain, however, that variations such as the addition of a new stage in ontogeny are undergone, and that this implies the occurrence of something really quite new. Therefore such a new stage can arise only from the germ-plasm, by the duplication, and in part variation, of the determinants of the preceding stage. If, for instance, the body of a Crustacean be lengthened by a segment, this must be due to a process of this kind, and in such a case it is intelligible enough that the new segment can be formed in the ontogeny only after the development of the older preceding one, for its determinants come from that, and are from the beginning so arranged that they are only liberated to activity by the formation of the preceding segment.

Now, if in the course of the phylogeny numerous new segments were added to the body of the Crustacean, the ontogeny would be materially prolonged, and condensation would become necessary in the interests of species-preservation. To bring this condensation about, whole series of segments which were added successively in the phylogeny succeeded each other with gradually increasing rapidity in the ontogeny, until finally they appeared simultaneously: the determinants of the segments n, n + 1, n + 2, ... n + x varied in regard to their liberating stimuli, and were roused to activity no longer successively, but simultaneously, in the cell complexes controlled by them. We have thus recapitulation, but with abridgement and compression, of the phyletic stages in the ontogeny. Thus in the nauplius of Leptodora we see the rudiments of five of the pairs of legs of the subsequent thorax ([Fig. 111], IV-VIII), and in the Zoæa larva the rudiments of six thoracic legs may be seen behind the already developed swimming-leg ([Fig. 114], VI-XIII).

But in the course of the phylogeny a segment may also become superfluous, and we know that it then degenerates and is ultimately eliminated altogether. Thus in a parasitic Isopod, which lives within other Crustaceans, a segment of the thorax is wanting in the relatively well-developed larva, and in the Caprellidæ among the Amphipod Crustaceans the whole abdomen of from six to seven segments has degenerated to a narrow, rudimentary structure. In such cases the gradual degeneration of the relative determinants has preceded step for step the degeneration of the part itself, and when this is complete the ontogeny shows nothing of what was previously present, and so we may speak of a 'falsification' of the phylogeny. But that the complete disappearance of the determinants only comes about with extreme slowness, so that whole geological periods are sometimes not enough for its accomplishment, we have already learnt from our study of rudimentary organs, instances of which can be demonstrated in every higher animal, bearing witness to the presence of the relevant organs or structures in the ancestors of the species.

We can infer with certainty, from the observational data at our disposal, that the disappearance of useless parts is regulated by definite laws; but it is too soon to attempt to formulate these laws, or even to trace them back to their mechanical causes. As we have already said, a much more comprehensive collection of facts, and above all one which has been made on a definite plan, is a necessary preliminary condition to this. But so much at least we may gather from the facts before us, that the degeneration of an organ begins at the final stage, and is transferred gradually backwards into the embryogenesis. Thus the two fingers of birds which have disappeared since Cretaceous times are still indicated in every bird-embryo, though they subsequently degenerate. In various mammals 'pre-lacteal tooth-germs' have been demonstrated in the jaws of embryos, which show us that not only did ancestors exist whose dentition was the modern 'milk-teeth,' but that still more remote ancestors possessed another set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth of the ancestors of the modern right whale (Balæna mysticetus) are only represented in the embryo of to-day in the form of dental pits. And, as we saw already, the Os centrale so characteristic of the wrist of lower vertebrates only appears in Man at a very early embryonic stage, and disappears again as such in the further course of the embryogenesis.