Science in Christian Perspective

BIOLOGY
Irving W. Knobloch, Ph.D.

Recapitulation may be regarded as fundamentally the result of the necessary passage from simplicity to complexity, from low to high organization, which is entailed by the metazoal sexual system of reproduction, with its single egg cell. The ~etention of visible organs or structures from lower ontogenies in a given ontogeny is only a special case of this general rule and probably depends on the presence in them of essential formative stimuli.-Joseph Needham, 1930

After some fifty years of advances in Biology, the basis of the Theory of Recapitulation, as formulated by Haeckel, can be shown to be unsound. Traditionally, this theory, or the Biogenetic Law, as it is sometimes called, stated that the individual in its life history passes from zygote to adult through a sequence of stages which correspond to the adult forms of its ancestors. Haeckel summed up this doctrine in the familiar phrases, "ontogeny recapitulates phylogeny". These sayings have adorned Biology texts for the better part of the first half of this century, showing how the notions of ancestry, heredity, and development were united in a three-fold manner: the ancestors created, heredity transmitted, and development repeated, the adult characteristics of past organisms.

The origins of this idea can be traced as far back as Aristotle, whose "scale of nature" captured the imaginations of biologists for centuries to come. He did not express recapitulation in its recent definitive form, to be sure, but the essence of it can be f ound in his writings. This was an ideal representative of the animal kingdom, arranging the adult forms of many species in a successive order with man as the pinnacle, and, as such, can be found in the writings of such men as William Harvey, John Hunter, and Charles Bonnet, as far along as the eighteenth century.

Gradually, though, the idea of comparing this animal series with the stages of individual growth became a natural one, and by it there were revealed similarities between the two successions. This was called the "law of parallelism" and was advocated by such continental figures as Serres, Kielmeyer, and Oken. It was widely prevalent until the time of Ctivier, and was looked upon favorably by Kant, Schelling, and Goethe, who were advocates of Natur-Philosophie, which had a strong following in Germany in the eighteenth and part of the nineteenth century. This school of thought had held that there is no gap between the inorganic and organic, so that nature is an organism which shows a graded series of forms from matter to man. Out of this thinking came the notion of an "ideal type", which does not exist in nature as a thing in-itself, but which is mirrored, though often indistinctly, in the life of every organism. As such, this conception of the animal kingdom, so different from the modern Linnean approach, was to be a, significant influence on the writings of Haeckel.

The first serious objection to this manner of thinking came from von Baer, who established the "principle of deviation" (1828) in place of the Meckel-Serres law of parallelism (1811-1834). On the basis of careful observations, he proposed the generalization that the similarity of two embryos of both higher and lower animals is dependent on the closeness of their kinship, the embryos of both higher and lower animals sharing a closer mutual resemblance than the mature forms.

Soon afterward, Fritz Muller became convinced, when working out the lif e-story of the Crustaceans, that the development of an individual is a historical document, with new stages added at the end. And so when Haeckel arrived on the scene (1866), the essential components of the idea were already in use, and he had but to revise and modify them in forming his famous dictum. According to him, the ancestral history is the actual cause of an individual's characteristics of development, so that phylogeny becomes the mechanical cause of ontogeny. Thus Haeckel attempted to combine the theory of parallelism, which is based on the Aristotelian idea of a scale of beings, and von Baer's theory of divergence, which says that the general characters of a large group of animals appears earlier in development than do the more specific. Similarities of structure he attributed to the actions of a common ancestry, and dissimilarities were said to be due to dif f erent lines of genealogical descent. And he explained the failure to discover adults corresponding to distinct embroyological stages as the extinction of ancestral species through natural selective agencies.

3. Critique of the Problem.

Now the evidence for Recapitulation, at first pause, is very great, for, indeed, there are many reminiscences of earlier types in embryonic development: There are many reminiscences of earlier types in embryonic development: There are distinct pharyngeal clefts in early stages of vertebrate growth that easily resemble the respiratory structures of fish; one can also find a tubular piscine heart, a cartilaginous endoskeleton, and a notochord all of which have their lower counterparts; the flat pleuro-nectid begins life with a shape just like any other fish, with its two eyed placed summetrically on either side of its head; and the limpet embryo first has a spiral gastropod shell which it later discards in favor of the adult conical variety. These are but a few easily found observations and they are incontrovertible.

In approaching this problem of why ancestral structures are repeated, it would seem that Zoologists sometimes tend to interpret phylogeny in terms of a morphologically graded series of contemporaneous types, with less emphasis placed on the palaeontological data. Haeckel made very little reference to such evidence as might be accumulated from the rocks, in a typical Platonic fashion, and he built up his phylogenies usually on ontogenetic data. Thus, his views differed from the Aristotelian parallelism largely by an extrapolation in time to make them conformable to the Darwinian point of view. The variations he recognized gave him trouble, to be sure, and these he called caenogenetic, if they could not be explained, as though they were vitiations of the ancestral record.

On this view, one would almost suspect the embryo of being disrespectful of its distinct ancestors, and had taken on the habit of playing games with the observer, if one were to fit the discrepancies in the record into a too strict Haecklian formulation. Thus there has appeared a rather complex vocabulary which is designed to explain the behavior of ontogenies when compared with the corresponding phylogenies, including "heterochrony", or the reversal of stages in evolution, "tachygenesis", or the shortening of parts of the life-history, and "lipogenesis", the dropping out of stages. But these are conceptual phenomena through and through, and, are arbitrary applications of an idealistic phylogeny upon biological units which operate in a space-time manifold. It cannot be established that an organism "drops out", or "shortens", or 'reverses" certain sequence in its life-history, but rather, it proceeds from start to finish in the shortest and most convenient manner.

But these emphases on the inadequacies of the Recapitulation Theory do not explain it. Indeed, the most fundamental question is why there is a repetition at all.

With the advent of the axial-gradients by Child in 1915, and the organizer theory of Hans Speman in 1928, there have come renewed insights into the interdependencies of the parts of living organisms in space and time. Certain embryonic structures, as the dorsal lip of the blastopore, have the ability to induce differentiation in surrounding tissue. The pronephros must be present to insure the appearance of the mesonephros and metanephros; and the notochord functions in laying down the vertebral column. And so many of the so-called embryological vestiges may have vital roles to fulfill in individual development. In many other ways the appearances of recapitulation could result from the conservatism in the methods of organ function and formation.

It will be recalled that Haeckel maintained that the ancestral adult stages were pushed back in the individual ontogeny. But he could not realize that this is practically impossible to explain genetically, since gene actions are manifested throughout ontogeny. A simple explanation for what he thought were the appearances of ancestral adult stages would be that the changes are not easily recognized in early periods, so that the tendency of new characters to appear in later development would be a statistical consequence of increasing complexity.

Then, too, the principle of the multiple effects of simple genes may be applied to explain the appearance of vestigal organs. There may be one gene which pleiotropically causes the primordium of an ancestral organ to be laid down, in addition to having other effects. Possibly the most interesting insight into the genetic aspects of the problem* has come from the work of D'Arcy Thompson and Huxley who point out how many ontogenetic phenomena may be explained in terms of genes controlling growth-rates.

As an example of bow the view-point concerning Recapitulation has been shifting since the turn of the century, is the following discusison of this theory (Shumway 1932:98) :

recapitulation, as well as the logical difficulties arising from an examination of the theory itself, seems to demand that the hypothesis be abandoned. Those of us who were reared in the phylogenetic tradition may see it go with a sigh of regret. Those of us charged with the responsibility of expounding the law of evolution to our classes will miss a familiar maxim, early learned, and a convenient skeleton on which to hang the discrete data of embryology. But there can be no excuse for continuing to impress plastic minds by means of discredited generalizations. Let us rather return to the laws of von Baer and explain the resemblances which these describe in terms of processes rather than of precedents. The resemblances may indicate close evolutionary relationships. Or they may result from convergent evolution. Only the methods of comparative anatomy, itself deductive, or palaeontology can indicate which. It is the function of the embryo to become an adult without looking backward on ancestral history. It is the business of the embryologist to describe the phenomena which he observes in terms of individual development without undue attention to what can be interpreted at most as reminiscences of evolution.

Recapitulation, thus, has turned out to be something quite unlike what it was first thought to be. It is not the mystical expression of some creative force in nature as the Nature-Philosophers lead by Kielmeyer, had thought; nor the mechanical pushing back of characters, as Weismann would have it; neither is it a simple expression of heredity, as Haeckel has obscurely maintained. The phenomena of recapitulation cannot be deduced from some simple general law, but rather they must be viewed as occurring in an organism developing in the most efficient manner possible, each stage of which is placed with careful reference to future development, as an individual expression of relative growth, and the fitting in of genetic actions into the life of the individual.

References

Needham, J. 1930. The Biochemical Aspect of the Recapitula tion Theory . Biochemical Reviews, 5:142-158.
Shumway, W. 1932. 1932. The Recapitulation Theory. Quar terly Review of Biology, 7:93-99.