Why should not the law run: the whole ancestral series must be reproduced in the development of each individual organism? We are now in a position to see the whole bearing of Haeckel’s idea, and at the same time to appreciate his careful restrictions of it.
First, let us see a little of the history of the matter. In the first third of the nineteenth century a number of pre-Darwinian ideas of evolution flitted about like ghosts in natural philosophy, as I have already said. The evolutionary ideas of Goethe and Lamarck are well known to-day. Another thinker of great influence was Lorentz Oken, who established the custom of holding scientific congresses. Oken had been constantly occupied with embryology, the science of the development of the individual organism. He was at all events acquainted with all that was known at the time on the subject. I open an old volume, wretchedly printed on blotting-paper, of Oken’s General Natural History for all Readers (1833), and turn to a passage in the fourth volume (the first to be issued) on page 470.
We read that the caterpillar of the butterfly resembles the animal form at a stage of development that lies below the insect—the worm. Oken says: “There is no doubt that we have here a striking resemblance, and one that justifies us in thinking that the development in the ovum is merely a repetition of the story of the creation of the animal groups.” Oken was quite aware that the chick in the egg had gill-slits like the fish. He bases his idea on that fact. He was very close indeed to the theory that Haeckel has so wonderfully elaborated. However, he was greeted with laughter. His theory was treated as an absurdity from 1833 to 1866. It cannot be denied that he was himself partly to blame for this. Oken made two serious mistakes. On both points Haeckel is perfectly clear and sound. Moreover, the theory of natural evolution that made it possible for us to speak of “ancestors” was still a Cinderella in the days of Oken. No sooner was it rehabilitated than the principle of the old theory of embryonic forms returned once more.
Darwin himself at once appealed to it, but it was reserved for Haeckel to develop its full importance. He corrected it in two particulars. Oken and his admirers had made an unfortunate mistake. They believed in a genealogical tree of all living things, but they conceived it on the lines of the old classification. Linné had enumerated in succession: mammals, birds, amphibia, fishes, insects, and worms. He put them in one straight line, which is certainly the best arrangement for general purposes. But when Oken came with the idea of natural evolution, he at once took this series as the outline of a genealogical tree. The mammals descended from the birds; the fishes from the insects; and so on. If that were really the case, the highest animals would be expected to reproduce all the animal and plant stages in the course of their embryonic development, on the lines of the theory. The human being would have to be, successively, not only a lizard and a fish, but even a bird, a beetle, a crab, and so on. This was by no means borne out by the facts, and so the theory seemed to be discredited.
Now let us glance at Haeckel’s genealogical tables. We find eight of them, artistically drawn, at the end of the second volume. The “genealogical tree” is given in the form of a branching tree, or as a huge forest-like growth of stems some of which only meet in the ultimate roots. There is no trace in Haeckel’s designs of the sort of Eiffel-Tower arrangement that the Linnean system involved. At the bottom we find the protists, the most primitive forms of life. From this point two parallel stems diverge, that of the animals and that of the plants; they never touch each other after this point, and so cannot be expected to be reproduced in the embryonic forms. Then the animal stem is split up almost at the root into at least five independent branches, each of which pursues its separate line of development. One culminates in the insects, above the worms and the crustacea. A totally independent stem issues in the vertebrates, and this in turn breaks into many different branches. Beyond the lizards, for instance, we find the development of the mammals and birds, which run on as separate and parallel lines. It was mere nonsense to expect a mammal in its embryonic development to assume the form of a bird, or a crab, or a beetle, or a mussel, or a medusa, even if the biogenetic law were established ten times over.
The second mistake made by Oken was to declare that, whatever it cost, the law must be observed everywhere. He examined the butterfly. It passed through two curious embryonic stages: first the caterpillar, then the pupa. The caterpillar corresponded to the worm; that might be plausibly contended. But the pupa also must stand for something. Between the worm and the insect in classification was the crustacean. It had a hard shell: so had the pupa. Consequently, the pupa is a reproduction of the crustacea-stage. Such were the bold chess-moves of the older theorist.
Haeckel first established that there was such a thing as the biogenetic law. There is a fundamental norm, which is made clear to us in embryology and can at the same time (remember the instance of the lizard-like teeth in the bird-embryo) give us most wonderful suggestions as to the line of ancestral development. But it has certain limitations, as we will now show.
The adaptations in the sense of the Darwinian laws have affected the animal’s embryonic life more and more, the higher the tree of life grew. The long recapitulation of the ancestral stages often came into conflict with the young individual’s need for protection. The result was that the biogenetic law found itself restricted by the Darwinian laws of adaptation. The too lengthy succession of ancestral portraits was abbreviated and compressed. Whole stages of embryonic or larval development were interpolated that had nothing to do with these ancestral portraits, but were destined for the protection of the fœtus. The butterfly-pupa is really an instructive instance of this description. It does not reproduce a crab-stage, nor has there been any stage in the ancestry of the butterfly when they lived throughout life in pupa-houses. The pupa is simply a later adaptation in the development of the butterfly, a protective stage in which it accomplishes the transition from the caterpillar-form in much the same way as the young bird develops under the protection of the hard egg-shell. Thus only a faint and shadowy trace has been left of the real ancestral forms, though this trace is an extremely instructive one. But we must not expect the impossible from it. In this way our naked and crude biogenetic law assumes a more finished and scientific form: the embryonic development of the individual is a condensed, abbreviated, and to some extent modified epitome of the evolutionary history of its ancestors. That is more modest, but it is a correct expression of the facts. The essential point of the older idea was not in itself wrong; all that was done was to explain the gaps, and leaps, and contradictions in it.
Now that Oken’s share in the theory has been properly appreciated, we may notice another little historical detail. In the period immediately after his time these ideas were ridiculed by men of science, great and small, but they were not exactly “done to death.” Agassiz, the most pronounced creationist and dualist of all the nineteenth-century zoologists, expounded them occasionally as a curious instance of the divine action. In fact, he looked upon the whole of zoology as a mystic cabinet of curiosities—the more curious the better. Thus he came to play with this idea and confirm it, but merely took it at first as a fine figure of speech. Agassiz is a tragical form. He survived Darwin, much in the same way that many an elegant mot-de-salon on the rights of man survived the French Revolution. Suddenly the whole structure of his ideas seemed to fall about him. Where he had played with roses, he now found torches. He reeled like a smitten man, and cried out against the horrid monsters that brought him pain and bitterness. His anxiety began with Darwin, even as regarded the question of the embryo. But there was another, a man far away in South America, that increased it—Fritz Müller.
Born in 1822, one of the finest pioneers in zoological work, Fritz Müller had wished to become a higher teacher, but had abandoned his plan on account of the oath that had to be taken by every servant of the State. In 1849 he wrote to the Ministry requesting that he might be allowed to dispense with the formula “So help me God, through Jesus Christ.” Meeting with a refusal, he went to South America, and began a solitary life as a student in the primitive forest, and sought to accumulate valuable zoological material. Darwin called him “the king of observers.” In 1864 he published an essay of ninety-four pages with the title For Darwin. He revived and improved the old idea of Oken’s and made fresh contributions to the natural history of the crustacea that were literally stupefying. We may say that the point that he believed he had established, in virtue of the law, in regard to the genealogical tree of the crustacea, was afterwards, with apparent justice, called into question, even by supporters of the law such as Arnold Lang. That, however, did not diminish the extent of his influence at the time. Haeckel has generously acknowledged how strongly he felt that influence himself. Nevertheless all that has been said about Haeckel’s priority in fully applying and shaping the law, and in its final formulation, is perfectly correct.