This original development from the ovum, first into an embryo with the form of a small globe or, more correctly, an oval body consisting of two layers of cells and having a hole at one pole—in other words, a creature with nothing but skin, stomach, and mouth—was found, curiously enough, in other animals besides the medusæ, corals, and sponges. We have the same course of development in representatives of the most varied groups of animals. There are worms, star-fishes, crabs, and snails that develop in the same way. In fact, it was proved in this very year (1867) that the lowest of the vertebrates, the amphioxus (or lancelet), develops in the same way. And this was not all. In the ontogeny of all the higher animals right up to man (inclusive) we find a state of things that most closely resembles the same development. At all events, the fertilised ovum gives rise in all cases to a cluster of cells; this cluster forms something like a flattened or elongated vesicle with a single-layered wall; the single layer of cells is doubled, and in the building up of the body one half makes the external coat or skin and the other half the internal lining or membrane. Haeckel reflected on the whole of the facts, and drew his conclusions. This very curious agreement in the earlier embryonic forms must be interpreted in terms of the biogenetic law. In the case of the higher animals the forms have been profoundly modified by cenogenesis. In the lower animals they are almost or altogether a pure recapitulation of the real primitive course of the development of the animal kingdom. In the earliest times animals were evolved in something like the following way. First, the primitive unicellular protozoa came together and formed crude social bodies, clusters of cells that kept together, but had no special division of labour. As all the members in the cluster pressed to the surface, in order to obtain their food, they came to form, not a solid mass of cells, but a hollow vesicle with a wall of cells. Then the first division of labour set in. Certain cells, those that were situated at the anterior pole, and so were better placed to receive the floating food as the animal moved along, became the eating-cells of the group; they provided nourishment for the others, as the nutritious sap circulated through all the cells in the cluster, as we find in the case of the siphonophores. As these feeding-cells multiplied rapidly at the fore part of the animal, a depression was formed at that pole of the body. In the end the ball or vesicle was doubled in upon itself, until it came to have the form of a cup with a double-layered wall. Externally were the cells in the skin that effected movement and feeling, and afforded protection; inside, forming the internal wall, were the eating- or stomach-cells. An opening remained at the top—the opening of the cup or vase-like body. The food entered by it: it was virtually the “mouth.” Thus was formed a primitive multicellular animal with division of labour. If we imagine it attaching itself to the bottom by its lower pole, we can see that it would easily become a sponge of the simplest kind, a polyp, a coral, or, detaching itself once more, a medusa. If we imagine it swimming ahead in the water or creeping along the ground in such a way as to assume a bilateral symmetrical structure, like a tube, with right and left, back and belly, and an anus behind, we have a worm. This worm developed, under the action of the Darwinian laws, into a star-fish in one case, a crab or insect in another, a snail or mussel in another, and lastly into the amphioxus, which led on through the vertebrates to the human frame. But the mysterious series of forms always remained in the development of the individual from the egg, pointing more or less clearly to the earlier stages: ovum, cluster of cells, ball, two cell-layers in a cup-shaped form, skin, stomach, and mouth. All animals that exhibit this primitive scheme belong to one great stem. It was not until this skin-stomach-mouth animal was formed that the tree branched out—evolving into sessile, creeping, swimming, and other forms. Let us give a name to this phylogenetic (ancestral) form, which stands at the great parting of the ways in the animal world, as embryology proves. Leaving aside its innumerable relatives in the primitive days, it must have differed essentially from all other living things at the time—all the protists and the plants—by its possession of a skin, stomach, and mouth. Gaster is the Greek for stomach. Let us, therefore, call this primitive parent of all the sponges polyps, medusaæ, worms, crustacea, insects, snails, mussels, cephalopods, fishes, salamanders, lizards, birds, mammals, and man, the gastræa, the primitive-stomach or primitive-gut animal. The corresponding embryonic form may be distinguished from it as the gastrula. There are still many living species of animals that are very little higher in organisation than the gastræa-form. The Pemmatodiscus gastrulaceus, discovered by Monticelli in 1895, corresponds entirely to it. And the gastrula is found, as I said, with astonishing regularity in its precise gastræa-form in representatives of all the higher groups of animals.
That is an outline of the famous gastræa-theory, that Haeckel discovered when he was engaged in studying the calcisponges. It was first published in his large Monograph on the Calcispongiæ in 1872, elaborated in his Studies of the Gastræa-theory in 1873, 1875, and 1876 (published in one volume in 1877), and generally expounded, together with the biogenetic law, in (amongst other works) his polemical essay, “The aims and methods of modern embryology” (1875). This discovery, in Haeckel’s opinion, now made the biogenetic law a real search-light in the exploration of the obscure past. It indicated a third critical point in the great genealogical tree. Already we had the root (the monera) and the crown (man); now we had the point from which the various real animal stems radiated like the umbellate branches of a single large bloom. Through it the Darwinian system had been converted into the greatest practical reform of animal classification. If this gastræa-theory was correct, it was an incalculable gain for zoology. The difficulty of it, on the other hand, lay in the infinite modifications of the embryonic processes in detail that had been brought about by cenogenesis; almost everywhere this had more or less obscured the original features. On the whole it gave rise to the greatest and most far-reaching discussion that has taken place in zoology for the last thirty years, apart from the Darwinian theory itself. To-day, at the close of these three decades, there are only two alternatives. One is that there is still an absolutely mysterious and hidden law of ontogeny, that compels countless animals over and over again to pass through these embryonic forms and assume a likeness to the gastræa. After all the eagerness with which the whole school of embryologists opposed to Haeckel have sought, up to our own day, to establish such a direct law, we have not yet got the shadow of a clear formulation of it. The other alternative is that Haeckel is right in believing that he has discovered the correct formula in his phylogenetic interpretation of embryonic processes in accordance with the biogenetic law. If that is so, the gastræa-theory is the crown of all his labours in technical zoology proper. Let us wait another thirty years.
The scientific controversy over the gastræa-theory was in full swing when Haeckel entered upon another bold experiment in the direction of the biogenetic law. He thought it would be useful, instead of framing wider hypotheses, to take one single instance of one of the highest animals, and trace the whole parallel of its embryonic and ancestral development down to its finest details. It would serve as an excellent object-lesson. He would take it, not from some remote corner of the system, such as the sponges or medusæ, but from the very top of the tree, where palingenesis and cenogenesis seemed to have culminated in an inextricable confusion. But what example could be more appropriate and effective than the most advanced of all living things—man. He would write a monograph on man on an entirely new method; would show ontogeny and phylogeny confirming each other down to the smallest detail. It was another great enterprise. And this particular subject was so interesting that it would appeal strongly to the general readers of his History of Creation as well as to the academic scientists. Man was a subject of such obviousness and importance to the layman that in this case there was really no professional limitation of interest at all. Every detail in the most technical treatment of the subject would be taken into account, and evoked his strongest sympathy.
When Haeckel had fully matured this plan, he produced his Anthropogeny.[[6]] The word, founded on the Greek, means the “genesis” or “evolution of man.”
[6]. The fifth edition is translated into English, with all the plates and illustrations, under the title of The Evolution of Man. [Trans.]
The work is a very able combination of two different aims. On the one hand it affords the technical student the outline of a wholly new and distinctive manual of human embryology (up to a certain extent) and general anatomy; and this is intimately bound up by his method with a kind of historical introduction to general anthropology. At the same time the book forms a second part of the History of Creation. It builds up the most important chapter of the later work, from the philosophical point of view, namely, that which deals with the origin of man, into a fresh volume; and it represents the first popular treatment of embryology on broad philosophic lines—a thing that had never been attempted before. Springing up from this double root, the work is certainly one of the most successful things in the whole of Haeckel’s literary career. Moreover, it is not merely a compendium of a larger work, like the History of Creation. In spirit and form it is an original work, and gives his very best to the reader. As far as its general effect is concerned, the double-address of the work has had its disadvantages. The academic students who were hostile to it have once more selected for attack certain excrescences and gaps that were merely due to the exigencies of popular treatment. On the other hand, the general reader found it, in spite of the popular form, on which Herculean labour had been spent—one has only to think of the details of embryology—a book that was not to be “read” in the ordinary sense of the word, but studied. The first edition appeared in 1874. A fifth edition has now been published, equipped with the finest illustrations, both from the artistic and the scientific point of view, that have ever appeared in a popular work on embryology. We find in the Anthropogeny all that the nineteenth century has learned or surmised with regard to the ancestral history of mankind. Even the gastræa-theory—the gastræa belonging to man’s direct ancestry—is dealt with in popular fashion as far as this was possible.
When the Anthropogeny was published Haeckel’s public position became more stormy than ever. In professional circles a number of the embryologists had taken up an attitude of opposition to him; the most heated of them attacked his popular works continually on the ground that he was popularising, not the real results of official science, but his own personal opinions. There was a great deal of truth in that. The only question was, which would stand best with the future, his or their personal opinion? It does not alter the subjectivity of opinions that a few people here and there combine and pretentiously constitute themselves into a “science.” Posterity will deal coolly enough with their collective decisions. It will take every man of science as an individual, and merely ask which of them came nearest to the truth. The name, the official science, will pass into the grave with many titles and decorations. All that will remain in men’s minds is the star of the personality in its relation to the great constellation of contemporary human truth. However, as regards the particular embryological attacks of these opponents, it seems to me to-day especially characteristic that such people are more and more abandoning the idea that it is only a question of contesting certain particular deductions of Haeckel’s within the limits of Darwinism. They find themselves increasingly compelled to throw Darwinism overboard altogether. Instead of its attempts to explain phenomena they are putting forward a confused claim of “direct mechanical explanations,” or relying on the sonorous old phrase, started in 1859, an “immanent law of evolution,” or retreating into a despairing attitude of “I don’t know.” These clearer divisions will make it very much easier for posterity to pass its judgment on the situation.
After the embryologists we have a considerable group of opponents on the anthropological side. The objections of these anthropological critics have in the course of time narrowed down to the single argument that no transitional form between man and the ape has yet been discovered. And for many years now this position has not been held on serious scientific grounds, but rather on ingenious and strained hypotheses. Because we now have, in the bones found at Java by Eugen Dubois in 1894, the remains of a being that stands precisely half-way between the gibbon and man. Hence what is called the anti-Darwinian and especially anti-Haeckelian school of anthropology to-day is mainly distinguished for its preference of more risky and more subtle hypotheses instead of plain conclusions from obvious facts. Finally, there is the theological opposition to Haeckel that increased with every book in which he put his ideas before the general public and helped them (in their boundless professional wisdom) to realise the danger of the situation.
The year 1877 was a critical one in this respect. In the middle of his struggles Haeckel retained all the simplicity of his nature. He saw that the idea of evolution was triumphing over all obstacles and rapidly securing the allegiance of the best men of the time. On the 18th of September, 1877, he spoke of this with unrestrained delight at the scientific congress at Munich. He described the theory of evolution as “the most important advance that has been made in pure and applied science.” Then Rudolf Virchow delivered a speech at the same congress.
There is no doubt whatever that in the period since Virchow had indicated a neutral field in 1863, in which science might effect “its compromise,” Haeckel had boldly invaded that province. In the previous year he had published a little work called The Perigenesis of the Plastidules, or the Generation of Waves in Vital Particles. It was delivered in lecture-form at the medical-scientific congress at Jena in November, 1875, and then printed on the occasion of Seebeck’s jubilee, May 9, 1876. Possibly it is the least known of all Haeckel’s works, though in my opinion it is one of the most valuable in regard to the prophetic breadth of its intuition. It essays to establish a theory of heredity. In dealing with this deepest mystery of life psychic factors are pressed into service without reserve. Not only is the cell-soul put into prominence, but the cell in turn is resolved into a number of smaller units, the plastidules. Each plastidule is then conceived as a psychic unity. The souls of the plastidules are endowed with memory; that is the root of heredity. They learn; that is the psychological expression of adaptation. The little work offers a suggestion of a psychology of Darwinism that may very well become the nucleus of the whole Darwinian structure in the twentieth century. But at the time it was quite obvious that a man with such ideas as these was breaking with lusty fist through the sacred net that spread before Virchow’s reserved province. The hour had come, therefore, for Virchow to feel that he must expel the idea of evolution from the whole field of science, and not merely from embryology and anthropology.