The History of Biological Theories

Darwin took up these ideas anew. He again asserted that the animal is characterized, not by its form but by its method of life, and that all the qualities of living organisms are the direct result of their interaction with the environment. It was, therefore, natural for him to devote little attention to morphology. His attention was focused upon the conditions under which life exists, and when he put forward what he believed to be a higher view of Nature it was certainly a more novel one. He tells us how the old conceptions aroused by such phrases as 'a vertebrate animal', 'a mammal', 'a species', 'unity of plan' dis- appeared. They were replaced by new pictures, as the animal in its greedy search for food, the universal struggle for existence, interbreeding between organisms. With these new colours Darwin repainted the face of Nature.

Haeckel did not attain to Darwin's vision, but he was full of enthusiasm for Darwin's pictures because of the newness of their colours. These colours alone attracted him, and remaining faithful to the older conceptions of science he spent himself in introducing a new nomenclature more consistent with the doctrine of evolution. Phylogeny and evolutionary morphology were begun by Haeckel and Gegenbaur; they still contained the ideas of the older morphology, but these ideas were expressed in new terms.

If we listen to Haeckel's words we find this difficult to believe, but his deeds make it perfectly obvious. First of all, hear his words. 'All the facts concerning the morphology of any organism, concerning its anatomy and its development, the tectological as well as the promorphological facts of its anatomical structure all these are the inevitable out- come offerees which work according to definite mechanical laws' (Generelle Morphologie). This suggests that if we could only study those mechanical forces, if we could visualize their action, our vision would develop into a picture of the organism. Haeckel made no attempt to do this, however. He gave us not the faintest clue to the mechanism that could produce an amoeba, a moneron even, still less a man. We cannot discover these 'mechanical causes' with the aid of the microscope, the scalpel, or even by philosophy ; and these were the methods of study which Haeckel had taken over from his teachers the idealistic morphologists.

Apart even from this, we can trace the influence of his predecessors in the words quoted. Haeckel points out, it is true, that 'physiology and psychology will have to receive genetic explanations'; in practice, however, he only applies his explanations to the facts of morphology and embryology, the two branches of biology which were being actively studied at that time. In this new 'Darwin- ian morphology' of his he does not suggest that two animals, or two plants, are related because they have been formed by the action of the same mechanical forces, nor because they have been acted upon by a similar environ- ment, but because their structures, both as embryo and as adult, are similar. Is not this the method of Cuvier, of Goethe, of Geoffroy the old 'comparative method' ? Haeckel used all these ideas, nor did he endow them with any wider meaning, though he asserted that 'relationship' had only meant 'similarity' in the past, and that, if we follow Darwin's model, it will now signify a true blood- relationship.

Homologous organs, Haeckel said, were, according to the earlier view, merely those which were similar in structure, while analogous organs had similar functions. Now, he claimed, we look upon homologous organs as those which are descended from a common ancestral organ, while analogous organs represent similar adaptations to a common environment. But how does he recognize these blood-relationships, these inherited and adaptive structures ? Simply by comparative methods, just as formerly similarities, homologies, and analogies were recognized.

Haeckel called this morphology, redressed in a genetic terminology, phylogeny. Phylogeny is the study of the evolution in time of the various plant and animal races (Greek phy Ion = race). He admits quite openly that his phylogeny is nothing more than the old systematic biology. Haeckel's phylogeny follows in practice the following fundamental rules. The nature of an organism is revealed by its form. By comparing both fully developed and embryonic forms we find that they have certain characters in common. We collect these characters and form an abstract whole, a plan. The animal which reveals this plan in its most elementary form is called the 'ancestor' of the organisms we are comparing, or is 'the type nearest to the ancestral form*. For example, a group of animals is found to possess a common character they are all unicellular. The amoeba and the infusorians belong to this group, but the amoeba has no definite organs, while the infusorians possess certain specialized structures. Hence the amoeba more closely resembles the primitive animal form, and the ancestors of all animals were there- fore like an amoeba.

This was how Haeckel drew up his genealogical trees schemes of classification arranged in tree-like form, the lower parts representing ancestral types, the branches younger and younger descendants until, at the ends of the smallest twigs, the names of those animals presumed to be the most modern were written. Haeckel hoped by his genealogical trees to overthrow Cuvier's idea of 'types'. Cuvier taught that animals were formed on one of four definite plans. Darwin's theory had suggested another view: no longer were we to believe in a 'plan' for the animal organism ; each animal was simply a collection of individual characters which had evolved in the course

of time. But the abandonment of this idea of 'types' was theoretical rather than practical. The Darwinians believed that 'types' exist, even as Cuvier had done, and Haeckel acknowledges this quite openly. 'Darwin had assumed that there are a few fundamental types in both the plant and the animal world in each kingdom perhaps four or five such. 'In all our systems of classification we follow this idea. The 4-7 phyla or groups into which we still subdivide the animal kingdom are the groups which all zoologists have recognized since the days of Von Baer and Cuvier they have been called "plans", "principal forms", "branches", "groups", &c., &c,' (Natiirlicbe Schopfungs- geschichte).

The construction of genealogical trees was a favourite occupation of Haeckel. Most biologists adopted this method, and it became the custom during the seventies and eighties of last century to close every systematic or anatomical investigation with a genealogical tree; this was either drawn or described in words. Some condemned this method from the very beginning. In 1876 du Bois Reymond said that such trees would have about as much value in the eyes of posterity as has Homer's story of the pedigree of his heroes ! Huxley, who first liked them, later rejected them. In recent times these genealogical trees have been quietly abandoned by most biologists, and Haeckel is held up to ridicule as if he had been the only sinner. Haeckel is also accused of having drawn up a genealogical tree for Man. We forget that Darwin drew up one in his Descent of Man ; that Wundt published a tree showing the evolution of the sense organs ; that Gegenbaur published one for the vertebrate limb, Romanes for the human soul, Spenser for social customs, Schleicher for human speech. Open any text-book of anatomy, of zoology, of systematic biology written in the eighties. If it has any pretence to be called scientific it will assume that Haeckel's ideal, the genealogical tree, is the final goal of investigation.

The work of von Baer (1828) caused the idea that the organism, in the course of its individual development from the egg, passes through the lower animal forms, to be discarded by the biologists of the thirties and forties. They accepted Baer's idea, based on the philosophy of Aristotle, that development is merely an increasing differ- entiation an advance from the general to the special. This conception is closely linked with the idealistic view of life. It affirms, for example, that in the human egg the whole man is contained as a potentiality, an idea; that, during development, this idea assumes a more and more concrete form until it finds its complete expression in the adult individual. When investigators adopted the mechanistic and materialist view of life they ceased to understand Baer's point of view.

Then there was a very extraordinary change. Von Baer was still a famous man; no one criticized his ideas about development. But he was famous as the discoverer of the theory of Meckel and Serres the very theory that he had always opposed! It was Darwin who gave him this new fame. It is true that there is a superficial resemblance between the two theories. Von Baer said that the embryos of different animals are alike because they are developing on the same plan. Meckel asserted that, as it develops, every animal passes through the (adult) phase of certain of the animals below it in the scale of existence. Von Baer compared embryos with other embryos ; Meckel compared embryos with adult forms.

Darwin was probably the first to confuse the two views. In the Origin of Species he cites von Baer's statement that the 'embryos of mammalia, of birds, lizards, and snakes, probably also of chelonia, are in their earliest states exceedingly like one another' to prove his hypothesis that the animal, in the course of its individual development, climbs up its own genealogical tree. This view seems, it is true, to be different from that held by Meckel, for it refers to the phylogenetically older, while Meckel's is concerned with the systematically 'lower'. But, used in this way, 'older' and 'lower' are merely two words for the same idea, and the difference is only an apparent one.

Darwin's view soon began to bear fruit. Sir J. Lubbock, a merchant with cultured tastes and a near neighbour of Darwin's, set out to prove that insects, in the course of their development from the egg, pass through stages representing their own phylogeny. He soon recognized, however, that this idea could not be applied to the butter- fly group, for all the facts are against the assumption that the caterpillar was ever a fully developed organism. The theory was applied with greater success, for a time at least, to the Crustacea. The eggs of most Crustacea, like those of butterflies, turn into larvae, and the larvae undergo many changes of form before the adult form is reached. Fritz Muller, a German teacher living in Brazil, examined the larvae of many Crustacea, particularly that type known as Nauplius. He declared that this form does not appear in the larval stages of certain species, but that in those another larval stage occurs, the so-called Zoea stage. In his For Darwin (1864) he asserted that the Nauplius, the Zoea, and other crustacean larvae represent phylo- genetic stages in the ancestry of the higher Crustacea, and that from these we can see to-day what the ancestors of the modern Crustacea were like; that, in short, the present-day Crustacea repeat in their individual develop- ment the history of their race, though in an abbreviated form. In those forms in which the Nauplius stage is lacking, the process of development has become so cur- tailed that the Nauplius stage has been completely sup- pressed. Darwin appreciated the originality of Miiller's work, and had it translated into English. Haeckel saw even more in Miiller's method, and, following it, he evolved his fundamental law of biogenesis.

The Nauplius theory was not accepted for long. To- day the Nauplius is not considered of more value for elucidating the phylogeny of the Crustacea than is the caterpillar for throwing light on the evolution of the butterfly. If the Nauplius really represents an ancient form, out of which the modern Crustacea have been evolved, then those Crustacea with the simplest morpho- logy should be most like the Nauplius for the simplest should be the most primitive. But the structures that the simplest crabs have in common are more numerous than those they share with Nauplius (they all have, for example, segmented bodies). So that, according to this theory, the Nauplius must represent a more modern form than do the simplest crabs.

Before the Nauplius theory was abandoned A. Dohrn tried to make it more complete by adding the Zoea theory. He assumed that the Nauplius is the ancestral form of all Crustacea, while the Zoea is the form from which the higher members of the group have evolved. But this, too, had soon to be abandoned. By widening the application of the Nauplius theory Haeckel developed his general law. Every animal during its ontogeny that is, its development from the egg to its final form passes through the same stages as its race passed through in its evolution from the single cell. Embryonic stages, larval stages, and the like represent in the essentials of their structure features inherited from a time when the animal was less highly developed than it is to-day.

The law is only applicable to the more salient features of structure. The development of the embryo is very much shortened when compared with the evolution of the race ; in a few hours or a few days it passes through a series of forms which took millions of years to evolve. Hence the ontogenetic story is very much simplified. The individual life-history skips many stages which occurred in the ancestral progress; others are much altered and curtailed.

Man is developed from a single cell, the fertilized egg. This shows, says Haeckel, that the human race has been evolved from a single-celled organism. The human embryo develops, on the posterior portion of the sides of the head, rudimentary structures which resemble the gills of fishes ; so Man must have been, once upon a time, an animal that breathed by gills. On the other hand we cannot, from the fact that the human embryo is united to the uterus by the placenta, infer that some fully developed organism once bore a placenta. This organ represents an adaptation of the embryo to life in utero.

Haeckel called those embryonic organs which represent ancestral characters and have a phylogenetic significance Palingenetic (Greek = again produced) ; those which repre- sent recent adaptations to embryonic life, &c., are Caeno- genetic (Greek ^recently developed). As we follow the development of the embryo we must distinguish between the paliiigenetic and the caenogenetic features. The former tell us the history of the evolution of the organism, the latter how the animal and its embryo have been gradu- ally adapted to new conditions. 'Ontogeny', says Haeckel, 'is a brief and incomplete recapitulation of phylogeny.' If this be true we should be able to deduce the history of any living form from the study of its ontogeny. All that we have to do is to learn to distinguish between the palingenetic structures and the caenogenetic ones. Haeckel formulated the following rules for our guidance:

(1) The earlier an organ appears in the life of an embryo the older it must be. (2) When comparing any series of embryos or larvae we shall probably find they have certain characters in common. The more varied the adult types which have the same common embryonic structure the more likely is it to be true that the structure is one derived from a common ancestor. This was the method by which Haeckel attempted to provide embryology with a new terminology. An exami- nation of his biogenetic law will convince us that it could not lead us very far. For how does Haeckel seek to dis-

tinguish between palingenetic characters and the caeno- genetic ones ? Simply by the method of comparison, that is, by von Baer's method ; only, von Baer said 'homo- logous' instead of 'palingenetic', 'analogous' instead of 'caenogenetic'. Was anything else possible ? How could any one hope to read the past by studying what is happen- ing to the embryos of to-day ? If such a thing were possible we ought, from the study of modern philology, to be able to guess at the contents of the lost manuscripts of the ancient world; archives would lose their purpose if history could be reconstructed from the study of modern times. Biologists were led into error by the impassioned way in which Haeckel affirmed the truth of his theory. They did not notice how impossible his inferences were ; they set to work to collect facts which were to prove the validity of his biogenetic law, to see how far the law was capable of application to other fields, and to use the theory in support of Darwinism. 1

Ten years after the appearance of the Origin of Species Huxley published a monograph on the Medusae, in which he pointed out that their bodies are two-layered, and that a similar two-layered stage is invariably found in the embryos of vertebrate animals during the early stages of embryonic development. In both cases the layers stand 'in the same physiological relationship' to each other. In the Medusae the outer layer forms the epidermis and the muscle, the inner the digestive tract and the reproductive organs. According to von Baer, similar organs arise from the corresponding layers of the vertebrate embryo.

When Huxley's book appeared many other forms with two-layered bodies were known ; most marine larvae went through such two-layered stages ; from these the coelen- 1 An historical review and criticism of the fundamental law of biogenesis is given by T. H. Morgan in his Evolution and Adaptation^ New York, 1903, p. 60 sq.; also by Hurst in his 'Biological Theories', iii. : The Recapitulation Theory/ Nat. Sc., ii, 1893. terata, echinoderms, worms and molluscs develop. On these facts Haeckel built up his gastraea theory.

From the frequent occurrence of the two-layered gastrula stage he concluded that all many-celled animals pass through this stage in the course of their embryonic development. The gastrula may be pictured as like a rubber ball with one side pushed in. He concluded that the first stable form of the multicellular animal must have been like a gastrula. He called this hypothetical ancestor of all many-celled animals a gastraea. 1 All multi-cellular animals, then, are descended from the 'Gastraea'. These descendants developed along two lines ; the one form, Trotascus,' was the starting-point of the coelenterata ; the other was Trothelmis', from which the worms and all higher animals have evolved. Protascus was a free-swimming form, and retained, as did the coelen- terata, its descendants, the radial symmetry of the gastrula. Prothelmis, on the other hand, crept over the oor of the ocean; its descendants became bilaterally symmetrical.

We may quote Haeckel, and give the following phylo- genetic series to illustrate the further evolution of the two forms Protascus and Prothelmis. 2. (a) Coelenterata, in which the body remains two- layered throughout life. These comprise the lower forms, in which there is never any trace of a third layer ; (here are included the simple sponges, hydroid polyps, and simple jelly-fish); and higher forms, in which so-called 'mesoderm 5 cells begin to appear between the two primi- tive layers ; these cells never form a complete layer. Here are included the corals, the higher jelly-fish, and flatworms.

2. (b) Coelomata, in which there is, between the outer and inner layer, a space formed between a double middle layer; this space is the coelom. To this group all the higher animals belong. Haeckel's gastraea theory was soon attacked. Claus, a well-known Viennese zoologist, criticized it in the very year of its publication. He said, firstly, that, although almost all animals do pass through a gastraea stage in their development, this gastraea is not produced in the same way in all these animals, hence it is absurd to suppose that it represents some common ancestral type of organization. Further, there is absolutely no palaeontological evidence in support of the suggestion that Protascus and Prothelmis ever existed; the oldest-known animals are not radially symmetrical, as they should be according to this theory, but bilaterally symmetrical.

Many agreed with the first of these criticisms. Never- theless the theory was generally accepted, and for a time it dominated embryology. Each objection was explained by the assumption that adaptations to new environmental conditions had led to deviations from the typical gastrula form. The popular idea of the method of the scientist is that he assembles a series of definite facts, upon which he founds his theory. We see that this is not always the case. It is not true that the facts which told against the gastraea theory were unknown when the theory was propounded ; or that the theory was gradually discredited as the facts which contradicted it were gradually accumulated, until it had finally to be abandoned. Everything important that has ever been cited against the theory was known when the theory was first put forward ; nevertheless it was widely accepted. To-day some still accept it ; others do not. Those who have abandoned it have not done so because the facts contradict it, but because their attention has been diverted into new channels by the modern inquiry into the phenomena of regeneration.

The facts of regeneration are difficult to reconcile with the Gastraea theory. Some investigators, it is true, have endeavoured to show that, when an organism regenerates a lost limb, it does so by processes similar to those by which that limb was developed from the egg-cell. But regeneration is often a very different matter from em- bryonic development the processes involved are not the same, and yet the two eventually lead to the same result. It is possible to produce artificial mutilations without

Eaying the least regard to the 'fundamental biogenetic iw'. How, then, can this be as fundamental as Haeckel believed it to be ? In 1861, during the interminable controversy as to the significance of the cell theory, Max Schultze, a German zoologist, propounded his definition of the cell : 'a mass of protoplasm provided with a nucleus.' We might have ex- pected that investigators would now devote themselves to the task of trying to discover why bodies are composed of cells ; why every cell must have a nucleus ; why the cell can only increase by division, and so on. But the theory of evolution gave a different direction to the study of the cell. Darwin's followers held that questions concerning the ultimate nature of living phenomena savoured of metaphysics, although equally fundamental questions were being studied by the physicists. Science, they held, is only concerned with the collection of facts concerning struc- ture, with the sorting of types into evolutionary series, and with devising structures which would fill in the gaps in such a series.

It was in this spirit that Haeckel studied the cell. For him the cell, with its protoplasm and nucleus, was an advanced structure. He set out to imagine something still simpler, and distinguished between cells and cytodes, which consist of a substance (Plasson), in which nucleus and protoplasm are as yet undifferentiated, while he gave the name coenocytes to masses of protoplasm containing many nuclei. Organisms made up of cytodes were called Moneras by Haeckel. He asserted that large numbers of them exist and that they represent the first step in the pro-

cess by which non-living matter becomes endowed with life (Monographie der Moneren, 1868 ; Studien iiber M oner en und andere Protisten, Leipzig, 1870). Haeckel believed that monera 'crystallize' from non- living matter. These monera, together with the simplest protozoa, form, according to Haeckel, the boundary between the plant and the animal kingdoms. It looked as if this theory of the monera was to receive striking confirmation. When the first Atlantic cable was laid there was found on the ocean floor a gelatinous slime which, according to Huxley, consisted of small particles of transparent jelly, mixed with little calcareous granules. Huxley thought the jelly to be the protoplasm of some primitive organism, which he named Bathybius Haeckelii y and placed among the monera. During the voyage of the Challenger (1879) it was, however, shown that the Batky- bius is but an inorganic jelly. Huxley hastened to re- pudiate his discovery while Haeckel consoled himself for a time with a similar discovery in the neighbourhood of Greenland.