The History of Biological Theories

The palaeontologists prided themselves upon discover- ing a large number of these intermediate types. Among these was Archaeopteryx, a creature supposed to link up the birds and the reptiles. The extinct reptilian dinosaurs were also believed to be intermediate between the same two groups. The Dipnoi or Mudfish are a group, contain- ing both living and extinct forms, which suggest how a transition may have occurred from fishes to amphibians. In 1901 Fleischman published a critical survey of all these 'intermediate forms 5 . He showed, from many instances, that the early enthusiasm had abated. The lack of success made him critical of the whole science of phylogeny. He went back to the ideas of Leibnitz, and asserted that problems relating to the origin of living animals do not come within the province of an exact science.

There could be no more whole-hearted condemnation of the search for transitional types. It may seem to us that Fleischmann's scepticism was too sweeping. He was dealing with the vertebrates, and the results obtained from the study of vertebrate phylogeny are certainly un- satisfactory. What, then, is to be our attitude to the phylogeny of the invertebrates ? for it is well known that results obtained in this field of inquiry are even less definite. Concerning the vertebrates we know this much, at least that the fish came before the reptile and the reptile before the bird and the mammal. The study of the morphology of invertebrates has told us still less about their phylogeny ; we cannot even say which group came first.

A list of the more important forms which have figured as 'transitional' in Darwinian speculation is instructive. Catallacta (transitional between the Protozoa and Metazoa). Coeloplana Metscknikowii and Ctenoplana Kovalevskii (Clenophora Flatworms). Trochosphaera aequatorialis (Rotifera Annelida). Hexarthra polyphera (Rotifera Arthropoda). Dinophilus (Rotifera Anne- lida) . Balanoglossus (Echinodermata Vertebrates) . Limulus (Trilobites Arachnids). Peripatus (Insects Worms). Scolopendrella (Insects Myriopoda). Proneo- menea (Molluscs Worms). Phoronis (Worms Brachio- pods). Tunicates (Invertebrates Vertebrates). Ampki-

0,mr(Tunicates Fishes). Ganoiden(Sharks bony fishes). Dipneusten (Fish Amphibians). Dinosaurs (Reptiles Birds). Archaeopteryx (Birds Reptiles). Various extinct hoofed animals. The Gibbon ; Pithecanthropus (The Ape- man), &c. The results obtained from the study of plants are no better. We do not know any form which represents a real transition between the Algae and the Mosses, nor one between the Gymnosperm and the Angiosperm. The belief that such forms ever existed has been frankly abandoned, and this has led to a theory of 'salta- tions' of advance by sudden leaps.

Biologists have made many attempts to discover the elements from which the bodies of plants and animals are formed. Goethe believed that the vertebra and the leaf, respectively, represent such elements; Schleiden saw the elements of structure in the cell ; Owen held to the homo- logous organs as the unit structures. There are in fact many ways of subdividing the organic body. Two in particular have found favour with anato- mists the division according to functional significance and the division according to morphological significance.

The fore-limbs of the horse, the whale, and the bird are morphologically equivalent, but they have very different functions. The fins of whales and fishes are modified to perform similar functions, though morphologically dis- tinct. Even greater functional similarity, combined with morphological dissimilarity, is seen in the wings of birds and butterflies respectively. Haeckel and Gegenbaur believed that homologous organs have been inherited from a common ancestor. If the primitive mammal was a four-footed animal, then the limbs of all mammals are homologous.

Similar organs for which we cannot postulate a common organ are, according to the same view, analogous; they have been derived from different rudimentary structures, but, as the result of external conditions, they have de- veloped a certain similarity. The Darwinians considered that the following are special cases of homology or of analogy: (a) Analogous Variations. 'Distinct species', according to Darwin, 'present analogous variations, so that a Variety of one species often assumes a Character proper to an allied Species, or reverts to some of the Characters of an early Progenitor' (Origin of Species). These characters are not such as are derived from a common ancestor ; they have been acquired independently as the result of some similarity of inner constitution. Darwin gives as an example of this the variation in structure exhibited by our domestic pigeons. The most distinct breeds present subvarieties with feathers on the feet a character not possessed by the original rock pigeon. This is an analogous variation in two or more distinct races, due to those races having inherited the same constitution and the same tendency to variation. The palaeontologists generally call this type of variation by another name ; a series of forms which are structurally related, but have not acquired these.structures from a common ancestry, is called a 'parallel series', or, following a suggestion emanating from Ray Lankester, 'homoplastic'. In recent times considerable importance has been attached to the study of Homoplasy through the work of H. F. Osborn, the American palaeontologist. 1

(b) Convergent characters are, according to Darwin, those similar characters which are found in animals widely separated in systematic position, but evolved under the stimulus of a similar environment. The compound eyes of the Crustacea and of the Insects are examples of such characters, for presumably their common ancestor did not possess any such compound eye. Hence, although these eyes are similar in structure, they are not to be called homologous.

1 Henry F. Osborn, 'Homoplasy as a law of latent or potential Homology,' Amer. Natur. 1902. The precise difference between convergence, homology, and parallelism is, however, often difficult to define. Latterly a new definition of convergence has been given which is rather different from Darwin's. The term is used to include any similarity of structure exhibited by widely divergent forms. Under this title are included, for example, the snake-like forms in the Permian Stego- cephalia (Dolichosoma) ; in the eels and some other fishes ; in those snake-like Amphibia, the Coecilia; in the slow- worm; in the cretaceous sea-serpents, and in the snakes themselves. Here, too, are included the various forms of teeth in different groups of animals ; the different types of shell ; different external coverings in the form of armour, horny scales, &c.

(c) Analogies were functional resemblances, in the eyes of the morphologists of the old school. Such similarities have not been thoroughly examined, and it is the general view that such resemblances are not important that the similarity between the flight of the bat, the insect, and the bird is not a matter of very deep significance. The words 'flying', 'swimming', 'running' represent analogies of this type, and we speak of 'a similarity between the organs of flight', &c.

It may be that, in these phrases, we are only describing very superficial resemblances; perhaps a more thorough study of the flight of a bird, the gallop of a horse, the method of swimming of a whale would reveal some very deep-seated resemblances between these three apparently different motions a resemblance possibly much greater than any which exists between the wing movements of a bird and a butterfly, or the swimming movements of a whale and a fish. We should then be able to discuss functional homologies just as we now discuss structural ones we might even discover a deep-seated relationship between the two.

It seems strange that the Darwinists did not attach more importance to questions of physiology in discussing the significance of organs, if we recall their fundamental hypothesis of the importance of adaptation. But the earlier morphological ideas were still exerting a great influence, and they believed that function could be deduced from the study of structure. We need only refer to a few scattered attempts to formulate a truly physiological interpretation of animal structure.

Dohrn, the founder of the Naples Zoological station, originated the hypothesis known as the 'Principle of Functional Exchange' (1875). This suggests that every organ has several innate functions a main one and several other subsidiary ones. The limbs, for example, are pri- marily organs for walking, but they may serve for jump- ing, swimming, grasping, and so on. It may happen that the chief function falls into abeyance, while a minor one replaces it and becomes in its turn the principal function. This is followed by structural alterations in the organ concerned.

Dohrn did not develop his ideas further. He made no attempt to discover whether walking is really the primary function of all limbs ; nor did he attempt to elucidate the relation between the so-called primary and secondary functions. His ideas soon passed into oblivion. Julius von Sachs, the botanist, formulated a very similar theory (1893), which exerted a more lasting influence, and which we may still trace in modern theories of organogeny. Sachs believed that many qualities are latent in plants, which are revealed only under certain exceptional external condi- tions. He believed, for example, that the properties of epiphytic roots are possessed to some extent by all roots. 1 Sachs succeeded in making the potato, the vetch, the pea, and the maize live epiphytically. He thus demonstrated that a change in habit can be brought about by a change in external condition.

Sachs was convinced that many similarities between plants can be thus explained, different plants having re- 1 Epiphytes are plants like the tropical orchids which live on fr$es without, however, being parasitic upon them. They live almost entirely upon what they can obtain from the air. acted in the same way to the same external stimulus. He called resemblances thus produced 'parallelisms of habit'. Among his examples of this phenomenon are : the simi- larity between the leaves of me higher plants and similar structures of mosses and algae; the close resemblance between all the fleshy Cactaceae and the Euphorbias ; the likeness between Rosa berberidifolia and the Barberry, between Geranium triste and the Umbelliferae. In these last two the resemblance is so close that we might almost speak of mimicry. Sachs also studied the problem of homology in the plant world; he considered that these 'phylogenetic parallelisms' were to be regarded as ex- amples of analogous variation or homoplasy.

Strasburger was another botanist who studied simi- larities in the plant world. He distinguished between architypic and phylotypic homologies (1902). The first group have been inherited from a common ancestor. Cladodes (leaf-like branches), certain thorns, and some tendrils are examples of these architypic homologous structures; they are all modifications of one ancestral form the shoot. Phylotypic homologies are those innate resemblances developed because the same law is acting a law inherent in the organism. An example of this type of resemblance is that similarity of structure found among the growing points of so many multicellular plants. Strasburger made a third and final group of 'pantypic homologies'. These comprise those similarities of structure common to all organisms the method of nuclear division, for example, which is substantially the same for all animals and all plants.

There has been a constant endeavour on the part of biologists to obtain some further analysis of the body of the living organism. As there was no possibility of abstract analysis, all that could be attempted was a rough empirical classification of those organs and tissues which are obvious to the eye. Genetic morphologists, like their predecessors, made no attempt to formulate any abstract system according to which the body might be analysed into its component parts. They were concerned with origins. In their dis- cussions of the origin of the various animal groups they used the methods which were already in use. They com- pared organs, endeavoured to find out which were to be regarded as homologous, and to arrange the homologous ones in series, according to their degree of differentiation. Each series began with the simplest form and passed through forms of ever-increasing complexity, to end in the most complex. The brain was studied in this way, and the brains of the most diverse vertebrates were arranged in one ascending series. It began with Amphioxus, passed on to the Lamprey, and so it continued, to end finally with the human brain. Such a series was called a phylogenetic series it was supposed to throw light on the evolution of the organ and was interpreted historically. The 'simplest 5 was thought to be the 'oldest' or 'original* form; the complex was called 'advanced' or 'more highly evolved'.

Following the lead of Gegenbaur some workers included in their series only mature organs; others included em- bryonic and larval forms; the two schools arrived at different ideas of the 'history' of the organ. Such series were constructed, not for whole organs only, but also of their parts, of tissues and of cells. Haeckel, for example, suggested a 'phylogeny of the tissues' ; others wrote on the. evolution of the pigments of the skin, on the evolution of sensory cells, of certain muscles, &c.

Darwin's teaching destroyed the belief in the fixity and therefore the definiteness of species, genera, and families. It also destroyed the belief in natural 'types'. There was a tendency among his followers to increase the number of groups into which the higher animals were divided. This seemed somehow to lend support to the view that it does not matter whether we assume there are four or ten of them! We no longer believe that Cuvier, with his dis-

tinction of four main types, had attained complete finality; but the opposite view, that there are no types, has only gained a theoretic consent. In practice there was very soon a return to type teaching. It is almost universally, though tacitly, assumed in bio- logical text-books that organisms must have had a poly- phyletic origin. We assume that there are various groups of plants and animals which are fundamentally different and which have been separated through their whole history. There is, however, no unanimity about the number of these lines of descent. Darwin originally assumed that there have been four or five such lines in each of the two kingdoms plant and animal but he did this without stating the reasons which led him to quote this number. Later he came to the conclusion that even these eight or ten ancestral forms had been originally derived from one primeval organism.

Haeckel also changed his views. He believed at first that living organisms had had a polyphyletic origin ; but later he assumed a monophyletic one. He asserted that the two views are not fundamentally different. In this he was right to a certain extent, for later morphologists have not been able to give any definite reason for accepting the one view rather than the other. Nevertheless, modern scientists seem usually to favour the view that the animal world has had a polyphyletic origin. Julius von Sachs has shown very clearly that this polyphyletic theory is only Cuvier's old 'type' idea in a modern dressing. Von Sachs apparently did not realize that Owen had adopted the idea, and had given the name 'Archetype' to that funda- mental plan which underlies each of the animal groups. This idea seems to have come back into Sachs* mind while studying the problems of phylogeny, and he formu- lated quite independently his definition of the archetypus, as he called it. 1 By this term he denoted a phylogenetic series of plants which have had a common origin; he assumed that each archetype is formed according to a

plan which is characteristic of the group. This is only a repetition of Owen's definition of his types. Sachs cites the following archetypes, i.e. plants which have formed separate evolutionary series since their initia- tion: (4) Conjugateae (and Bacillariaceae), including some green algae and others. (6) Archegoniateae (which include almost all the green plants from the Algae (for example, Coleochaete) to the Phanerogams). The theory of the polyphyletic origin of the animal world is only Cuvier's type idea given a new and historical interpretation. Our modern systems of classification are also very like those of Cuvier. Compare the modern sub- divisions of the animal kingdom with Cuvier's types (Vertebrates, Molluscs, Articulata, Worms, Radiata). We do not find the word type in the modern classifications. The number of sub-divisions has been increased. We recognize an important innovation in the introduction of the Protozoa, but otherwise the whole picture fits very well into the frame of Cuvier's scheme.

I cite four examples of modern classification. The authors are all, with the exception of Fleischmann, up- holders of Haeckel's views. Korschelt and Heider, for example, divide the animal kingdom into a large number of small groups yet the only 'improvement', as compared with Cuvier's, is that they have increased the number of subdivisions. Cuvier's Radiata (groups 2-4, 14 and 15), Mollusca (groups 25-29), Articulata (10-13 an d 16-24), and Vertebrata are still easily recognizable.

Purely morphological studies have fallen into disrepute to-day, and it is not only the Darwinian aspects of those studies which are out of favour. In the early days of Darwinism morphology was shaken to its foundations by the genetic theory. Haeckel infused fresh life into it when he began the construction of genealogical trees, and for a time every morphologist followed his example. To-day this branch of science is paying the penalty for this ; it is accused of inexactness the accusation levelled against the whole Darwinian teaching. Physiology is the modern science ; it has long been threatening to displace morphology altogether.

Darwinian embryology was completely dominated by the fundamental law of biogenesis. The embryologist's task was to discover the past history of each organism from its ontogeny. It is not possible to elucidate this history directly, but we can do so indirectly if we accept Haeckel's law that embryonic development is but a shortened and modified repetition of phylogenetic evolution. Francis Balfour, the sagacious English scientist, thus described the task of the embryologist (1880) :

'In the department of Phylogeny the following are the more important points aimed at : (a) To test how far Comparative Embryology brings to light ancestral forms common to the whole of the Metazoa ... by com- paring the embryos and larvae of the different forms. It supposes the ovum itself to represent the unicellular an- cestral form of the Metazoa, and hence deduces that all the Metazoa have descended from an Amoeba-like form. (b) To see how far modern larval forms may be interpreted as the ancestral type as reminiscent of the progenitors of the group. For example, the trochosphere larva was considered by Ray Lankester to be common to the Molluscs, Vermes, and Echino- dermata, and hence our phylogenetic studies lead us to affirm that the Trochosphere may have once existed as an adult animal, and that from it all the above groups may have evolved.

(c) To see how far these larval forms agree with living or fossil forms in the adult stage. When, for example, we find that the rotifers "have many points of resemblance to the trochosphere", we infer that the trochosphere also gave rise to the Rotatoria. (J) To find how far organs appear in the embryo or larva which either atrophy or become functionless in the adult group, as do the gill-slits in the embryos of the higher Craniates. (e) To find in how far such organs represent structures inherited from the ancestral form, and in how far they represent adaptations to new conditions of life.

If these organs are found in a permanent condition in some lower form, performing some definite function, then we may con- clude that the organs now only present in the embryo once passed through this stage of development. The second department of comparative embryology is con- cerned with the origin of organs . . . with the origin and homo- logies of the germinal layers . . . the origin of the primary tissues and their relation to the germinal layers . . . the origin and develop- ment of the more complicated organs and systems of organs.'

This programme was followed by the embryologists. Using comparative data, they constructed their genea- logical trees. They also tried to find out from which of the cells of the segmented egg each organ and tissue of the body was developed. Many organs and tissues were invested with a temporary importance ; they were believed to represent some phylogenetically transitional stage in the evolution of the animal world. The gastrula stage, to which we have already referred in our chapter on Haeckel, became prominent in discussion. The larvae of the marine Planarians; the larvae called Pilidia of certain nemertine worms ; the larva called a Trochosphere all these, and many others, have been dis- cussed as the reputed ancestors of a larger or smaller group of organisms.

Among embryonic organs which were considered very important were the segmentally arranged glands found in certain worms (from these, glands found in arthropods and vertebrates were believed to have evolved) ; the cleft foot of certain crabs ; the small number of segments in the larvae of certain millipeds (these larvae were thought to be the ancestors of the insects) ; the gill-slits in the higher vertebrates, and so on. Balfour's Embryology (1880) are based upon the theory of embryonic layers. We have seen already that both the gastrula and the hypothetical gastraea consist of two embryonic layers ectoderm and endoderm. Wherever these layers occurred they were held to be homologous, i.e. inherited from a common ancestry ; and those tissues and organs which were derived from the same embryonic layer were regarded as related structures.