The History of Biological Theories

on palaeontology contains little but empty classification, soul-deadening genealogical trees concerning animals with strange names which indicate here a tooth, there a bone ; while relegated to the introduction or to the final chapter there will be a few words of praise or criticism of Darwin's theory of natural selection. To what end did such animals ever exist ? What did they look like ? What sort of lives did they lead ? Why did they become extinct ? What is the meaning, for ex- ample, of the replacing of the dominant tree-like Crypto- gams by the Phanerogams ? Palaeontology gives no answer. Instead of being the leading biological science, she is as cold and lifeless as the stones which she piles up in our museums.

Steinmann seems to have realized this and complained that palaeontology has no method of its own. He holds that it must emancipate itself from the merely classificatory, and reforming itself, adopt the method of the historian. If the historical relationships between animals are kept well in mind there will be less need for the palaeontologist to^turn his attention to questions of anatomical relation- ship. 1 He will not conclude, because of the resemblance between them, that mammals are all descended from some primitive mammal. From the fact that, in the tertiary epoch, mammals replaced the mesozoic reptiles, he, as palaeontologist and historical writer, will conclude that the mammals are the descendants of the reptiles that, in fact, certain groups of mammals are descended from certain definite reptilian groups that the Icthyosauriwere replaced by dolphins, the Plesiosauri by sperm whales, the Thalattosauri by the whales. He will reject the prevailing theory that Trilobites became extinct at the end of the palaeozoic period, and will show how they were transformed into^the Isopoda (e.g. the Woodlouse), into Decapoda and Cirripedes, into Spiders, Insects, and Fishes.

1 G. Steinmann, Die geologischen Grundlagen der Abstammungslebre, 1908. attention to the hypotheses of Cope and Brunner. They believed that an animal can undergo a change in the features which characterize its race, its family, or its order, without undergoing a simultaneous alteration in its specific characters. The greatest objection to Steinmann's work is that he merely substituted another series of eventualities for those suggested by Darwin. Is it necessary for the palaeonto- logist to believe that the mammals of the tertiary period represent the very much transformed descendants of mesozoic reptiles, before he can discuss their succession ? Must the historical writer know from which of the subjects of the French king Marat, Danton, and other leaders of the Revolution were descended before he can report upon that Revolution ?

The Darwinian theories never became absolutely pre- dominant in palaeontology. Many famous palaeontologists sided with Darwin. Among them were von Zittel in Germany, Gaudry in France, Neumayer in Austria, Marsh in America, Huxley in England, Kovalevsky in Russia. Nevertheless there were always the dissidents to whom Darwin's ideas were inacceptable, e. g. von Ettinghausen and Cope, who developed views which were, from the very beginning, absolutely unorthodox.

In 1867 Waagen of Vienna made a distinction between variations and mutations. The first term he took to denote variations in the members of species belonging to the same geological epoch this would include, for example, the variations among the individuals of any existing species. Mutations, on the other hand, were to include those gradual changes presumably always proceeding in the same direction which a form may undergo in the course of several geological epochs.

Neumayer, who was also working in Vienna, made an exhaustive study of molluscs which gave much support to this view. Neumayer first believed that these ideas of variation and mutation would fit into the Darwinian frame. When they were further investigated they led to a new conception of a species. Mutations were studied in a great variety of forms and many tables were con- structed. These differ from phylogenetic trees in that they contain no hypothetical forms. Further, they take into account the similarity of the whole body, and not of isolated organs. Lastly, they generally run in parallel columns, without branching. We may give the following table, which illustrates the palaeontological evolution of the Proboscidia, as an example of this type of work. 1

According to this table the mastodons fall into two series: those which lie next to each other across the table (e.g. M. arvernensis and M. Borsoni^ or M. longirostris and M. turicensis) are related as species of the same genus; those which follow each other down the table represent mutations in a succession of forms. These chronological series represent a new departure in systematic biology they can only be defined histori- cally. The genus represents an ideal abstraction from many similar species. The series (Deperet calls it a phylum) is connected in a quite definite manner with some definite epoch of the world's history, and its members must be ordered in one definite sequence. Many of these series show evolution according to definite laws inherent in the organism. They demonstrate the continued differentia- tion of certain structures, an increase in the size of the body, and often recognizable periods of youth, maturity, and decline. The extinction of an organism reveals itself as due to internal causes.

This idea of series is sometimes extended to larger groups of organisms. The Ammonoid forms Goniatiideae, Ceratitidae, and Ammonitidae were formerly grouped as families, each one, it was thought, having been evolved from some original stock. Now they are regarded as merely developmental stages in the whole Ammonoid series; this contains a very great variety of forms, which represent varying levels in an evolutionary series which is advancing along several parallel lines.

Although these tables introduce a new idea, and replace the older genealogical trees, nevertheless they are nothing but a concrete expression of Darwinian ideals of the endeavour to replace the ideas of systematic biology by the idea of an historical series. The hypothesis upon which all such series are based, is the old one that each muta- tion has been gradually evolved from its predecessor. This in itself is enough to indicate the source from which all such work has sprung.

V\7THEN he is considering natural selection Darwin \^ speaks of Nature almost as if she were alive : 'We behold the face of nature with gladness, we often see superabundance of food ; we do not see or we forget that the birds which are idly singing round us mostly live on insects or seeds and are thus constantly destroying life ; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey ; we do not always bear in mind that, though food may now be superabundant, it is not so at all seasons of each recurring year' (Origin of Species).

Darwin was here merely following the lead of contem- porary political economists, and applying their ideas in the world of science. He showed one factor at work a struggle. He interprets this as the force par excellence which causes evolution, and then considers how this force works, in conjunction with variation, heredity, correlation, &c. Given Nature, and the constant struggle for existence this struggle, if there is variation too, will result in the survival of the fittest.

This word 'result' has a double meaning. It is used first of all to denote those objective realities which succeed one another in time (after the struggle for existence comes the survival of the fittest). It also denotes a logical deduc- tion (if we consider the struggle for existence, we must arrive at the idea of the survival). All through Darwinism this logical peculiarity recurs; each idea denotes both a logical abstraction and an actual object or event. It is impossible to misinterpret such words as 'Zoology' and 'the Animal World', to confuse science with the objects of science. But such words as Phylogeny, Ontogeny, are used by Darwinists to denote both a science and its object. So natural selection denotes both an idea and an event.

We 'explain* phenomena either by showing what has caused them, or by demonstrating the reason for their existence. We do the latter when we show why a certain thing must be as we find it, and not otherwise, why, for example, the sum of the angles of a triangle must be equal to 1 80. Our explanation is referred to causes, when we show what has produced an event ; for example, the cause of a musical note is a vibrating string. (In addition to all this, teleological and moral explanations exist, which we are not considering here.) 'Natural Selection' is used at one and the same time to express both an objective cause and a subjective reason it is meant to be interpreted thus. Actually it only denotes a subjective reason for we cannot see it active and at work in Nature. Darwin does not cite one case where natural selection is actually taking place ; and when he speaks of it, he generally begins with such words as 'Let us assume that '

This peculiarity of Darwinism, which differentiates it from all other scientific theories, was soon pointed out by its opponents. The Duke of Argyll called it 'abstraction personified 5 , but as it is no real abstraction from separate phenomena, it would perhaps be better described as 'deduction personified'. Darwinists never free themselves from these difficulties. They continually assert that natural selection can pro- duce nothing new and yet are constantly forced to assume that it has been the cause of differences in structure. They recognize that we cannot study natural selection in actual operation. 'The saddest thing of all', said Weis- mann, 'is that, in scarcely one case, can we say whether a certain deviation is useful or not ; there seems no prospect of our ever being able to do so.' Recently Plate has spoken in almost the same words: 1 'It is practically impossible to estimate the value, for purposes of selection, of any single object; often we cannot even tell whether an apparently useful organ is worthy of selection or no. From

1 L. Plate, Selektionsprinzip und Probleme der Artbildung. Ein Handbucb des Darwinismus, 1908. this we see that the theory of selection is not one whose validity can be tested by the examination of isolated instances ; but it is a logical deduction from such general facts of experience as variation, overpopulation, and the struggle for existence/ A further peculiarity of the theory is that it looks upon progress as a purely passive thing. By life Man has always meant an activity, a development of power, an effect upon the surroundings. Progress in the world was believed to depend upon endeavour, upon the struggle for ideals, upon the force of the individual, and we are as a matter of fact still convinced that this is so. Indirectly, Darwin opposed this idea. Life itself can do nothing; according to him it is only a plaything in the hands of the most diverse external factors. There is no stepping forward, one is only pushed forward. If a man has some advantage over others he cannot develop it and make it of value by his own efforts ; and at his death all that he has struggled for dies too. The only individual who can hope for victory is the one who produces a numerous progeny that is the teaching of the selection theory.

We cannot give the history of the development of this idea of natural selection, for there has been no develop- ment ; the idea is an abstract one, and means to-day just what it meant in Darwin's time. We shall find this all the more striking when we remember that this idea of selection has always been the central idea round which discussion has ranged. There is no doubt that the conditions under which animals live are altering before our eyes. In Scotland the song-thrush is being ousted by the missel-thrush. In Australia our bees are gradually replacing the native stingless bee. Round us the little singing bird Serinus bortulanus is constantly appearing formerly it was never seen here (i.e. in South Germany).

But what is the nature of the struggle that leads in the one case to victory, in the other to destruction ? Of that we know very little. An extraordinary light is thrown on the anthropomor- phic views of the nineteenth century by the ideas about artificial selection which were then held. Without any ex- perimental proof whatever, Darwin claimed that Nature follows essentially the same methods as does Man, when he wishes to produce a new race. A few of the older critics challenged this assertion, and demanded its foundation. Even now investigators do not seem to realize how such a statement cries out for proof.

CARL NAGELI, the well-known German botanist (1817-91), was a scientist with an established reputa- tion when Darwin published his theory. He had done a considerable amount of work on Schleiden's cell-theory and on minute anatomy, and was well known as one of the early supporters of the new materialistic views. He knew the attractions of the Naturfkilosophie, and en- thusiasm for Oken's idealism had induced him to forsake medicine for botany. He had gained an insight into Hegelian ideas from Hegel's own lectures ; indeed, Nageli was accused by Schleiden of being nothing but a follower of Hegel. Influenced by the materialism of the day, he had abandoned the idealistic standpoint, and when Darwin appeared Nageli was prepared to welcome him. But no enthusiasm for the new ideas could quite destroy the influence of those youthful ideals.

In 1884 Nageli published a book on the evolution of the living world, which bore the bold title of A Mechanico- physiological Theory of Evolution. It dealt very fully with the phylogeny of plants, and contained in addition a considerable amount of scientific materialism, including theories of the origin of the atom, and concerning mole- cular forces. Nevertheless, he ends by claiming that Darwin paid too little attention to the whole organization of the body, and he asserts that some force tending towards per- fection must be the still deeper principle which underlies natural selection. He agrees with the older morphologists in distinguishing between the original plan and the adaptive characters of a plant. The plan makes the rose a member of the Rosaceae, the mistletoe of the Loran- thaceae, the tulip a monocotyledonous plant ; adaptation is exhibited in the colour of the rose and the peculiar

suckers of the mistletoe, for the environment has stamped these characters on them. According to Nageli, life, once formed from lifeless matters, contained an urge driving it towards ever higher forms towards perfection ; even as the forces within the solar system have driven the whole system since the begin- ning of time. He believed that, as the result of the inter- play of chemical and physical forces, simple cells are constantly being formed from lifeless matter; that their powers of growth, reproduction, inheritance, and varia- bility follow from the mechanical structure of this living substance ; and that evolution to a high form must follow as the result of the possession of these powers.

Man is ultimately descended from the oldest formed cells, the monkey from somewhat younger cells, the modern infusoria have arisen quite recently. Hence modern monkeys are not the descendants of those of the tertiary period ; they have been evolved from lower forms, and their descendants will in time become human. But by that time present-day humanity will have advanced further. The world of plants and animals thus resembles a garden containing many branching trees; these con- stantly produce new shoots, which represent the fauna and flora of to-day ; they grow, driven by an inner urge and pruned by natural selection. If there were no struggle for existence an enormous number of forms would arise, which would interfere with one another. As the gardener prunes away certain branches, thus preventing his bushes from growing into shapeless and tangled masses, so natural selection kills off those forms which are not following the predominating line of evolution.

It is easy to see traces of vitalistic ideas behind Nageli's theories. He invented the idea of invisible living units Mizellen. He was so sure of their existence that he could estimate their size. They have a volume equal to one three-millionth part of a cubic centimetre, and a live seed contains 4,000 billion Mizellen; he even described their chain-like arrangement in living substance. Proto- plasm is formed from them, and is of two kinds. One (Idioplasm) contains all qualities in nuce and 4s like a microscopic picture of the macroscopic individual'. The other (Stereoplasm) is different in different parts of the body, and each sample contains only certain of the qualities which characterize the individual to which it belongs. Nageli cited laws which he believed to underlie evolution, driving it along its appointed road.

(a) The law of phylogenetic union. Following this law unicellular organisms divide and become multicellular. (V) The law of complexity, differentiation, and reduc- tion. Similar cells become differentiated and form tissues ; at the same time, the development of the individual organ- ism is simplified, certain steps of its phylogenetic evolu- tion being omitted. Nageli's attempt to introduce the principle of 'an evolu- tion towards perfection' into biological theory, and to drive natural selection into a position of secondary im- portance, was not favourably received. His mechanistic views were ignored by Darwin and his school and they accused him of mysticism; others like Wigand who understood these 'mystical' ideas drawn from an earlier view of morphology, accused him of materialism.

Nageli himself found it difficult to reconcile his idea of organic continuity with the fact of the constancy of species ; yet he fully believed in this constancy, and cited it against the Darwinian theory. In spite of all these difficulties, biological thought was strongly influenced by Nageli; notably many thinkers endorsed the opinion that some deeper principle than Natural Selection must be found to account for evolution. Weismann adopted Nageli's view of two kinds of proto- plasm when formulating his theory of heredity and the germplasm. Askenasy accepted the principle of per- fectibility. 1

1 E. Askenasy, Beitrdge zur Kritik der Darwinischen Lebrc, 1872. and post-Darwinian epochs. He owes much to Nageli. He is in favour of the mechanistic view ; believes that the study of phylogeny is the highest goal of science, and that through it the idealistic (or, as he calls it, the 'scholastic' morphology) will be discredited. He distinguishes between those characters (organic) due to a formative force and those others (adaptive) due to reaction with the environ- ment. Certain views expressed by De Vries concerning 'elementary' species, and the differences between inheri- table variations and non-inheritable fluctuations, are also founded on Nageli's views.

Nageli was the first important scientist who, relying solely on intellectual arguments, nevertheless rejected empiricism, and with him began the reaction against it. He says very definitely that he offers his rationalistic theory, which is purely German in origin, in place of the English work, which is nothing but a mere collection of facts. In those days it seemed strange to contrast the two in this way ; yet here was a suggestion which was destined to be much more fully developed.

WE have already pointed out the difference between the old and the new biology the one idealistic, the other naturalistic. The cell theory underwent a similar change. When Schleiden and Schwann discovered that a body is composed of innumerable cells, they struck a blow at any idea of the 'unity' of the whole organism. The cell became the unit. Virchow went even further, and developed his idea of a cell republic. He taught that disease does not attack the whole organism, but that it is localized in certain groups of cells. The cell became for biochemistry and biophysics what the atom is for chemistry; general ideas were abandoned. The cell was used to 'explain' everything inheritance, variation, the processes of life, the nature of sex, even the soul and immaterial phenomena.

Many biologists, even to-day, believe that all the problems of the living body can be reduced to problems of the single cell. Hertwig, for example, having published his work, Cells and Tissues ('Zelle und Gewebe'), gave to later editions the title General Biology ('Allgemeine Biologic'). In accordance with the usual scientific tendency of the times, the cell-theory when first formulated was accepted by all workers as a dogma. No attempt was made to investigate the nature of the cell or its relation to the whole organism. From 1840 until the end of the century we may search the records in vain for any worker who believed in any theory in place of the cell-theory.

The great histologists of the time Virchow, Kolliker, Strasburger, &c., accepted Schleiden's vague and sketchy views, and made it their task 'to find out of what elements those cell structures, which still seem to us simple, are composed', as Kolliker expressed it. Research was focused upon the protoplasm and the nucleus, and an elaborate microscopic technique was evolved, for which very careful and laborious methods of staining were required. We will pass in review a few of the more general con- stituents of the cell, and then summarize the theories propounded concerning them.

The cell may assume many forms. Some have a volume of several cubic centimetres (e.g. the egg-cell of the bird) ; some are so small as to be invisible with any microscope (e.g. some bacteria). Cells may be spherical, thread-like, plate-like, branched, &c. Driesch has pointed out that in any given organ the size and form of the cells composing any one of its elements remains constant. Others have asserted that the size of the cells in any given tissue element varies round a definite mean. Cells may possess certain organs cilia and flagella by means of which they move ; coloured eye-spots, by which they see ; a cell-mouth, for the absorption of nutriment ; fine muscle fibres, &c. Those cells which form part of a tissue are, however, usually simpler in structure.