The History of Biological Theories

Darwin discarded these ideas, and diverted the whole investigation into other channels. There were, however, certain scientists who did not follow Darwin's lead. Naudin in France and Mendel in Mahren followed the old line of investigation, and paid no attention to the change in public opinion. Naudin believed that hybrids must be looked upon as unnatural forms, in which the characters of the parents do not unite, but remain lying side by side, only to fall asunder when further breeding takes place. The professor of the Brunn Gymnasium, an Augustine priest, also held to the old views, but he gave to them a deeper significance. He imagined (a) that the organism is made up of a number of characters, some- what as substances are composed of atoms ; (V) that in hybrids between related forms the characters of the parents do not fuse, but lie side by side; (c) that of these characters as a rule only one develops while the other is latent. Into each sex cell (ovum and pollen grain) of the descendants only one of every such pair of characters enters ; so the characters of the grandparents form new combinations in these cells; in every egg, in

every pollen grain, there is a group of paternal and a group of maternal characters. The number of possible combina- tions of these characters will follow the mathematical laws of probability. If the hybrid propagates itself further by self-fertiliza- tion every pollen grain unites with an egg ; if these happen to contain the same group of characters a descendant ('a mongrel 5 ) is produced which has some of the characters of the grandfather and some of the grandmother ; but it possesses each such character in its pure form; such a form is a constant one. If, however, the characters in the egg and in the fertilizing pollen grain are different, a form (hybrid) arises, in which double characters appear again. These can then only be separated by further self-fertilization. By continued self-fertilization among the hybrids we can calculate mathematically how many different crosses will be produced in each generation. If n is the number of characters in which the two parents chosen for hybridization differ, then 3 is the number of all the possible different kinds of crosses which will be produced. From the hybridization of two varieties of peas which differ from each other in three character- istics we get in the second generation 3 3 , or 27 different forms, of which 2 3 or 8 will be constant forms. The difference between this conception of characters and that of the orthodox Darwinists is seen very clearly in the difference between Galton's and Mendel's theories of heredity. According to Galton, who in this respect was only expressing the views of his time, the characteristics of the parents have a resultant effect on the descendant, like that of two forces working simultaneously on one body. The result is uniform, and nobody could resolve it into its components unless he knew the nature of those components from some other source.

Following out this idea further, Galton believed that every individual is influenced, in a greater or less degree, by all his ancestors, so that no ancestor can reappear pure in any descendant. According to Mendel, however, the characters of the parents are combined in the descendants, but do not fuse, so that they exist there side by side, and can separate again in future generations. No fusion of characters takes place, and hence the ancestor can reappear unadulterated in one of his descendants.

The theories of Naudin and of Mendel aroused no interest, though the latter based his on accurate ex- periments. They appeared when the teaching of Darwin was at its very zenith, and the Darwinian theory did not touch upon this idea of the resolution of a plant into unit characters. True, Mendel did draw the attention of Nageli to his experiments, but even the latter could not gather anything from them which seemed to him im- portant. There are yet further reasons for this neglect. Mendel's experiments were carried out without a micro- scope and with cultivated plants ; this in itself stamped him as mediocre and amateurish. He published his dis- coveries in a little known journal. Further, the fact that he was a monk was not without significance. How could such a one contribute anything in support of the prevailing Darwinian teaching ? His work was only rediscovered in recent years, long after his death. Mendel's theory, which was based on experiments upon one species of plant, forms to-day the basis of the modern science of heredity. New ideas were built up on his theory as a foundation. A large number of facts, botanical and zoological, from medical science, and even from psychiatry, have been collected which seem to support it. In this work of collation limits beyond which the theory does not seem to hold have often been encountered. All this work has, however, begun seriously to undermine the foundations of Darwinism.

TDEALISTIC morphology was flourishing at the be- JLginning of the nineteenth century. Its methods were comparative, its subject-matter the bodies of mature animals and plants, its goal the understanding of the nature of these forms. A few isolated scientists tried to break through the boundaries within which this subject was confined; they retained the old methods, but they extended the subject-matter, taking abnormal forms into consideration. Many leaders of thought in the realm of morphological science among them Geoff roy St. Hilaire,

Meckel, and De Candolle furthered this endeavour, Geoffroy turned to the study of human monstrosities in an effort to find further evidence in favour of his principle of unity in organic design. Meckel, with true German industry, collected all the known facts about pathological structures in man, and produced a work which, in relation to the knowledge of that period, is as important as was Darwin's later work dealing with an analogous subject his Monograph on the Variations of Animals and Plants under Domestication. De Candolle, from the study of vegetable monstrosities, came to very definite conclusions about the normal structure of plants.

When morphology began to claim less attention the work of Schleiden and von Baer became increasingly important ; mechanistic and causal explanations of form began to be discussed. Just at that moment (1852) An Anatomical and Physiological Survey of the Animal World was produced by two German zoologists, C. Berg- mann and R. Leuckart. These authors set out to in- clude the physiology of structure in all discussions. The following words indicate their programme :

'If we have been successful in unravelling any thread, from the tangled skein of causative factors which lies behind the evolution of animal form, then morphology will henceforth become a part of physiology. Just as, to-day, we strive to discover the complex interplay of forces which leads to certain forms of crystal, or to the formation and differentiation of the cell, so, in the future, we shall endeavour to invent new lines of work, which will enable us to investigate those causes which influence the arrangement of organs; hence, at some future time, we shall be able to work towards a physiology of plastic form.'

As can be seen, the two authors did not attempt to realize their programme; they were content to suggest that there is a connexion between the structure and the function of an organ, and they left to the future the task of explaining the organ by elucidating its cause. This suggestion of Bergmann and Leuckart was simply overwhelmed and lost sight of in the flood of Darwinism ; morphology was supplemented, not by the study of physiology, but by the study of ontogeny. Under the leadership of Gegenbaur and Haeckel it was gradually changed to a discussion of genealogical descent. The next task was to describe the embryonic development of each form ; and because each organism begins as an egg, which segments into two cells, into four, and so on, until finally tissues and organs are formed, attempts were now made to determine from which cell each part of the body arises. The comparative method, it is true, had been renounced. Nevertheless this new science was called comparative, to distinguish it from human anatomy and embryology on the one hand, and from systematic biology on the other. Henceforward, however, scientists were not endeavouring to formulate general ideas on the basis of comparison. They were searching in their comparisons for the causes of structural change. The fundamental law of biogenesis had gained such universal acceptance that no one thought of looking for these causes elsewhere than in antecedent structural conditions. As the historical fact that candles were used in the catacombs is the reason why they are burnt on altars to-day, so the cause of the gill-clefts in

mammalian embryos was supposed to be the fact that the ancestors of the mammals had once been fishes. The hypothesis that all multicellular organisms have been evolved from an amoeba was believed to explain why every animal begins its developments from an egg-cell. Haeckel had no hesitation in calling such causes 'mechan- istic'; still less so as he believed that the only possible opposition to his theory was that opposition which was inspired by theology. His views were eagerly accepted by all orthodox embryologists, and, during the 'eighties, embryological science was studied most enthusiastically. Haeckel's pupils hold fast to his teaching even to-day. One of them, O. Hertwig, says:

'Is not the fact that the eggs and sperms are elementary organ- isms or cells in itself causal ?' Hertwig, it is true, is looking for the forerunners which determine the various stages in ontogeny in those forms which preceded the present tmes only by hours or minutes, rather than in those which preceded them by millions of years. In principle, however, he is of the same opinion as Haeckel. The egg is the cause of segmentation. This, in turn, leads to the embryo. The embryo is the cause of the mature form, even as to-day is the cause of to-morrow. He hopes by giving a very minute description of the successive stages in development to attain to a very exact knowledge of causes a knowledge of almost astronomical precision and exactitude.

Haeckel's elucidation of causes did not give any general satisfaction, however. It clings too closely to the mere details of practical experience, and denies all rights to that more soaring intelligence which strives to realize what is persistent in a world where all is changing, and what are the universal laws which underlie all such change. He scornfully challenges every attempt to understand onto- geny when he says 'each of these simple ontogenetic processes of unfolding is the result of an extremely complicated series of historical events. It is causally

determined by the thousands of phylogenetic changes, by the in- numerable hereditary and adaptive alterations, which the ancestors of the organism in question have undergone during the course of millions of years' (E. Haeckel, Ziele und Wege der heutigen Entwick- lungsgescbickte, 1875). Does not this suggest that the organism of to-day merely represents an accumulation of chance occurrences, with which intelligence has had nothing to do ? Intellect, however, could not be thus thrust aside for very long. Attempts were soon in evidence, not only to describe, but also to understand, development. The Leipzig embryologist His, an opponent of Haeckel, tried to explain embryology in accordance with the views of Bergmann and Leuckart. He declared that Haeckel's phylogenetic methods tend to make us avoid a direct explanation, and that we must try to understand why each succeeding embryonic stage is the necessary sequel of the one which precedes it. His believed that the mechanical causes of the phenomena of development will be found if we examine the processes of growth. The embryo of a mammal is at first like an elastic plate ; it grows unequally, and hence crumples like a piece of damp paper. These first folds mark the limits of the various parts of the body, and these parts form themselves subsequently by further foldings. When we have found the law which governs the growth of the various parts of the embryo we shall be able, aided by this mechanical theory of folding, to understand the whole development. It will not then be necessary to explain the different embryonic stages historically.

His endeavoured to obtain experimental proof of his theory of foldings, but his experiments consisted merely of very rough analogies; he examined the mechanical bending of wooden plates, and drew attention to the crumpling of layers of rock phenomena so far removed from those of development that it is not surprising that his theory found few ready to accept it. This was the case, even though several scientists, among them Kolliker, had given their support to the thesis that development must be explained directly.

The zoologist Goette, who was also incited to this work by Haeckel's theory, spoke in favour of a direct explana- tion of embryonic development (1875). In contrast with His, however, he suggested a theory which was vitalistic in its tendency. The 'principle of form' must govern development, he said. He expressed his thoughts in such an obscure style, however, that his theory attracted even less attention than did that of His. In 1880 the anatomist Rauber made another attempt, in a paper on The Creation and Destruction of Form, to replace HaeckePs theory by some more exact hypothesis. In this work, pursuing the line of thought suggested by Bischoff the embryologist (under whom he had studied), he examined and discussed the monstrosities which often occur among healthy fish embryos. Bischoff was an idealistic morphologist, and was directly under the in- fluence of Geoffroy and Meckel. All that Rauber did was to develop these theories further, but, being a disciple of the new mechanistic conception of life, he referred to Lotze and not to Bischoff when discussing the philosophical foundation of his theory. He considered that Lotze was a philosopher who was underrated by the Germans, and that his work was very important for embryologists ; for this antagonist of the theory of vital force had used facts obtained from ontogeny to prove that a strictly mecha- nistic view is an intellectual necessity; development is merely irregular growth, he had asserted growth which results in a number of secondary changes of position

'which partly appear to be the result of displacements, protrusions, invaginations or extensions, and in part are really produced in one of these ways, being then the result of mechanical tensions and pressures.' Lotze had also compared the processes of folding off of the embryo from the ovum with analogous processes in the earth's crust, as His did later. Lotze, then, according to Rauber, must be referred to in any attempt to enlarge upon and perfect Haeckel's views. We must seek 'a know- ledge of the forces, or the systems of forces, which enable the germ to embody all the forms mentioned ; to pass from the first form, through all the transitional stages, until it reaches its final phase' (A. Rauber, Formbildung und Formstorung, &c.).

A new science, a 'cell-mechanics', must be founded and ontogeny merged in developmental mechanics. The ele- ments of this mechanics, according to Rauber, are cell- divisions, cell-growth, cell-migration, and differentiation. In a later treatise Rauber modified these ideas in one respect. He no longer believed that cells are the factors which chiefly determine the course of development. He believed that the form of the mature individual is the guiding principle which directs this development.

These writings of His, Rauber, and Goette were not important because of any new concrete discoveries which they proclaimed. What characterized them was the scheme for a new science which they promulgated. But who was to follow the suggested programme if not the authors themselves ? The rest of the scientific world had its own programme, which Haeckel had forced upon it; this programme had first to be depreciated in the eyes of the scientist before he would be ready to consider the introduction of a new one into his laboratories. Not every scientist, however, was ready to welcome such a change. Roux was the first to be successful in the task of gaining a hearing for the new ideas.

In the early 'eighties Wilhelm Roux, then Professor at Halle, suggested that there may be an internal struggle for existence between the various elements of the indi- vidual body. This suggestion helped to broaden the pre- vailing ideas about Natural Selection. embryological views would really lead us, if followed to their logical conclusion. His very early work, done in 1879, already gave evidence of the line of thought which he was destined to follow later. This early work was an investigation into the causes of the branching of blood- vessels. In 1885 be began some experiments on the de- velopment of a tadpole from an egg which had been damaged, and in this work his point of view was expressed even more clearly. In 1894 a new journal began to appear The Archives for Developmental Mechanics ('Archiv fur Entwicklungsmechanik') in which the results of this new branch of science could be made public.

Roux is Haeckel's pupil. His problems are the same problems, his line of thought is that of the keen protago- nist of Monism. Development the development of the bodily machine is the subject-matter of all their work. Their object is to ascertain the causes of this development. Roux believes in a mechanistic world, even as Haeckel does. But what were principles and postulates to the master-mind of Haeckel, principles and postulates which were enunciated with great decision, it is true, were treated as if they were concrete facts by his pupil.

Haeckel's theories dealt with the subject-matter of idealistic morphology, but his aim was always to deduce causal and mechanistic explanations of the facts. His facts were the facts of systematic zoology ; his desire was to confirm his theories by a study of development. In practice he began with the study of form, but in his theories he stressed function as the primary phenomenon. Roux had not imbibed the ideas of idealistic morphology in his youth; he did not hesitate to fit his facts into Haeckel's philosophical scheme, and the result was that he presented, in what he believed to be a concrete form, the ideals which had been preached by Haeckel. In place of the much more difficult phylogenetic development, Roux examined the ontogenetic development, and relegated the historical facts to a place of secondary importance. He, too, endeavoured to discover the causes of form, but he

claimed to have perfected Haeckel's method ; for he did not rely upon comparisons only, but upon actual experi- ments. He was, like his teacher, a believer in the mechan- istic theory, but he did not believe in 'far-fetched speculations on atomic oscillations' or in 'explaining* the soul through nervous activity, or the like : he tried to demonstrate his facts concretely, and to show that these mechanical forces reveal themselves as pressures, tensions, and flexures during the course of embryonic development. This endeavour to put Haeckel's ideals into practice, however, caused Roux unwittingly to overstep the boundaries of orthodox Darwinism. Roux no longer believed that the task of the scientist is to give an intel- ligent picture of what is actually happening in nature. Science which merely gives a photographic picture of Nature did not satisfy him. In all his works there is an endeavour to understand Nature, to know how she works in her magnificent workshops. And so he did not think very highly of the so-called 'descriptive' embryology of the previous decade. There had been no attempt in that work to discover the causes of events. (The idealists of a still earlier age had, it will be remembered, raised the same objection to mere descriptive embryology.) Roux rejected it because the descriptive method can give us no certainty that its assertions are justified. Such certainty can only come from experiment.

Roux now began to assert the rights and claims of Reason claims energetically denied by the evolutionists. This was not the old Reason of the idealists, which was seeking for 'logical causes', and for 'ideas' which revealed themselves in Nature. It was the reason of Darwin and of Mill, which strives to ascertain what are the actual objective causes of all events, those causes which precede the event, and are themselves preceded and caused by still earlier phenomena.

The difference between Roux's views and those of the orthodox evolutionists will be clearer if we contrast them with some of the objections and criticisms which emanated from Hertwig. Hertwig was likewise a pupil of Haeckel, but he has a much less independent mind. He declares, first of all, that he cannot imagine how Roux can hope that this attempt to discover the causes which lie behind phenomena will lead to anything intrinsically new, for it has been for long the avowed goal of science to elucidate the main causes of events : 'The theory of development, as it has been presented to us, does not teach us bare, unconnected facts; it presents us rather with a series of facts, which stand in an absolutely essential and causal relationship to each other/ I Certainly they do, we answer to this assertion, which is so very characteristic of the old type of Darwinian argument ; but of what use is it for us to know that they are causally connected, if we do not understand what were the causes ? With unerring insight, Roux saw that those genetic ideas which affirm that the past is the cause of the present would, if true, provide us with a complete description of both external and internal development a complete cinematographic film of de- velopment, in short. This ideal did not satisfy him, however. He wanted to know by what forces each little particle in the germ is driven on along its allotted road.

We do not know what forces are present in the egg, what is their arrangement, nor how they are able to initiate the complicated series of changes which leads up to the development of a new individual ; nor do we know what combination of forces influences the further progress of development. In short, we have no idea why a highly complicated organism, with the structure typical of its kind, is formed from the relatively simple egg; nor why an organism, once formed in this way, remains comparatively unaltered in spite of the fact that the matter of which it is composed is con- stantly being renewed' (W. Roux, Programm und Forscbungsmetbo- den der Entwicklungsmecbanik, 1897, p. 15).

1 O. Hertwig, Zeit- und Streitfragen der Biologic, 1 897. In the same book he makes a statement which is typical of Darwinistic mode of thought : 'For the words Reason (Grund) and Inference (Folgerung) we can equally well substitute the words Cause (Ursache) and Effect (Wirkung).' He who sees no difference between a reason and a cause will naturally find nothing new in Roux' s work. At a later date Hertwig rejected Darwinism. See his Zur Abwehr dfs etbiscben, des sozialen, des politiscben Darwinismus > 1918. Einhorn gives an

Roux founded a new branch of science, which was to solve these problems and to ascertain the causes of organic form. Using the material already acquired by descriptive embryologists, we must, he affirmed, now search for the forces which preside over the formation of the organism from the egg. Such an experimental science will surpass descriptive embryology, as every science based on experi- ment surpasses one based on mere description. It will be a causal morphology, closely akin to physiology ; but while the object of the latter is to ascertain the causes under- lying the processes going on in the mature body, develop- mental mechanics will seek for the causes leading to the initial development of that body.

This endeavour to explain embryonic development causally and experimentally led Roux into further opposi- tion to orthodox Darwinism. In theory Roux remained a mechanist. He could not, however, refrain from ascribing to the form of the organism a greater significance than it had previously enjoyed. His attempts to analyse form definitely suggest that changes in form are extremely significant. After he had studied the facts of regenera- tion, and certain cases where development had been arti- ficially interfered with, he came to regard form as of even greater importance. Resulting from this Roux affirmed that chemistry and physics are of minor importance; the chemistry and physics of living substance are only distantly related to these problems of form.

This emphasis of the 'How' in processes of development very soon led the followers of the new movement to ignore the facts of normal development, although in the beginning they had aimed at explaining ordinary development. They soon came to regard all changes of form, whether normal or produced by artificial means, as of equal significance ; the change in form itself became the subject-matter of developmental mechanics. account of Hertwig's attitude to Darwinism in his book Erfabrungs- und De- szendenzlebre. Eine Kritik der Grundlagen der modernen Entwicklungslebre im allgemeinen und des biolog. Grundgesetzes im besonderen, 1924. The book contains a very good criticism of Darwinism.

Do the separate parts of the egg develop into the organs to which they give rise because of the action of innate forces (i.e. by self-differentiation) ? Or does the mature organism represent the result of action and re- action between itself and its environment, i.e. is it the outcome of a differentiation dependent partly on external factors ? Roux asked these questions, and regarded them as among the most important which developmental mechanics had to solve. Another equally important task for the scientist was to ascertain the place and time, where and when, each formative factor begins to influence the embryo. Roux sought to solve these problems by work on frogs' eggs. Earlier embryologists had observed that a definite relationship exists between the arrangement of the substances in the egg and the arrangement of the organs which develop from that egg. Weismann taught this in his theory of the germplasm, while His asserted that the future embryo is to a certain extent pre-formed. Roux now set to work to obtain experimental facts which would support this theory. He observed that the first segmentation, which divides the egg into two parts, generally coincides with the median plane of the future body ; he killed one of the two cells resulting from this segmentation, and, from the surviving one a half embryo, either a right or a left half, developed. Roux believed that, in this manner, he had proved experimentally that, at the first segmentation, each daughter cell represents one half of the future body. He used this fact to support his 'mosaic theory' of the egg, according to which the organs are pre-formed in the egg as rudiments lying side by side. These experiments were shown later to be inconclusive, and so the mosaic theory was not accepted. But they stirred other scientists to similar experiments, and the new subject gained many disciples.