The History of Biological Theories

Passive changes are not considered in our theoretical discussions. The active reactions of the organism to the influences of the environment are classified in various ways. They can be divided empirically into morphological, physiological, and psychological reactions. The morpho- logical ones can again be split up into different classes. Instead of giving a further classification, we will quote a few examples. Under variations due to habitat have been included (by Nageli and others) such small modifications in plant form as are brought about by differences in climate, in light, in intensity, in humidity, &c. Thus the common knotgrass (Polygonum ampbibium) appears in three varieties, a land form, a water form, and a sand- dune form. These differ from each other in the structure of the stem, in the shape of the leaves, and in their hairi- ness. The differences are to be regarded as the reaction of one and the same plant to different environments ; for by changing the amount of moisture we can change the water form into the land form or the sand-dune form. Zoologists introduced the name 'local varieties' for these minor differences in form. Almost every species which extends over a wide area has several local varieties. Thus the common goldfinch (Carduelis carduelis) exhibits the following forms : mountain goldfinches are always larger and more beautiful than garden or wood goldfinches, which again differ slightly from each other ; the southern birds are more intensively coloured and lighter on the underside than the northern ones (Brehm quotes them as another species); the eastern, namely those from the Volga area, are specially large, and are often classed as a special sub-species. In addition to those quoted a whole series of analogous deviations are known they are all characterized by the fact that they gradually pass one into the other.

Only in cases where species inhabit places separated from each other by insurmountable barriers do we fail to find forms transitional between the different local forms. Gulick describes such a case for the species of snail called Acbatinella. This exists in the Sandwich Isles in many forms. In some areas these change by insensible gradations into one another ; in others, where intervening areas are separated by mountains, &c., the species are very sharply differentiated.

Physiological variations include adaptation to a particular food, to a particular mode of respiration, to a particular temperature, to a particular method of moving, to living in a particular environment, &c. The phenomena of train- ing may be included among the psychological effects of the environment on the organism. Other scientists classify these facts according to the nature of the stimulus into reactions to light, to gravity, to temperature, to a dry atmosphere, to chemical influences, to climate, &c.

The so-called 'seasonal dimorphism' is a special case of the reaction of the organism to its environment. Wallace fave this name to the phenomenon noticed in those utterflies which live through two generations each year, one in the spring and one towards the end of the summer. The spring form differs from the summer form in the marking of the wings to such a degree that, before their connexion was recognized, they were placed in different species. This is indicated by the double names they bear to-day: Vanessa levana-prorsa, Antocbaris belia-ausonia, Lycaena polyspercbon-amyntas, and others. Seasonal dimor- phism had already been recognized at the beginning of the nineteenth century. In 1879 G. Dorfmeister proved experimentally that it is the difference between the spring and summer temperatures, which causes the difference in the markings of the butterflies ; for, by artificially cooling the chrysalis, he succeeded in obtaining the spring form from the summer one. The centrifugalizing of the chrysalis, or changing the atmosphere which it breathes, or lighting it with yellow light, brought about changes similar to those caused by a change of temperature. Recently many analogous cases have been cited. For example, if it de- velops in warmth and darkness the female chrysalis of the lemon moth (Gonopteryx rhamni L.) gives rise to butter- flies coloured like the male, while ordinarily the female butterfly is much lighter than the male. In the moth Doritis Apollo a similar experiment gave the opposite re- sult, for, influenced by cold, the males took on the colour of the females. Seasonal dimorphism also occurs among

the Protozoa, Rotatoria, and Cladocera; and, according to Von Wettstein (1900), it is seen too in certain flowers which bloom twice a year, e. g. in Alectorolophus, Gentiana, and Eupbrasia. All these researches into the influence of the environ- ment on the organism have made it apparent that it is impossible to deduce the true Nature of the organism in this way. We begin to see that no variations due to the influence of the outside world lead to a change into a new organism ; the organism stands revealed as something invariable, something whose fundamental nature cannot be altered ; the changes of form, and other similar changes, only show how it behaves under many and varying con- ditions. The recognition of this fact has led some biolo- gists to believe that variations in the organism brought about by its environment are merely direct adaptations. They point, for example, to the phenomenon of immunity ; the organism can to some degree counteract the effect of certain poisons by the production of anti-toxins. It has also the power to adapt itself to certain conditions of light and temperature, by characteristic regulations of its vital processes. Many modern researches bear upon this problem, and it is therefore not surprising that our ideas upon the subject are still far from clear. 1

1 H. Driesch discusses this question very fully in Die organischen Regulationen^ Leipzig, 1901. ONE of the most important and characteristic features which differentiates the living from the non-living, is the power of reproduction. No organism is formed by the action of material forces, but each one is pro- duced by a living predecessor. From the time of Aristotle, however, there have always been some who have main- tained this assumption to be incorrect, and that there are certain circumstances under .which spontaneous genera- tion may take place. Moreover, the method by which life arises from life, a phenomenon without analogy in in- organic nature, presents a problem which is just as obscure in the lowest types of life as in mankind.

Life renews itself in two ways : the first sexually, when two individuals are essential for the production of the offspring ; the second asexually, when one individual alone can produce another. In asexual reproduction a smaller or larger portion of the body separates itself from the mother organism, and by growth and differentiation develops into a new individual. This individual exists side by side with the mother organ- ism, which, in the meantime, has replaced the part cut off. If the organism divides into two roughly equal halves we speak of 'fission' ; 'budding' takes place, on the other hand, when the newly developed organism separates from the body of the old one as a comparatively small branch. Finally, if it is produced from a single cell, which is usually formed in a special organ of the mother body set apart for its production, we speak of reproduction by 'spores'.

In sexual reproduction there are always two, if not in- dependent individuals, then at least physiologically (sexu- ally) different organs, whose products unite: the male, which forms spermatozoa, and the female, in which the ova are formed. In exceptional cases one of the sexes (the male) can be suppressed. We then speak of partheno- genesis, when unfertilized females lay eggs capable of development. Among the higher animals, especially among mammals, parthenogenesis does not occur, although the eggs of birds and mammals often begin to segment without being fertilized. At a time when Darwin was still considering his theory, Hofmeister discovered that even the so-called sexless Cryptogams reproduce themselves sexually, but that here the sexual and asexual methods of reproduction alternate in a peculiar way. The green mosses form eggs and sperms, but the fertilized egg does not develop into another moss plant. It forms a capsule on a brown stalk, in which asexual spores are formed. These fall to the ground and eventually germinate to form a new sexual plant. Thus the little moss plant has two life periods : in the first it lives as a green plant, which forms sexual organs ; in the second as a brown capsule, which forms asexual spores.

The ferns, horsetails, and lycopods all go through these two phases, though in them the sexual individuals are small and inconspicuous, while the asexual, on the other hand, are large, being (in the case of ferns) the actual fern plants, which bear asexual spores. Among flowering plants, the first, or sexual phase, is very much reduced. While the algae reproduce themselves now sexually, now asexually, the higher plants, from the mosses upwards, follow an ordered alternation of the two methods of reproduction ; the higher the plant, the more developed is its asexual phase, and the more reduced the sexual phase becomes. The meaning of this alternation is by no means clear.

More recently the analysis of the behaviour of the chromosomes during fertilization has shown that there are specific sex-determining chromosomes ; further, that the inheritance of sex follows the same Mendelian rules as does the inheritance of any other bodily character. This does nothing, however, to help in the understanding of the whole phenomenon of sex. The philosophy of sex, to which man has always devoted much thought, has passed to-day into the chromosome theory. Much of Aristotle's philosophical system origi- nated in the recognition of the difference between the two sexes; this gave him his ideas about matter and form. In the female are embodied the passive principles, in the male the active, creative, formative principles. Even Harvey allowed himself to be influenced by these ideas, and he compared the female uterus with the brain ; as the latter possesses the power to form images of external objects, so the uterus whose ideas are the eggs forms them in the image of the fertilizing male. In the speculations of the evolutionists of the eighteenth century the broader aspects of sex were absolutely neglected. The result of the dis- covery of eggs and spermatozoa was that the true nature of the problem was obscured. They imagined that they could answer all questions on the subject by examining those structures. The theory that the complete man lies already enclosed within the ovum or the spermatozoon suggested that one sex, either the male or the female, represented a superfluous, purposeless creation of mother nature !

The German romantic philosophers looked with wonder upon the phenomena of sex. Their most fundamental idea, that of polarity, was often inseparable from the idea of the contrast between the sexes. Even Schopenhauer devoted a special chapter to observations on the meta- physics of sexual love. Led by the poets, and by Goethe the man of the world, these philosophers were able to appreciate the fateful power of the differences between the sexes. Since Darwin's time, however, biologists have not considered the subject of any paramount importance. It is true that Darwin based his theory of sexual selection on the differences between the male and female of the same species. This theory, however, lacks most of the beauty which characterizes living nature. He only saw

in these differences secondary adaptations to the external conditions of life. Since then this subject has lost much of its significance. Blind to the processes of actual life, and carried away by their observations of microscopic structure, biologists have tended to look upon the problem of sex, under which, according to some philosophers, all the problems of the world lie hidden, as merely a problem of chemistry and of cell structure. From the fact that the spermatozoon and the ovum are both cells, it was in- ferred that there is no essential difference between them. By considering the sexual cells from which they originate, instead of the adult individuals, in all the fullness of their life and struggle, they concluded that there is no essen- tial difference between man and woman ; the differences which actually exist between them are, according to these theorists, merely special adaptations for the purpose of facilitating the union of the spermatozoon with the ovum.

'All the contrivances connected with sex are variations upon one and the same theme ; firstly, they enable the sex cells to come together, and secondly, they insure that the egg shall be nourished and kept in safety. We call the one set of contrivances "male", the other "female''. All these relationships are of a secondary nature, and have nothing to do with the real essence of fertiliza- tion ; this is the union of two cells, and is therefore purely a cell phenomenon. In these views we agree with Weismann, Rich, Hertwig, Strasburger, and Mallpas, who have expressed similar opinions' (O. Hertwig, Allgemeine Eiologie^ 1902).

On this view one question alone remains: what is the meaning of the process of fertilization itself? In the simplest forms of life, as, for instance, the bacteria, there were originally no sexual differences. These developed gradually, and began in the fusion of two otherwise similar cells. To facilitate conjugation one cell gradually assumed a passive role, and the task of accumulating food ; the other became more active, hence smaller, and sought out the former. Thus began the differentiation between ovum and spermatozoon. When, later, multicellular organisms de- veloped, the process of reproduction was taken over by a

few cells, and for the purpose of facilitating conjugation the two sexes became differentiated in various directions. This is the way in which Strasburger, Maupas, and Weis- mann accounted for the development of sexual differences. The latter also thought that these differences in sex play an important part in bringing about variation. The offspring inherits some characteristics from its father, others from its mother, and hence embodies a new combination of characters. Others have given such obvious explanations of all the facts connected with sexual life that there seems to be nothing which is beyond the comprehension of these scientists ! Do we ask what is the basis of sex-love ? Jaeger puts forward the hypothesis that it consists in a similarity between the exhalations of the male and the female, and in a chemical attraction set up by these exhalations. Pfeffer has actually succeeded in obtaining a proof of this hypothesis in the case of certain plants. Mantegazza also gave a very similar explanation. Others, like Nageli, have considered that the attraction is electrical in nature.

But why do two cells strive to unite ? why the electricity and the chemical attraction ? The reason is not a very abstruse one ! According to some scientists cell conjugation developed from a kind of cannibalism. One cell devoured its neighbour, became strong, passed on the capacity for devouring its neighbour to its successors, and so conjuga- tion began. Jacques Loeb suggests (1906) that fertiliza- tion has the following significance: the spermatozoon brings into the ovum certain chemical substances which hasten segmentation ; this can, however, be brought about without the help of the spermatozoon, merely by the influence of certain chemicals. A little potassium chloride or cooking salt is a substitute for the male element, as has been shown at any rate in Echinidae worms, starfish, and other animals. A mechanical stimulus (as has been demon- strated on the frog) may act in the same way.

Boveri (1902), on his side, compared the egg to a watch which has not been wound up ; fertilization simply winds the spring, and this makes segmentation possible. Accord- ing to him the essential factor is the centrosome, which enters the ovum with the spermatozoon. For Herbert Spencer also the object of fertilization was no mystery; life is like a constantly moving wave ; the beginning of life resembles the heaving surface of the water ; it becomes calmer and calmer as development proceeds ; in the ovum such a great peace prevails that a new impulse must come to it from outside; the fertilizing spermatozoon is like a stone thrown into a pond ; life is set in motion again, and the power for a new period of development is given. 1

When we contemplate the activities and struggles of the Universe, it would seem as if the antithesis between male and female plays the most important part in the whole drama. The most beautiful and the most vile in practical life, in philosophy, and in literature, is developed under the spell of this antithesis. It is the inevitable inspiration of the poet. In every religion we find in its metaphysical foundations some solution of the question of the relation between man and woman.

But the men of science can only tell us of centrosomes and chromosomes; their exalted wisdom has revealed nothing more. Tant pis pour elle \ 1 A systematic account of the problems of sex is given by P. Geddes and J. Thomson in The Evolution of Sex, 1899; ^. Dante, La Sexualite, 1899. H. His gives the history of the subject in 'Die Theorien der geschlechtlichen Zeugung', Archiv.fur Antbropologie^ iv, 1870 and 1872 (incomplete). TDEALISTIC morphologists stopped short, in their JLanalysis of animal and plant bodies, at the idea of organs; they did not consider the morphological aspects of this idea in any detail, for they saw in the organ only an instrument for life's physiological activities. True, there were some attempts at a more abstract analysis. The ideas of homology and of analogy, of segmentation, the vertebral theory of the skull, the theories about meta- morphosis, the division of the plant into root, stem, leaf, and trichome, and indeed the whole distinction between morphological and physiological processes all these repre- sent endeavours to combat the opinion that the organism is nothing but a set of instruments for the performing of certain living processes. Yet no morphologist succeeded definitely and of set purpose in freeing himself from the domination of physiology. Lastly, excessive stressing of the general aspects of all these questions killed every attempt at any more detailed analysis of organic form.

Darwinism, as expounded by Haeckel, was made to include the whole of idealistic morphology, but it gave to this a very one-sided physiological bias. According to Darwin bodily structure is determined by the practical necessities of life. The discovery of the cell, an element which cannot be deduced either from the facts of general anatomy or of physiology, marks the first onslaught upon organography. The work on the cotyledon, the new theories of heredity and of variability, all helped to rob the morphology, which was merely organography, of its original importance, and to force it into the background.

The discussion of heredity and of variability led to a new conception, namely, to the idea of characters which are material, and in a sense unchanging, entities. The first advance in this direction is seen in Darwin's theory of pangenesis. When he published it, it was not clear to any one that he was attacking organic morphology that morphology which, rejuvenated and equipped with a new evolutionary terminology, his friend Haeckel had just sent out into the world anew. People welcomed Haeckel's work, but Darwin's theory of little invisible bodies suspended in the blood, which streamed through the entire body in order to settle in the sex-cells, was found very far-fetched. In spite of these facts, however, Darwin's theory exerted the more lasting influence. Great thinkers like Nageli, Galton, and Weismann believed in the invisible bodies, and sought to explain how, with the aid of these bodies, the character- istics of the parents were transmitted to the children. Of what do these characteristics consist ? Darwin did not puzzle over this question ; he regarded as characteristics of the organism such diverse factors as, for instance, the length of its foot, the number of its teeth, the position of a blood-vessel or a single muscle, its intellectual capacity, courage, skin colour, and the ability to speak! Carried away by the materialistic tendencies of his time he could not help thinking of each of these characteristics as a little body, or group of little bodies.

It was not noticed that Darwin's resolution of the organism into characters represented a different concep- tion of life from that which thought of organs, tissues, and cells. Haeckel simply ignored Darwin's discussion, as if it had never been. Weismann, a more acute scientist, took it up and distinguished between two different kinds of character the inborn and the acquired. But, although much ink was spilt in the dispute about the inheritance of these characters, no one attempted to solve the problem : What is a character ? It is true that Weismann dared to try to find out how many inherited characters one organ- ism (Daphnia) possessed; he always believed, however, that the number could be deduced a 'priori.

Meantime other scientists were seeking to obtain a more accurate conception of these characters. Cope taught that the same character can occur in different varieties and species. A mixing of the characters through crossing was also spoken of, but, in the current theories of the day, it was still assumed that organs and their parts represent the only conceivable elements into which an organism can be resolved. This idea was only gradually shaken off by thinkers who have striven to get a clearer conception of 'characters'.

The analysis of the organism into its characters has become a uniform, systematic study with a set pro- gramme. Since Mendel the outlines of such a study have come into view in the modern speculations on heredity, in the researches into hybridization, in the theories re- lating to the sudden formation of new forms, in De Vries' theory of mutation and in Johannsen's teaching about genes. De Vries, on his part, endeavoured to reconcile the cell theory with the theory of separate hereditary charac- ters. He, too, believed in small hypothetical bodies which have these characters shut up within them ; but even in his early work he laid more emphasis on the characters than on the material bodies which contained them. He proceeded to show that the same attribute, e.g. Chloro- phyll, is present in one plant and absent in a nearly related plant, and is, therefore, not necessarily linked up with other attributes. The same shape of leaf, the same alkaloid, can be characteristic of different species of plants. Hence he inferred that every species consists of an aggregate of many characters, and that these can be repeated in different combinations in the various species. The task of the biologist, then, is to discover these characters.

'It is clear, therefore, that the characteristic structure of each species is the result of a combination of inherited characters, and that most of these are repeated in numberless other varieties. According to this view every species is an extremely complicated structure, and the whole organic world is the product of an infinite variety of combinations and permutations of a relatively small number of characters' (E. Baur, Einfiibrung in d. experimented Vererbungslebre, 1919).

Recently this idea of characters has been further defined. By a character we do not necessarily mean anything material or structural, nothing, in short, actually existing ; not an organ, or a colour, or the like, but such a feature as the specific and characteristic mode of reacting to a stimulus, the development, for example, of a definite colour in the flower. The theory of unit characters has been advanced by the study of the phenomena of hybridization, even more than by the mutation theory. Among all living things it is the almost universal rule that for the production of a new organism two individuals are indispensable. In forms which possess the organs of both sexes, two of those hermaphrodites will usually unite for fertilization, even though self-fertilization is possible, and can take place when necessary. This fact was first observed in the eighteenth century by the German botanist Sprengel working on plants. He taught that nature, in various ways, hinders the direct fertilization of the pistil by pollen from the same flower, while it aids cross-fertilization by pollen borne on another flower.

Darwin accepted this hypothesis. It agreed with his observations on the relation between flowers and insects. Many experiments led him to assert that 'no organic being fertilizes itself for a perpetuity of generations ; but that a cross with another individual is occasionally perhaps at long intervals of time indispensable' (Origin of Species). It seems as if a very great similarity, or very near rela- tionship between the sexes, hinders effective fertilization, and that a certain degree of dissimilarity is necessary to render possible a revitalization by the male or the female. But, if this dissimilarity passes a certain optimum, fertilization becomes more difficult again, or even im- possible. And yet very dissimilar animals and plants can often be crossed successfully. The general rule is that two crossed varieties will produce offspring, but that a cross

between two species, on the other hand, remains unfruit- ful ; there are, however, numerous exceptions to this rule. Darwin paid a great deal of attention to the subject of hybridization. He thought the organism consisted of the sum of its characteristics, just as a number is from one point of view the sum of other numbers which lose their identity and become completely merged in the total. Darwin imagined that, in a hybrid between two forms, the characters are merged in this mathematical way; so that, in the hybrid, it is not the characters of the parents which appear, but the resultant of the fusion of those characters. In addition to this he tried at great length to prove that hybrids between species only differ in degree from those between varieties or between individuals, and that the sterility of the hybrid must be explained as due to secondary causes.

Darwin's relative, Francis Galton, accepted these views in every detail and enlarged upon them. Galton took it for granted that all characters merely represent quanti- tative deviations from some mean value, and that, in any cross, these will be combined in the descendant according to the mathematical theory of probability. Calculations into which we need not enter led him to the conclusion that the descendant always inherits a quarter of his characteristics from each parent, one-sixteenth from each grandparent, one-sixty-fourth from each great-grand- parent, and so on. The same rule must apply in the crossing of forms which are not very closely related ; if a cross once takes place the blood of both parents becomes so intimately mixed in the offspring that it soon becomes quite impossible to tell, from examination of the de- scendants, what the original ancestral forms were like. Nature was searched for hybrids which, having arisen in this manner, had propagated themselves as apparently new forms ; many were found among plants (Verbascum^ Salix, Hieracium, Rosa, &c.).

Meantime, another conception of hybridization was arising, though this was not yet referred to in the con- temporary literature. The subject was being very actively studied during the first half of the nineteenth century. In England the significance of the experiments of Knight and Herbert was being actively discussed. In Holland Gartner had put forward a whole series of new experiments and new theories; in France Naudin was studying the phenomena of hybridization. The objects of these experi- ments were very varied. In some the aim was to discover whether hybrids between two varieties follow different laws from those governing the hybrids between species. In others the aim was to see if we can produce new species by hybridization. In still others, to determine whether the hybrid is more like the father or the mother, and so on. There was a strong inclination to regard hybrids as un- natural products, which do not pass on the characters inherited from both parents, but whose descendants tend to revert to one of the parental types.