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Darwin and Modern Science (1909)

Edited by A.C. Seward


XIII. THE INFLUENCE OF ENVIRONMENT ON THE FORMS OF PLANTS.

By GEORG KLEBS, PH.D.
Professor of Botany in the University of Heidelberg.

B
he dependence of plants on their environment became the object of scientific research when the phenomena of life were first investigated and physiology took its place as a special branch of science. This occurred in the course of the eighteenth century as the result of the pioneer work of Hales, Duhamel, Ingenhousz, Senebier and others. In the nineteenth century, particularly in the second half, physiology experienced an unprecedented development in that it began to concern itself with the experimental study of nutrition and growth, and with the phenomena associated with stimulus and movement; on the other hand, physiology neglected phenomena connected with the production of form, a department of knowledge which was the province of morphology, a purely descriptive science. It was in the middle of the last century that the growth of comparative morphology and the study of phases of development reached their highest point.

The forms of plants appeared to be the expression of their inscrutable inner nature; the stages passed through in the development of the individual were regarded as the outcome of purely internal and hidden laws. The feasibility of experimental inquiry seemed therefore remote. Meanwhile, the recognition of the great importance of such a causal morphology emerged from the researches of the physiologists of that time, more especially from those of Hofmeister (Hofmeister, "Allgemeine Morphologie", Leipzig, 1868, page 579.), and afterwards from the work of Sachs. (Sachs, "Stoff und Form der Pflanzenorgane", Vol. I. 1880; Vol. II. 1882. "Gesammelte Abhandlungen uber Pflanzen-Physiologie", II. Leipzig, 1893.) Hofmeister, in speaking of this line of inquiry, described it as "the most pressing and immediate aim of the investigator to discover to what extent external forces acting on the organism are of importance in determining its form." This advance was the outcome of the influence of that potent force in biology which was created by Darwin's "Origin of Species" (1859).

The significance of the splendid conception of the transformation of species was first recognised and discussed by Lamarck (1809); as an explanation of transformation he at once seized upon the idea--an intelligible view--that the external world is the determining factor. Lamarck (Lamarck, "Philosophie zoologique", pages 223-227. Paris, 1809.) endeavoured, more especially, to demonstrate from the behaviour of plants that changes in environment induce change in form which eventually leads to the production of new species. In the case of animals, Lamarck adopted the teleological view that alterations in the environment first lead to alterations in the needs of the organisms, which, as the result of a kind of conscious effort of will, induce useful modifications and even the development of new organs. His work has not exercised any influence on the progress of science: Darwin himself confessed in regard to Lamarck's work --"I got not a fact or idea from it." ("Life and Letters", Vol. II. page 215.)

On a mass of incomparably richer and more essential data Darwin based his view of the descent of organisms and gained for it general acceptance; as an explanation of modification he elaborated the ingeniously conceived selection theory. The question of special interest in this connection, namely what is the importance of the influence of the environment, Darwin always answered with some hesitation and caution, indeed with a certain amount of indecision.

The fundamental principle underlying his theory is that of general variability as a whole, the nature and extent of which, especially in cultivated organisms, are fully dealt with in his well-known book. (Darwin, "The variation of Animals and Plants under domestication", 2 vols., edition 1, 1868; edition 2, 1875; popular edition 1905.) In regard to the question as to the cause of variability Darwin adopts a consistently mechanical view. He says: "These several considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or, to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability." ("The variation of Animals and Plants" (2nd edition), Vol. II. page 242.) Darwin did not draw further conclusions from this general principle.

Variations produced in organisms by the environment are distinguished by Darwin as "the definite" and "the indefinite." (Ibid. II. page 260. See also "Origin of Species" (6th edition), page 6.) The first occur "when all or nearly all the offspring of an individual exposed to certain conditions during several generations are modified in the same manner." Indefinite variation is much more general and a more important factor in the production of new species; as a result of this, single individuals are distinguished from one another by "slight" differences, first in one then in another character. There may also occur, though this is very rare, more marked modifications, "variations which seem to us in our ignorance to arise spontaneously." ("Origin of Species" (6th edition), page 421.) The selection theory demands the further postulate that such changes, "whether extremely slight or strongly marked," are inherited. Darwin was no nearer to an experimental proof of this assumption than to the discovery of the actual cause of variability. It was not until the later years of his life that Darwin was occupied with the "perplexing problem...what causes almost every cultivated plant to vary" ("Life and Letters", Vol. III. page 342.): he began to make experiments on the influence of the soil, but these were soon given up.

In the course of the violent controversy which was the outcome of Darwin's work the fundamental principles of his teaching were not advanced by any decisive observations. Among the supporters and opponents, Nageli (Nageli, "Theorie der Abstammungslehre", Munich, 1884; cf. Chapter III.) was one of the few who sought to obtain proofs by experimental methods. His extensive cultural experiments with alpine Hieracia led him to form the opinion that the changes which are induced by an alteration in the food-supply, in climate or in habitat, are not inherited and are therefore of no importance from the point of view of the production of species. And yet Nageli did attribute an important influence to the external world; he believed that adaptations of plants arise as reactions to continuous stimuli, which supply a need and are therefore useful. These opinions, which recall the teleological aspect of Lamarckism, are entirely unsupported by proof. While other far-reaching attempts at an explanation of the theory of descent were formulated both in Nageli's time and afterwards, some in support of, others in opposition to Darwin, the necessity of investigating, from different standpoints, the underlying causes, variability and heredity, was more and more realised. To this category belong the statistical investigations undertaken by Quetelet and Galton, the researches into hybridisation, to which an impetus was given by the re- discovery of the Mendelian law of segregation, as also by the culture experiments on mutating species following the work of de Vries, and lastly the consideration of the question how far variation and heredity are governed by external influences. These latter problems, which are concerned in general with the causes of form-production and form- modification, may be treated in a short summary which falls under two heads, one having reference to the conditions of form-production in single species, the other being concerned with the conditions governing the transformation of species.

I. THE INFLUENCE OF EXTERNAL CONDITIONS ON FORM-PRODUCTION IN SINGLE SPECIES.

The members of plants, which we express by the terms stem, leaf, flower, etc. are capable of modification within certain limits; since Lamarck's time this power of modification has been brought more or less into relation with the environment. We are concerned not only with the question of experimental demonstration of this relationship, but, more generally, with an examination of the origin of forms, the sequences of stages in development that are governed by recognisable causes. We have to consider the general problem; to study the conditions of all typical as well as of atypic forms, in other words, to found a physiology of form.

If we survey the endless variety of plant-forms and consider the highly complex and still little known processes in the interior of cells, and if we remember that the whole of this branch of investigation came into existence only a few decades ago, we are able to grasp the fact that a satisfactory explanation of the factors determining form cannot be discovered all at once. The goal is still far away. We are not concerned now with the controversial question, whether, on the whole, the fundamental processes in the development of form can be recognised by physiological means. A belief in the possibility of this can in any case do no harm. What we may and must attempt is this--to discover points of attack on one side or another, which may enable us by means of experimental methods to come into closer touch with these elusive and difficult problems. While we are forced to admit that there is at present much that is insoluble there remains an inexhaustible supply of problems capable of solution.

The object of our investigations is the species; but as regards the question, what is a species, science of to-day takes up a position different from that of Darwin. For him it was the Linnean species which illustrates variation: we now know, thanks to the work of Jordan, de Bary, and particularly to that of de Vries (de Vries, "Die Mutationstheorie", Leipzig, 1901, Vol. I. page 33.), that the Linnean species consists of a large or small number of entities, elementary species. In experimental investigation it is essential that observations be made on a pure species, or, as Johannsen (Johannsen, "Ueber Erblichkeit in Populationen und reinen Linien", Jena, 1903.) says, on a pure "line." What has long been recognised as necessary in the investigation of fungi, bacteria and algae must also be insisted on in the case of flowering plants; we must start with a single individual which is reproduced vegetatively or by strict self-fertilisation. In dioecious plants we must aim at the reproduction of brothers and sisters.

We may at the outset take it for granted that a pure species remains the same under similar external conditions; it varies as these vary. IT IS CHARACTERISTIC OF A SPECIES THAT IT ALWAYS EXHIBITS A CONSTANT RELATION TO A PARTICULAR ENVIRONMENT. In the case of two different species, e.g. the hay and anthrax bacilli or two varieties of Campanula with blue and white flowers respectively, a similar environment produces a constant difference. The cause of this is a mystery.

According to the modern standpoint, the living cell is a complex chemico- physical system which is regarded as a dynamical system of equilibrium, a conception suggested by Herbert Spencer and which has acquired a constantly increasing importance in the light of modern developments in physical chemistry. The various chemical compounds, proteids, carbohydrates, fats, the whole series of different ferments, etc. occur in the cell in a definite physical arrangement. The two systems of two species must as a matter of fact possess a constant difference, which it is necessary to define by a special term. We say, therefore, that the SPECIFIC STRUCTURE is different.

By way of illustrating this provisionally, we may assume that the proteids of the two species possess a constant chemical difference. This conception of specific structure is specially important in its bearing on a further treatment of the subject. In the original cell, eventually also in every cell of a plant, the characters which afterwards become apparent must exist somewhere; they are integral parts of the capabilities or potentialities of specific structure. Thus not only the characters which are exhibited under ordinary conditions in nature, but also many others which become apparent only under special conditions (In this connection I leave out of account, as before, the idea of material carriers of heredity which since the publication of Darwin's Pangenesis hypothesis has been frequently suggested. See my remarks in "Variationen der Bluten", "Pringsheim's Jahrb. Wiss. Bot." 1905, page 298; also Detto, "Biol. Centralbl." 1907, page 81, "Die Erklarbarkeit der Ontogenese durch materielle Anlagen".), are to be included as such potentialities in cells; the conception of specific structure includes the WHOLE OF THE POTENTIALITIES OF A SPECIES; specific structure comprises that which we must always assume without being able to explain it.

A relatively simple substance, such as oxalate of lime, is known under a great number of different crystalline forms belonging to different systems (Compare Kohl's work on "Anatomisch-phys. Untersuchungen uber Kalksalze", etc. Marburg, 1889.); these may occur as single crystals, concretions or as concentric sphaerites. The power to assume this variety of form is in some way inherent in the molecular structure, though we cannot, even in this case, explain the necessary connection between structure and crystalline form. These potentialities can only become operative under the influence of external conditions; their stimulation into activity depends on the degree of concentration of the various solutions, on the nature of the particular calcium salt, on the acid or alkaline reactions. Broadly speaking, the plant cell behaves in a similar way. The manifestation of each form, which is inherent as a potentiality in the specific structure, is ultimately to be referred to external conditions.

An insight into this connection is, however, rendered exceedingly difficult, often quite impossible, because the environment never directly calls into action the potentialities. Its influence is exerted on what we may call the inner world of the organism, the importance of which increases with the degree of differentiation. The production of form in every plant depends upon processes in the interior of the cells, and the nature of these determines which among the possible characters is to be brought to light. In no single case are we acquainted with the internal process responsible for the production of a particular form. All possible factors may play a part, such as osmotic pressure, permeability of the protoplasm, the degree of concentration of the various chemical substances, etc.; all these factors should be included in the category of INTERNAL CONDITIONS. This inner world appears the more hidden from our ken because it is always represented by a certain definite state, whether we are dealing with a single cell or with a small group of cells. These have been produced from pre-existing cells and they in turn from others; the problem is constantly pushed back through a succession of generations until it becomes identified with that of the origin of species.

A way, however, is opened for investigation; experience teaches us that this inner world is not a constant factor: on the contrary, it appears to be very variable. The dependence of VARIABLE INTERNAL on VARIABLE EXTERNAL conditions gives us the key with which research may open the door. In the lower plants this dependence is at once apparent, each cell is directly subject to external influences. In the higher plants with their different organs, these influences were transmitted to cells in course of development along exceedingly complex lines. In the case of the growing-point of a bud, which is capable of producing a complete plant, direct influences play a much less important part than those exerted through other organs, particularly through the roots and leaves, which are essential in nutrition. These correlations, as we may call them, are of the greatest importance as aids to an understanding of form-production. When a bud is produced on a particular part of a plant, it undergoes definite internal modifications induced by the influence of other organs, the activity of which is governed by the environment, and as the result of this it develops along a certain direction; it may, for example, become a flower. The particular direction of development is determined before the rudiment is differentiated and is exerted so strongly that further development ensues without interruption, even though the external conditions vary considerably and exert a positively inimical influence: this produces the impression that development proceeds entirely independently of the outer world. The widespread belief that such independence exists is very premature and at all events unproven.

The state of the young rudiment is the outcome of previous influences of the external world communicated through other organs. Experiments show that in certain cases, if the efficiency of roots and leaves as organs concerned with nutrition is interfered with, the production of flowers is affected, and their characters, which are normally very constant, undergo far-reaching modifications. To find the right moment at which to make the necessary alteration in the environment is indeed difficult and in many cases not yet possible. This is especially the case with fertilised eggs, which in a higher degree than buds have acquired, through parental influences, an apparently fixed internal organisation, and this seems to have pre-determined their development. It is, however, highly probable that it will be possible, by influencing the parents, to alter the internal organisation and to switch off development on to other lines.

Having made these general observations I will now cite a few of the many facts at our disposal, in order to illustrate the methods and aim of the experimental methods of research. As a matter of convenience I will deal separately with modification of development and with modification of single organs.

I. EFFECT OF ENVIRONMENT UPON THE COURSE OF DEVELOPMENT.

Every plant, whether an alga or a flowering plant passes, under natural conditions, through a series of developmental stages characteristic of each species, and these consist in a regular sequence of definite forms. It is impossible to form an opinion from mere observation and description as to what inner changes are essential for the production of the several forms. We must endeavour to influence the inner factors by known external conditions in such a way that the individual stages in development are separately controlled and the order of their sequence determined at will by experimental treatment. Such control over the course of development may be gained with special certainty in the case of the lower organisms.

With these it is practicable to control the principal conditions of cultivation and to vary them in various ways. By this means it has been demonstrated that each developmental stage depends upon special external conditions, and in cases where our knowledge is sufficient, a particular stage may be obtained at will. In the Green Algae (See Klebs, "Die Bedingung der Fortpflanzung...", Jena, 1896; also "Jahrb. fur Wiss. Bot." 1898 and 1900; "Probleme der Entwickelung, III." "Biol. Centralbl." 1904, page 452.), as in the case of Fungi, we may classify the stages of development into purely vegetative growth (growth, cell-division, branching), asexual reproduction (formation of zoospores, conidia) and sexual processes (formation of male and female sexual organs). By modifying the external conditions it is possible to induce algae or fungi (Vaucheria, Saprolegnia) to grow continuously for several years or, in the course of a few days, to die after an enormous production of asexual or sexual cells. In some instances even an almost complete stoppage of growth may be caused, reproductive cells being scarcely formed before the organism is again compelled to resort to reproduction. Thus the sequence of the different stages in development can be modified as we may desire.

The result of a more thorough investigation of the determining conditions appears to produce at first sight a confused impression of all sorts of possibilities. Even closely allied species exhibit differences in regard to the connection between their development and external conditions. It is especially noteworthy that the same form in development may be produced as the result of very different alterations in the environment. At the same time we can undoubtedly detect a certain unity in the multiplicity of the individual phenomena.

If we compare the factors essential for the different stages in development, we see that the question always resolves itself into one of modification of similar conditions common to all life-processes. We should rather have inferred that there exist specific external stimuli for each developmental stage, for instance, certain chemical agencies. Experiments hitherto made support the conclusion that QUANTITATIVE alterations in the general conditions of life produce different types of development. An alga or a fungus grows so long as all the conditions of nutrition remain at a certain optimum for growth. In order to bring about asexual reproduction, e.g. the formation of zoospores, it is sometimes necessary to increase the degree of intensity of external factors; sometimes, on the other hand, these must be reduced in intensity. In the case of many algae a decrease in light-intensity or in the amount of salts in the culture solution, or in the temperature, induces asexual reproduction, while in others, on the contrary, an increase in regard to each of these factors is required to produce the same result. This holds good for the quantitative variations which induce sexual reproduction in algae. The controlling factor is found to be a reduction in the supply of nutritive salts and the exposure of the plants to prolonged illumination or, better still, an increase in the intensity of the light, the efficiency of illumination depending on the consequent formation of organic substances such as carbohydrates.

The quantitative alterations of external conditions may be spoken of as releasing stimuli. They produce, in the complex equilibrium of the cell, quantitative modifications in the arrangement and distribution of mass, by means of which other chemical processes are at once set in motion, and finally a new condition of equilibrium is attained. But the commonly expressed view that the environment can as a rule act only as a releasing agent is incorrect, because it overlooks an essential point. The power of a cell to receive stimuli is only acquired as the result of previous nutrition, which has produced a definite condition of concentration of different substances. Quantities are in this case the determining factors. The distribution of quantities is especially important in the sexual reproduction of algae, for which a vigorous production of the materials formed during carbon-assimilation appears to be essential.

In the Flowering plants, on the other hand, for reasons already mentioned, the whole problem is more complicated. Investigations on changes in the course of development of fertilised eggs have hitherto been unsuccessful; the difficulty of influencing egg-cells deeply immersed in tissue constitutes a serious obstacle. Other parts of plants are, however, convenient objects of experiment; e.g. the growing apices of buds which serve as cuttings for reproductive purposes, or buds on tubers, runners, rhizomes, etc. A growing apex consists of cells capable of division in which, as in egg-cells, a complete series of latent possibilities of development is embodied. Which of these possibilities becomes effective depends upon the action of the outer world transmitted by organs concerned with nutrition.

Of the different stages which a flowering plant passes through in the course of its development we will deal only with one in order to show that, in spite of its great complexity, the problem is, in essentials, equally open to attack in the higher plants and in the simplest organisms. The most important stage in the life of a flowering plant is the transition from purely vegetative growth to sexual reproduction--that is, the production of flowers. In certain cases it can be demonstrated that there is no internal cause, dependent simply on the specific structure, which compels a plant to produce its flowers after a definite period of vegetative growth. (Klebs, "Willkurliche Entwickelungsanderungen", Jena 1903; see also "Probleme der Entwickelung", I. II. "Centralbl." 1904.)

One extreme case, that of exceptionally early flowering, has been observed in nature and more often in cultivation. A number of plants under certain conditions are able to flower soon after germination. (Cf. numerous records of this kind by Diels, "Jugendformen und Bluten", Berlin, 1906.) This shortening of the period of development is exhibited in the most striking form in trees, as in the oak (Mobius, "Beitrage zur Lehre von der Fortpflanzung", Jena, 1897, page 89.), flowering seedlings of which have been observed from one to three years old, whereas normally the tree does not flower until it is sixty or eighty years old.

Another extreme case is represented by prolonged vegetative growth leading to the complete suppression of flower-production. This result may be obtained with several plants, such as Glechoma, the sugar beet, Digitalis, and others, if they are kept during the winter in a warm, damp atmosphere, and in rich soil; in the following spring or summer they fail to flower. (Klebs, "Willkurliche Aenderungen", etc. Jena, 1903, page 130.) Theoretically, however, experiments are of greater importance in which the production of flowers is inhibited by very favourable conditions of nutrition (Klebs, "Ueber kunstliche Metamorphosen", Stuttgart, 1906, page 115 ("Abh. Naturf. Ges. Halle", XXV.) occurring at the normal flowering period. Even in the case of plants of Sempervivum several years old, which, as is shown by control experiments on precisely similar plants, are on the point of flowering, flowering is rendered impossible if they are forced to very vigorous growth by an abundant supply of water and salts in the spring. Flowering, however, occurs, if such plants are cultivated in relatively dry sandy soil and in the presence of strong light. Careful researches into the conditions of growth have led, in the cases Sempervivum, to the following results: (1) With a strong light and vigorous carbon-assimilation a considerably increased supply of water and nutritive salts produces active vegetative growth. (2) With a vigorous carbon-assimilation in strong light, and a decrease in the supply of water and salts active flower-production is induced. (3) If an average supply of water and salts is given both processes are possible; the intensity of carbon-assimilation determines which of the two is manifested. A diminution in the production of organic substances, particularly of carbohydrates, induces vegetative growth. This can be effected by culture in feeble light or in light deprived of the yellow-red rays: on the other hand, flower-production follows an increase in light-intensity. These results are essentially in agreement with well-known observations on cultivated plants, according to which, the application of much moisture, after a plentiful supply of manure composed of inorganic salts, hinders the flower-production of many vegetables, while a decrease in the supply of water and salts favours flowering.

ii. INFLUENCE OF THE ENVIRONMENT ON THE FORM OF SINGLE ORGANS. (A considerable number of observations bearing on this question are given by Goebel in his "Experimentelle Morphologie der Pflanzen", Leipzig, 1908. It is not possible to deal here with the alteration in anatomical structure; cf. Kuster, "Pathologische Pflanzenanatomie", Jena, 1903.)

If we look closely into the development of a flowering plant, we notice that in a given species differently formed organs occur in definite positions. In a potato plant colourless runners are formed from the base of the main stem which grow underground and produce tubers at their tips: from a higher level foliage shoots arise nearer the apex. External appearances suggest that both the place of origin and the form of these organs were predetermined in the egg-cell or in the tuber. But it was shown experimentally by the well-known investigator Knight (Knight, "Selection from the Physiological and Horticultural Papers", London, 1841.) that tubers may be developed on the aerial stem in place of foliage shoots. These observations were considerably extended by Vochting. (Vochting, "Ueber die Bildung der Knollen", Cassel, 1887; see also "Bot. Zeit." 1902, 87.) In one kind of potato, germinating tubers were induced to form foliage shoots under the influence of a higher temperature; at a lower temperature they formed tuber-bearing shoots. Many other examples of the conversion of foliage-shoots into runners and rhizomes, or vice versa, have been described by Goebel and others. As in the asexual reproduction of algae quantitative alteration in the amount of moisture, light, temperature, etc. determines whether this or that form of shoot is produced. If the primordia of these organs are exposed to altered conditions of nutrition at a sufficiently early stage a complete substitution of one organ for another is effected. If the rudiment has reached a certain stage in development before it is exposed to these influences, extraordinary intermediate forms are obtained, bearing the characters of both organs.

The study of regeneration following injury is of greater importance as regards the problem of the development and place of origin of organs. (Reference may be made to the full summary of results given by Goebel in his "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section IV.) Only in relatively very rare cases is there a complete re-formation of the injured organ itself, as e.g. in the growing-apex. Much more commonly injury leads to the development of complementary formations, it may be the rejuvenescence of a hitherto dormant rudiment, or it may be the formation of such ab initio. In all organs, stems, roots, leaves, as well as inflorescences, this kind of regeneration, which occurs in a great variety of ways according to the species, may be observed on detached pieces of the plant. Cases are also known, such, for example, as the leaves of many plants which readily form roots but not shoots, where a complete regeneration does not occur.

The widely spread power of reacting to wounding affords a very valuable means of inducing a fresh development of buds and roots on places where they do not occur in normal circumstances. Injury creates special conditions, but little is known as yet in regard to alterations directly produced in this way. Where the injury consists in the separation of an organ from its normal connections, the factors concerned are more comprehensible. A detached leaf, e.g., is at once cut off from a supply of water and salts, and is deprived of the means of getting rid of organic substances which it produces; the result is a considerable alteration in the degree of concentration. No experimental investigation on these lines has yet been made. Our ignorance has often led to the view that we are dealing with a force whose specific quality is the restitution of the parts lost by operation; the proof, therefore, that in certain cases a similar production of new roots or buds may be induced without previous injury and simply by a change in external conditions assumes an importance. (Klebs, "Willkurliche Entwickelung", page 100; also, "Probleme der Entwickelung", "Biol. Centralbl." 1904, page 610.)

A specially striking phenomenon of regeneration, exhibited also by uninjured plants, is afforded by polarity, which was discovered by Vochting. (See the classic work of Vochting, "Ueber Organbildung im Pflanzenreich", I. Bonn, 1888; also "Bot. Zeit. 1906, page 101; cf. Goebel, "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section V, Polaritat.) It is found, for example, that roots are formed from the base of a detached piece of stem and shoots from the apex. Within the limits of this essay it is impossible to go into this difficult question; it is, however, important from the point of view of our general survey to emphasise the fact that the physiological distinctions between base and apex of pieces of stem are only of a quantitative kind, that is, they consist in the inhibition of certain phenomena or in favouring them. As a matter of fact roots may be produced from the apices of willows and cuttings of other plants; the distinction is thus obliterated under the influence of environment. The fixed polarity of cuttings from full grown stems cannot be destroyed; it is the expression of previous development. Vochting speaks of polarity as a fixed inherited character. This is an unconvincing conclusion, as nothing can be deduced from our present knowledge as to the causes which led up to polarity. We know that the fertilised egg, like the embryo, is fixed at one end by which it hangs freely in the embryo-sac and afterwards in the endosperm. From the first, therefore, the two ends have different natures, and these are revealed in the differentiation into root-apex and stem-apex. A definite direction in the flow of food-substances is correlated with this arrangement, and this eventually leads to a polarity in the tissues. This view requires experimental proof, which in the case of the egg-cells of flowering plants hardly appears possible; but it derives considerable support from the fact that in herbaceous plants, e.g. Sempervivum (Klebs, "Variationen der Bluten", "Jahrb. Wiss. Bot." 1905, page 260.), rosettes or flower-shoots are formed in response to external conditions at the base, in the middle, or at the apex of the stem, so that polarity as it occurs under normal conditions cannot be the result of unalterable hereditary factors. On the other hand, the lower plants should furnish decisive evidence on this question, and the experiments of Stahl, Winkler, Kniep, and others indicate the right method of attacking the problem.

The relation of leaf-form to environment has often been investigated and is well known. The leaves of bog and water plants (Cf.Goebel, loc. cit. chapter II.; also Gluck, "Untersuchungen uber Wasser- und Sumpfgewachse", Jena, Vols. I.-II. 1905-06.) afford the most striking examples of modifications: according as they are grown in water, moist or dry air, the form of the species characteristic of the particular habitat is produced, since the stems are also modified. To the same group of phenomena belongs the modification of the forms of leaves and stems in plants on transplantation from the plains to the mountains (Bonnier, "Recherches sur l'Anatomie experimentale des Vegetaux", Corbeil, 1895.) or vice versa. Such variations are by no means isolated examples. All plants exhibit a definite alteration in form as the result of prolonged cultivation in moist or dry air, in strong or feeble light, or in darkness, or in salt solutions of different composition and strength.

Every individual which is exposed to definite combinations of external factors exhibits eventually the same type of modification. This is the type of variation which Darwin termed "definite." It is easy to realise that indefinite or fluctuating variations belong essentially to the same class of phenomena; both are reactions to changes in environment. In the production of individual variations two different influences undoubtedly cooperate. One set of variations is caused by different external conditions, during the production, either of sexual cells or of vegetative primordia; another set is the result of varying external conditions during the development of the embryo into an adult plant. The two sets of influences cannot as yet be sharply differentiated. If, for purposes of vegetative reproduction, we select pieces of the same parent-plant of a pure species, the second type of variation predominates. Individual fluctuations depend essentially in such cases on small variations in environment during development.

These relations must be borne in mind if we wish to understand the results of statistical methods. Since the work of Quetelet, Galton, and others the statistical examination of individual differences in animals and plants has become a special science, which is primarily based on the consideration that the application of the theory of probability renders possible mathematical statement and control of the results. The facts show that any character, size of leaf, length of stem, the number of members in a flower, etc. do not vary haphazard but in a very regular manner. In most cases it is found that there is a value which occurs most commonly, the average or medium value, from which the larger and smaller deviations, the so-called plus and minus variations fall away in a continuous series and end in a limiting value. In the simpler cases a falling off occurs equally on both sides of the curve; the curve constructed from such data agrees very closely with the Gaussian curve of error. In more complicated cases irregular curves of different kinds are obtained which may be calculated on certain suppositions.

The regular fluctuations about a mean according to the rule of probability is often attributed to some law underlying variability. (de Vries, "Mutationstheorie", Vol. I. page 35, Leipzig, 1901.) But there is no such law which compels a plant to vary in a particular manner. Every experimental investigation shows, as we have already remarked, that the fluctuation of characters depends on fluctuation in the external factors. The applicability of the method of probability follows from the fact that the numerous individuals of a species are influenced by a limited number of variable conditions. (Klebs, "Willkurl. Ent." Jena, 1903, page 141.) As each of these conditions includes within certain limits all possible values and exhibits all possible combinations, it follows that, according to the rules of probability, there must be a mean value, about which the larger and smaller deviations are distributed. Any character will be found to have the mean value which corresponds with that combination of determining factors which occurs most frequently. Deviations towards plus and minus values will be correspondingly produced by rarer conditions.

A conclusion of fundamental importance may be drawn from this conception, which is, to a certain extent, supported by experimental investigation. (Klebs, "Studien uber Variation", "Arch. fur Entw." 1907.) There is no normal curve for a particular CHARACTER, there is only a curve for the varying combinations of conditions occurring in nature or under cultivation. Under other conditions entirely different curves may be obtained with other variants as a mean value. If, for example, under ordinary conditions the number 10 is the most frequent variant for the stamens of Sedum spectabile, in special circumstances (red light) this is replaced by the number 5. The more accurately we know the conditions for a particular form or number, and are able to reproduce it by experiment, the nearer we are to achieving our aim of rendering a particular variation impossible or of making it dominant.

In addition to the individual variations of a species, more pronounced fluctuations occur relatively rarely and sporadically which are spoken of as "single variations," or if specially striking as abnormalities or monstrosities. These forms have long attracted the attention of morphologists; a large number of observations of this kind are given in the handbooks of Masters (Masters, "Vegetable Teratology", London, 1869.) and Penzig (Penzig, "Pflanzen-Teratologie, Vols I. and II. Genua, 1890-94.) These variations, which used to be regarded as curiosities, have now assumed considerable importance in connection with the causes of form- development. They also possess special interest in relation to the question of heredity, a subject which does not at present concern us, as such deviations from normal development undoubtedly arise as individual variations induced by the influence of environment.

Abnormal developments of all kinds in stems, leaves, and flowers, may be produced by parasites, insects, or fungi. They may also be induced by injury, as Blaringhem (Blaringhem, "Mutation et traumatismes", Paris, 1907.) has more particularly demonstrated, which, by cutting away the leading shoots of branches in an early stage of development, caused fasciation, torsion, anomalous flowers, etc. The experiments of Blaringhem point to the probability that disturbances in the conditions of food-supply consequent on injury are the cause of the production of monstrosities. This is certainly the case in my experiments with species of Sempervivum (Klebs, "Kunstliche Metamorphosen", Stuttgart, 1906.); individuals, which at first formed normal flowers, produced a great variety of abnormalities as the result of changes in nutrition, we may call to mind the fact that the formation of inflorescences occurs normally when a vigorous production of organic compounds, such as starch, sugar, etc. follows a diminution in the supply of mineral salts. On the other hand, the development of inflorescences is entirely suppressed if, at a suitable moment before the actual foundations have been laid, water and mineral salts are supplied to the roots. If, during the week when the inflorescence has just been laid down and is growing very slowly, the supply of water and salts is increased, the internal conditions of the cells are essentially changed. At a later stage, after the elongation of the inflorescence, rosettes of leaves are produced instead of flowers, and structures intermediate between the two kinds of organs; a number of peculiar plant-forms are thus obtained (Cf. Lotsy, "Vorlesungen uber Deszendenztheorien", Vol. II. pl. 3, Jena, 1908.) Abnormalities in the greatest variety are produced in flowers by varying the time at which the stimulus is applied, and by the cooperation of other factors such as temperature, darkness, etc. In number and arrangement the several floral members vary within wide limits; sepals, petals, stamens, and carpels are altered in form and colour, a transformation of stamens to carpels and from carpels to stamens occurs in varying degrees. The majority of the deviations observed had not previously been seen either under natural conditions or in cultivation; they were first brought to light through the influence of external factors.

Such transformations of flowers become apparent at a time, which is separated by about two months from the period at which the particular cause began to act. There is, therefore, no close connection between the appearance of the modifications and the external conditions which prevail at the moment. When we are ignorant of the causes which are operative so long before the results are seen, we gain the impression that such variations as occur are spontaneous or autonomous expressions of the inner nature of the plant. It is much more likely that, as in Sempervivum, they were originally produced by an external stimulus which had previously reached the sexual cells or the young embryo. In any case abnormalities of this kind appear to be of a special type as compared with ordinary fluctuating variations. Darwin pointed out this difference; Bateson (Bateson, "Materials for the study of Variation", London, 1894, page 5.) has attempted to make the distinction sharper, at the same time emphasising its importance in heredity.

Bateson applies the term CONTINUOUS to small variations connected with one another by transitional stages, while those which are more striking and characterised from the first by a certain completeness, he names DISCONTINUOUS. He drew attention to a great difficulty which stands in the way of Lamarck's hypothesis, as also of Darwin's view. "According to both theories, specific diversity of form is consequent upon diversity of environment, and diversity of environment is thus the ultimate measure of diversity of specific form. Here then we meet the difficulty that diverse environments often shade into each other insensibly and form a continuous series, whereas the Specific Forms of life which are subject to them on the whole form a Discontinuous Series." This difficulty is, however, not of fundamental importance as well authenticated facts have been adduced showing that by alteration of the environment discontinuous variations, such as alterations in the number and form of members of a flower, may be produced. We can as yet no more explain how this happens than we can explain the existence of continuous variations. We can only assert that both kinds of variation arise in response to quantitative alterations in external conditions. The question as to which kind of variation is produced depends on the greater or less degree of alteration; it is correlated with the state of the particular cells at the moment.

In this short sketch it is only possible to deal superficially with a small part of the subject. It has been clearly shown that in view of the general dependence of development on the factors of the environment a number of problems are ready for experimental treatment. One must, however, not forget that the science of the physiology of form has not progressed beyond its initial stages. Just now our first duty is to demonstrate the dependence on external factors in as many forms of plants as possible, in order to obtain a more thorough control of all the different plant-forms. The problem is not only to produce at will (and independently of their normal mode of life) forms which occur in nature, but also to stimulate into operation potentialities which necessarily lie dormant under the conditions which prevail in nature. The constitution of a species is much richer in possibilities of development than would appear to be the case under normal conditions. It remains for man to stimulate into activity all the potentialities.

But the control of plant-form is only a preliminary step--the foundation stones on which to erect a coherent scientific structure. We must discover what are the internal processes in the cell produced by external factors, which as a necessary consequence result in the appearance of a definite form. We are here brought into contact with the most obscure problem of life. Progress can only be made pari passu with progress in physics and chemistry, and with the growth of our knowledge of nutrition, growth, etc.

Let us take one of the simplest cases--an alteration in form. A cylindrical cell of the alga Stigeoclonium assumes, as Livingstone (Livingstone, "On the nature of the stimulus which causes the change of form, etc." "Botanical Gazette", XXX. 1900; also XXXII. 1901.) has shown, a spherical form when the osmotic pressure of the culture fluid is increased; or a spore of Mucor, which, in a sugar solution grows into a branched filament, in the presence of a small quantity of acid (hydrogen ions) becomes a comparatively large sphere. (Ritter, "Ueber Kugelhefe, etc." "Ber. bot. Gesell." Berlin, XXV. page 255, 1907.) In both cases there has undoubtedly been an alteration in the osmotic pressure of the cell-sap, but this does not suffice to explain the alteration in form, since the unknown alterations, which are induced in the protoplasm, must in their turn influence the cell-membrane. In the case of the very much more complex alterations in form, such as we encounter in the course of development of plants, there do not appear to be any clues which lead us to a deeper insight into the phenomena. Nevertheless we continue the attempt, seeking with the help of any available hypothesis for points of attack, which may enable us to acquire a more complete mastery of physiological methods. To quote a single example; I may put the question, what internal changes produce a transition from vegetative growth to sexual reproduction?

The facts, which are as clearly established from the lower as for the higher plants, teach us that quantitative alteration in the environment produces such a transition. This suggests the conclusion that quantitative internal changes in the cells, and with them disturbances in the degree of concentration, are induced, through which the chemical reactions are led in the direction of sexual reproduction. An increase in the production of organic substances in the presence of light, chiefly of the carbohydrates, with a simultaneous decrease in the amount of inorganic salts and water, are the cause of the disturbance and at the same time of the alteration in the direction of development. Possibly indeed mineral salts as such are not in question, but only in the form of other organic combinations, particularly proteid material, so that we are concerned with an alteration in the relation of the carbohydrates and proteids. The difficulties of such researches are very great because the methods are not yet sufficiently exact to demonstrate the frequently small quantitative differences in chemical composition. Questions relating to the enzymes, which are of the greatest importance in all these life-processes, are especially complicated. In any case it is the necessary result of such an hypothesis that we must employ chemical methods of investigation in dealing with problems connected with the physiology of form.

II. INFLUENCE OF ENVIRONMENT ON THE TRANSFORMATION OF SPECIES.

The study of the physiology of form-development in a pure species has already yielded results and makes slow but sure progress. The physiology of the possibility of the transformation of one species into another is based, as yet, rather on pious hope than on accomplished fact. From the first it appeared to be hopeless to investigate physiologically the origin of Linnean species and at the same time that of the natural system, an aim which Darwin had before him in his enduring work. The historical sequence of events, of which an organism is the expression, can only be treated hypothetically with the help of facts supplied by comparative morphology, the history of development, geographical distribution, and palaeontology. (See Lotsy, "Vorlesungen" (Jena, I. 1906, II. 1908), for summary of the facts.) A glance at the controversy which is going on today in regard to different hypotheses shows that the same material may lead different investigators to form entirely different opinions. Our ultimate aim is to find a solution of the problem as to the cause of the origin of species. Indeed such attempts are now being made: they are justified by the fact that under cultivation new and permanent strains are produced; the fundamental importance of this was first grasped by Darwin. New points of view in regard to these lines of inquiry have been adopted by H. de Vries who has succeeded in obtaining from Oenothera Lamarckiana a number of constant "elementary" species. Even if it is demonstrated that he was simply dealing with the complex splitting up of a hybrid (Bateson, "Reports to the Evolution Committee of the Royal Society", London, 1902; cf. also Lotsy, "Vorlesungen", Vol. I. page 234.), the facts adduced in no sense lose their very great value.

We must look at the problem in its simplest form; we find it in every case where a new race differs essentially from the original type in a single character only; for example, in the colour of the flowers or in the petalody of the stamens (doubling of flowers). In this connection we must keep in view the fact that every visible character in a plant is the resultant of the cooperation of specific structure, with its various potentialities, and the influence of the environment. We know, that in a pure species all characters vary, that a blue-flowering Campanula or a red Sempervivum can be converted by experiment into white-flowering forms, that a transformation of stamens into petals may be caused by fungi or by the influence of changed conditions of nutrition, or that plants in dry and poor soil become dwarfed. But so far as the experiments justify a conclusion, it would appear that such alterations are not inherited by the offspring. Like all other variations they appear only so long as special conditions prevail in the surroundings.

It has been shown that the case is quite different as regards the white- flowering, double or dwarf races, because these retain their characters when cultivated under practically identical conditions, and side by side with the blue, single-flowering or tall races. The problem may therefore be stated thus: how can a character, which appears in the one case only under the strictly limited conditions of the experiment, in other cases become apparent under the very much wider conditions of ordinary cultivation? If a character appears, in these circumstances, in the case of all individuals, we then speak of constant races. In such simple cases the essential point is not the creation of a new character but rather an ALTERATION OF THIS CHARACTER IN ACCORDANCE WITH THE ENVIRONMENT. In the examples mentioned the modified character in the simple varieties (or a number of characters in elementary species) appears more or less suddenly and is constant in the above sense. The result is what de Vries has termed a Mutation. In this connection we must bear in mind the fact that no difference, recognisable externally, need exist between individual variation and mutation. Even the most minute quantitative difference between two plants may be of specific value if it is preserved under similar external conditions during many successive generations. We do not know how this happens. We may state the problem in other terms; by saying that the specific structure must be altered. It is possible, to some extent, to explain this sudden alteration, if we regard it as a chemical alteration of structure either in the specific qualities of the proteids or of the unknown carriers of life. In the case of many organic compounds their morphological characters (the physical condition, crystalline form, etc.) are at once changed by alteration of atomic relations or by incorporation of new radicals. (For instance ethylchloride (C2H5Cl) is a gas at 21 deg C., ethylenechloride (C2H4Cl2) a fluid boiling at 84 deg C., beta trichlorethane (C2H3Cl3) a fluid boiling at 113 deg C., perchlorethane (C2Cl6) a crystalline substance. Klebs, ("Willkurliche Entwickelungsanderungen" page 158.) Much more important, however, would be an answer to the question, whether an individual variation can be converted experimentally into an inherited character—a mutation in de Vries's sense.

In all circumstances we may recognise as a guiding principle the assumption adopted by Lamarck, Darwin, and many others, that the inheritance of any one character, or in more general terms, the transformation of one species into another, is, in the last instance, to be referred to a change in the environment. From a causal-mechanical point of view it is not a priori conceivable that one species can ever become changed into another so long as external conditions remain constant. The inner structure of a species must be essentially altered by external influences. Two methods of experimental research may be adopted, the effect of crossing distinct species and, secondly, the effect of definite factors of the environment.

The subject of hybridisation is dealt with in another part of this essay. It is enough to refer here to the most important fact, that as the result of combinations of characters of different species new and constant forms are produced. Further, Tschermack, Bateson and others have demonstrated the possibility that hitherto unknown inheritable characters may be produced by hybridisation.

The other method of producing constant races by the influence of special external conditions has often been employed. The sporeless races of Bacteria and Yeasts (Cf. Detto, "Die Theorie der direkten Anpassung...", pages 98 et seq., Jena, 1904; see also Lotsy, "Vorlesungen", II. pages 636 et seq., where other similar cases are described.) are well known, in which an internal alteration of the cells is induced by the influence of poison or higher temperature, so that the power of producing spores even under normal conditions appears to be lost. A similar state of things is found in some races which under certain definite conditions lose their colour or their virulence. Among the phanerogams the investigations of Schubler on cereals afford parallel cases, in which the influence of a northern climate produces individuals which ripen their seeds early; these seeds produce plants which seed early in southern countries. Analogous results were obtained by Cieslar in his experiments; seeds of conifers from the Alps when planted in the plains produced plants of slow growth and small diameter.

All these observations are of considerable interest theoretically; they show that the action of environment certainly induces such internal changes, and that these are transmitted to the next generation. But as regards the main question, whether constant races may be obtained by this means, the experiments cannot as yet supply a definite answer. In phanerogams, the influence very soon dies out in succeeding generations; in the case of bacteria, in which it is only a question of the loss of a character it is relatively easy for this to reappear. It is not impossible, that in all such cases there is a material hanging-on of certain internal conditions, in consequence of which the modification of the character persists for a time in the descendants, although the original external conditions are no longer present.

Thus a slow dying-out of the effect of a stimulus was seen in my experiments on Veronica chamaedrys. (Klebs, "Kunstliche Metamorphosen", Stuttgart, 1906, page 132.) During the cultivation of an artificially modified inflorescence I obtained a race showing modifications in different directions, among which twisting was especially conspicuous. This plant, however, does not behave as the twisted race of Dipsacus isolated by de Vries (de Vries, "Mutationstheorie", Vol. II. Leipzig, 1903, page 573.), which produced each year a definite percentage of twisted individuals. In the vegetative reproduction of this Veronica the torsion appeared in the first, also in the second and third year, but with diminishing intensity. In spite of good cultivation this character has apparently now disappeared; it disappeared still more quickly in seedlings. In another character of the same Veronica chamaedrys the influence of the environment was stronger. The transformation of the inflorescences to foliage-shoots formed the starting-point; it occurred only under narrowly defined conditions, namely on cultivation as a cutting in moist air and on removal of all other leaf- buds. In the majority (7/10) of the plants obtained from the transformed shoots, the modification appeared in the following year without any interference. Of the three plants which were under observation several years the first lost the character in a short time, while the two others still retain it, after vegetative propagation, in varying degrees. The same character occurs also in some of the seedlings; but anything approaching a constant race has not been produced.

Another means of producing new races has been attempted by Blaringhem. (Blaringhem, "Mutation et Traumatisme", Paris, 1907.) On removing at an early stage the main shoots of different plants he observed various abnormalities in the newly formed basal shoots. From the seeds of such plants he obtained races, a large percentage of which exhibited these abnormalities. Starting from a male Maize plant with a fasciated inflorescence, on which a proportion of the flowers had become male, a new race was bred in which hermaphrodite flowers were frequently produced. In the same way Blaringhem obtained, among other similar results, a race of barley with branched ears. These races, however, behaved in essentials like those which have been demonstrated by de Vries to be inconstant, e.g. Trifolium pratense quinquefolium and others. The abnormality appears in a proportion of the individuals and only under very special conditions. It must be remembered too that Blaringhem worked with old cultivated plants, which from the first had been disposed to split into a great variety of races. It is possible, but difficult to prove, that injury contributed to this result.

A third method has been adopted by MacDougal (MacDougal, "Heredity and Origin of species", "Monist", 1906; "Report of department of botanical research", "Fifth Year-book of the Carnegie Institution of Washington", page 119, 1907.) who injected strong (10 percent) sugar solution or weak solutions of calcium nitrate and zinc sulphate into young carpels of different plants. From the seeds of a plant of Raimannia odorata the carpels of which had been thus treated he obtained several plants distinguished from the parent-forms by the absence of hairs and by distinct forms of leaves. Further examination showed that he had here to do with a new elementary species. MacDougal also obtained a more or less distinct mutant of Oenothera biennis. We cannot as yet form an opinion as to how far the effect is due to the wound or to the injection of fluid as such, or to its chemical properties. This, however, is not so essential as to decide whether the mutant stands in any relation to the influence of external factors. It is at any rate very important that this kind of investigation should be carried further.

If it could be shown that new and inherited races were obtained by MacDougal's method, it would be safe to conclude that the same end might be gained by altering the conditions of the food-stuff conducted to the sexual cells. New races or elementary species, however, arise without wounding or injection. This at once raises the much discussed question, how far garden-cultivation has led to the creation of new races? Contrary to the opinion expressed by Darwin and others, de Vries ("Mutationstheorie", Vol. I. pages 412 et seq.) tried to show that garden-races have been produced only from spontaneous types which occur in a wild state or from sub-races, which the breeder has accidentally discovered but not originated. In a small number of cases only has de Vries adduced definite proof. On the other side we have the work of Korschinsky (Korschinsky, "Heterogenesis und Evolution", "Flora", 1901.) which shows that whole series of garden-races have made their appearance only after years of cultivation. In the majority of races we are entirely ignorant of their origin.

It is, however, a fact that if a plant is removed from natural conditions into cultivation, a well-marked variation occurs. The well-known plant- breeder L. de Vilmorin (L. de Vilmorin, "Notices sur l'amelioration des plantes", Paris, 1886, page 36.), speaking from his own experience, states that a plant is induced to "affoler," that is to exhibit all possible variations from which the breeder may make a further selection only after cultivation for several generations. The effect of cultivation was particularly striking in Veronica chamaedrys (Klebs, "Kunstliche Metamorphosen", Stuttgart, 1906, page 152.) which, in spite of its wide distribution in nature, varies very little. After a few years of cultivation this "good" and constant species becomes highly variable. The specimens on which the experiments were made were three modified inflorescence cuttings, the parent-plants of which certainly exhibited no striking abnormalities. In a short time many hitherto latent potentialities became apparent, so that characters, never previously observed, or at least very rarely, were exhibited, such as scattered leaf- arrangement, torsion, terminal or branched inflorescences, the conversion of the inflorescence into foliage-shoots, every conceivable alteration in the colour of flowers, the assumption of a green colour by parts of the flowers, the proliferation of flowers.

All this points to some disturbance in the species resulting from methods of cultivation. It has, however, not yet been possible to produce constant races with any one of these modified characters. But variations appeared among the seedlings, some of which, e.g. yellow variegation, were not inheritable, while others have proved constant. This holds good, so far as we know at present, for a small rose-coloured form which is to be reckoned as a mutation. Thus the prospect of producing new races by cultivation appears to be full of promise.

So long as the view is held that good nourishment, i.e. a plentiful supply of water and salts, constitutes the essential characteristic of garden- cultivation, we can hardly conceive that new mutations can be thus produced. But perhaps the view here put forward in regard to the production of form throws new light on this puzzling problem.

Good manuring is in the highest degree favourable to vegetative growth, but is in no way equally favourable to the formation of flowers. The constantly repeated expression, good or favourable nourishment, is not only vague but misleading, because circumstances favourable to growth differ from those which promote reproduction; for the production of every form there are certain favourable conditions of nourishment, which may be defined for each species. Experience shows that, within definite and often very wide limits, it does not depend upon the ABSOLUTE AMOUNT of the various food substances, but upon their respective degrees of concentration. As we have already stated, the production of flowers follows a relative increase in the amount of carbohydrates formed in the presence of light, as compared with the inorganic salts on which the formation of albuminous substances depends. (Klebs, "Kunstliche Metamorphosen", page 117.) The various modifications of flowers are due to the fact that a relatively too strong solution of salts is supplied to the rudiments of these organs. As a general rule every plant form depends upon a certain relation between the different chemical substances in the cells and is modified by an alteration of that relation.

During long cultivation under conditions which vary in very different degrees, such as moisture, the amount of salts, light intensity, temperature, oxygen, it is possible that sudden and special disturbances in the relations of the cell substances have a directive influence on the inner organisation of the sexual cells, so that not only inconstant but also constant varieties will be formed.

Definite proof in support of this view has not yet been furnished, and we must admit that the question as to the cause of heredity remains, fundamentally, as far from solution as it was in Darwin's time. As the result of the work of many investigators, particularly de Vries, the problem is constantly becoming clearer and more definite. The penetration into this most difficult and therefore most interesting problem of life and the creation by experiment of new races or elementary species are no longer beyond the region of possibility.



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