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

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

The 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.

XIV. EXPERIMENTAL STUDY OF THE INFLUENCE OF ENVIRONMENT ON ANIMALS.

By JACQUES LOEB, M.D.
Professor of Physiology in the University of California.

I. INTRODUCTORY REMARKS.

What the biologist calls the natural environment of an animal is from a physical point of view a rather rigid combination of definite forces. It is obvious that by a purposeful and systematic variation of these and by the application of other forces in the laboratory, results must be obtainable which do not appear in the natural environment. This is the reasoning underlying the modern development of the study of the effects of environment upon animal life. It was perhaps not the least important of Darwin’s services to science that the boldness of his conceptions gave to the experimental biologist courage to enter upon the attempt of controlling at will the life-phenomena of animals, and of bringing about effects which cannot be expected in Nature.

The systematic physico-chemical analysis of the effect of outside forces upon the form and reactions of animals is also our only means of unravelling the mechanism of heredity beyond the scope of the Mendelian law. The manner in which a germ-cell can force upon the adult certain characters will not be understood until we succeed in varying and controlling hereditary characteristics; and this can only be accomplished on the basis of a systematic study of the effects of chemical and physical forces upon living matter.

Owing to limitation of space this sketch is necessarily very incomplete, and it must not be inferred that studies which are not mentioned here were considered to be of minor importance. All the writer could hope to do was to bring together a few instances of the experimental analysis of the effect of environment, which indicate the nature and extent of our control over life-phenomena and which also have some relation to the work of Darwin. In the selection of these instances preference is given to those problems which are not too technical for the general reader.

The forces, the influence of which we shall discuss, are in succession chemical agencies, temperature, light, and gravitation. We shall also treat separately the effect of these forces upon form and instinctive reactions.

II. THE EFFECTS OF CHEMICAL AGENCIES.

(a) HETEROGENEOUS HYBRIDISATION.

It was held until recently that hybridisation is not possible except between closely related species and that even among these a successful hybridisation cannot always be counted upon. This view was well supported by experience. It is, for instance, well known that the majority of marine animals lay their unfertilised eggs in the ocean and that the males shed their sperm also into the sea-water. The numerical excess of the spermatozoa over the ova in the sea-water is the only guarantee that the eggs are fertilised, for the spermatozoa are carried to the eggs by chance and are not attracted by the latter. This statement is the result of numerous experiments by various authors, and is contrary to common belief. As a rule all or the majority of individuals of a species in a given region spawn on the same day, and when this occurs the sea-water constitutes a veritable suspension of sperm. It has been shown by experiment that in fresh sea-water the sperm may live and retain its fertilising power for several days. It is thus unavoidable that at certain periods more than one kind of spermatozoon is suspended in the sea-water and it is a matter of surprise that the most heterogeneous hybridisations do not constantly occur. The reason for this becomes obvious if we bring together mature eggs and equally mature and active sperm of a different family. When this is done no egg is, as a rule, fertilised. The eggs of a sea-urchin can be fertilised by sperm of their own species, or, though in smaller numbers, by the sperm of other species of sea-urchins, but not by the sperm of other groups of echinoderms, e.g. starfish, brittle-stars, holothurians or crinoids, and still less by the sperm of more distant groups of animals. The consensus of opinion seemed to be that the spermatozoon must enter the egg through a narrow opening or canal, the so-called micropyle, and that the micropyle allowed only the spermatozoa of the same or of a closely related species to enter the egg.

It seemed to the writer that the cause of this limitation of hybridisation might be of another kind and that by a change in the constitution of the sea-water it might be possible to bring about heterogenous hybridisations, which in normal sea-water are impossible. This assumption proved correct. Sea-water has a faintly alkaline reaction (in terms of the physical chemist its concentration of hydroxyl ions is about (10 to the power minus six)N at Pacific Grove, California, and about (10 to the power minus 5)N at Woods Hole, Massachusetts). If we slightly raise the alkalinity of the sea-water by adding to it a small but definite quantity of sodium hydroxide or some other alkali, the eggs of the sea-urchin can be fertilised with the sperm of widely different groups of animals, possibly with the sperm of any marine animal which sheds it into the ocean. In 1903 it was shown that if we add from about 0.5 to 0.8 cubic centimetre N/10 sodium hydroxide to 50 cubic centimetres of sea-water, the eggs of Strongylocentrotus purpuratus (a sea-urchin which is found on the coast of California) can be fertilised in large quantities by the sperm of various kinds of starfish, brittle- stars and holothurians; while in normal sea-water or with less sodium hydroxide not a single egg of the same female could be fertilised with the starfish sperm which proved effective in the hyper-alkaline sea-water. The sperm of the various forms of starfish was not equally effective for these hybridisations; the sperm of Asterias ochracea and A. capitata gave the best results, since it was possible to fertilise 50 per cent or more of the sea-urchin eggs, while the sperm of Pycnopodia and Asterina fertilised only 2 per cent of the same eggs.

Godlewski used the same method for the hybridisation of the sea-urchin eggs with the sperm of a crinoid (Antedon rosacea). Kupelwieser afterwards obtained results which seemed to indicate the possibility of fertilising the eggs of Strongylocentrotus with the sperm of a mollusc (Mytilus.) Recently, the writer succeeded in fertilising the eggs of Strongylocentrotus franciscanus with the sperm of a mollusc–Chlorostoma. This result could only be obtained in sea-water the alkalinity of which had been increased (through the addition of 0.8 cubic centimetre N/10 sodium hydroxide to 50 cubic centimetres of sea-water). We thus see that by increasing the alkalinity of the sea-water it is possible to effect heterogeneous hybridisations which are at present impossible in the natural environment of these animals.

It is, however, conceivable that in former periods of the earth’s history such heterogeneous hybridisations were possible. It is known that in solutions like sea-water the degree of alkalinity must increase when the amount of carbon-dioxide in the atmosphere is diminished. If it be true, as Arrhenius assumes, that the Ice age was caused or preceded by a diminution in the amount of carbon-dioxide in the air, such a diminution must also have resulted in an increase of the alkalinity of the sea-water, and one result of such an increase must have been to render possible heterogeneous hybridisations in the ocean which in the present state of alkalinity are practically excluded.

But granted that such hybridisations were possible, would they have influenced the character of the fauna? In other words, are the hybrids between sea-urchin and starfish, or better still, between sea-urchin and mollusc, capable of development, and if so, what is their character? The first experiment made it appear doubtful whether these heterogeneous hybrids could live. The sea-urchin eggs which were fertilised in the laboratory by the spermatozoa of the starfish, as a rule, died earlier than those of the pure breeds. But more recent results indicate that this was due merely to deficiencies in the technique of the earlier experiments. The writer has recently obtained hybrid larvae between the sea-urchin egg and the sperm of a mollusc (Chlorostoma) which, in the laboratory, developed as well and lived as long as the pure breeds of the sea-urchin, and there was nothing to indicate any difference in the vitality of the two breeds.

So far as the question of heredity is concerned, all the experiments on heterogeneous hybridisation of the egg of the sea-urchin with the sperm of starfish, brittle-stars, crinoids and molluscs, have led to the same result, namely, that the larvae have purely maternal characteristics and differ in no way from the pure breed of the form from which the egg is taken. By way of illustration it may be said that the larvae of the sea- urchin reach on the third day or earlier (according to species and temperature) the so-called pluteus stage, in which they possess a typical skeleton; while neither the larvae of the starfish nor those of the mollusc form a skeleton at the corresponding stage. It was, therefore, a matter of some interest to find out whether or not the larvae produced by the fertilisation of the sea-urchin egg with the sperm of starfish or mollusc would form the normal and typical pluteus skeleton. This was invariably the case in the experiments of Godlewski, Kupelwieser, Hagedoorn, and the writer. These hybrid larvae were exclusively maternal in character.

It might be argued that in the case of heterogeneous hybridisation the sperm-nucleus does not fuse with the egg-nucleus, and that, therefore, the spermatozoon cannot transmit its hereditary substances to the larvae. But these objections are refuted by Godlewski’s experiments, in which he showed definitely that if the egg of the sea-urchin is fertilised with the sperm of a crinoid the fusion of the egg-nucleus and sperm-nucleus takes place in the normal way. It remains for further experiments to decide what the character of the adult hybrids would be.

(b). ARTIFICIAL PARTHENOGENESIS.

Possibly in no other field of Biology has our ability to control life- phenomena by outside conditions been proved to such an extent as in the domain of fertilisation. The reader knows that the eggs of the overwhelming majority of animals cannot develop unless a spermatozoon enters them. In this case a living agency is the cause of development and the problem arises whether it is possible to accomplish the same result through the application of well-known physico-chemical agencies. This is, indeed, true, and during the last ten years living larvae have been produced by chemical agencies from the unfertilised eggs of sea-urchins, starfish, holothurians and a number of annelids and molluscs; in fact this holds true in regard to the eggs of practically all forms of animals with which such experiments have been tried long enough. In each form the method of procedure is somewhat different and a long series of experiments is often required before the successful method is found.

The facts of Artificial Parthenogenesis, as the chemical fertilisation of the egg is called, have, perhaps, some bearing on the problem of evolution. If we wish to form a mental image of the process of evolution we have to reckon with the possibility that parthenogenetic propagation may have preceded sexual reproduction. This suggests also the possibility that at that period outside forces may have supplied the conditions for the development of the egg which at present the spermatozoon has to supply. For this, if for no other reason, a brief consideration of the means of artificial parthenogenesis may be of interest to the student of evolution.

It seemed necessary in these experiments to imitate as completely as possible by chemical agencies the effects of the spermatozoon upon the egg. When a spermatozoon enters the egg of a sea-urchin or certain starfish or annelids, the immediate effect is a characteristic change of the surface of the egg, namely the formation of the so-called membrane of fertilisation. The writer found that we can produce this membrane in the unfertilised egg by certain acids, especially the monobasic acids of the fatty series, e.g. formic, acetic, propionic, butyric, etc. Carbon-dioxide is also very efficient in this direction. It was also found that the higher acids are more efficient than the lower ones, and it is possible that the spermatozoon induces membrane-formation by carrying into the egg a higher fatty acid, namely oleic acid or one of its salts or esters.

The physico-chemical process which underlies the formation of the membrane seems to be the cause of the development of the egg. In all cases in which the unfertilised egg has been treated in such a way as to cause it to form a membrane it begins to develop. For the eggs of certain animals membrane- formation is all that is required to induce a complete development of the unfertilised egg, e.g. in the starfish and certain annelids. For the eggs of other animals a second treatment is necessary, presumably to overcome some of the injurious effects of acid treatment. Thus the unfertilised eggs of the sea-urchin Strongylocentrotus purpuratus of the Californian coast begin to develop when membrane-formation has been induced by treatment with a fatty acid, e.g. butyric acid; but the development soon ceases and the eggs perish in the early stages of segmentation, or after the first nuclear division. But if we treat the same eggs, after membrane- formation, for from 35 to 55 minutes (at 15 deg C.) with sea-water the concentration (osmotic pressure) of which has been raised through the addition of a definite amount of some salt or sugar, the eggs will segment and develop normally, when transferred back to normal sea-water. If care is taken, practically all the eggs can be caused to develop into plutei, the majority of which may be perfectly normal and may live as long as larvae produced from eggs fertilised with sperm.

It is obvious that the sea-urchin egg is injured in the process of membrane-formation and that the subsequent treatment with a hypertonic solution only acts as a remedy. The nature of this injury became clear when it was discovered that all the agencies which cause haemolysis, i.e. the destruction of the red blood corpuscles, also cause membrane-formation in unfertilised eggs, e.g. fatty acids or ether, alcohols or chloroform, etc., or saponin, solanin, digitalin, bile salts and alkali. It thus happens that the phenomena of artificial parthenogenesis are linked together with the phenomena of haemolysis which at present play so important a role in the study of immunity. The difference between cytolysis (or haemolysis) and fertilisation seems to be this, that the latter is caused by a superficial or slight cytolysis of the egg, while if the cytolytic agencies have time to act on the whole egg the latter is completely destroyed. If we put unfertilised eggs of a sea-urchin into sea-water which contains a trace of saponin we notice that, after a few minutes, all the eggs form the typical membrane of fertilisation. If the eggs are then taken out of the saponin solution, freed from all traces of saponin by repeated washing in normal sea-water, and transferred to the hypertonic sea-water for from 35 to 55 minutes, they develop into larvae. If, however, they are left in the sea-water containing the saponin they undergo, a few minutes after membrane-formation, the disintegration known in pathology as CYTOLYSIS. Membrane-formation is, therefore, caused by a superficial or incomplete cytolysis. The writer believes that the subsequent treatment of the egg with hypertonic sea-water is needed only to overcome the destructive effects of this partial cytolysis. The full reasons for this belief cannot be given in a short essay.

Many pathologists assume that haemolysis or cytolysis is due to a liquefaction of certain fatty or fat-like compounds, the so-called lipoids, in the cell. If this view is correct, it would be necessary to ascribe the fertilisation of the egg to the same process.

The analogy between haemolysis and fertilisation throws, possibly, some light on a curious observation. It is well known that the blood corpuscles, as a rule, undergo cytolysis if injected into the blood of an animal which belongs to a different family. The writer found last year that the blood of mammals, e.g. the rabbit, pig, and cattle, causes the egg of Strongylocentrotus to form a typical fertilisation-membrane. If such eggs are afterwards treated for a short period with hypertonic sea-water they develop into normal larvae (plutei). Some substance contained in the blood causes, presumably, a superficial cytolysis of the egg and thus starts its development.

We can also cause the development of the sea-urchin egg without membrane- formation. The early experiments of the writer were done in this way and many experimenters still use such methods. It is probable that in this case the mechanism of fertilisation is essentially the same as in the case where the membrane-formation is brought about, with this difference only, that the cytolytic effect is less when no fertilisation-membrane is formed. This inference is corroborated by observations on the fertilisation of the sea-urchin egg with ox blood. It very frequently happens that not all of the eggs form membranes in this process. Those eggs which form membranes begin to develop, but perish if they are not treated with hypertonic sea- water. Some of the other eggs, however, which do not form membranes, develop directly into normal larvae without any treatment with hypertonic sea-water, provided they are exposed to the blood for only a few minutes. Presumably some blood enters the eggs and causes the cytolytic effects in a less degree than is necessary for membrane-formation, but in a sufficient degree to cause their development. The slightness of the cytolytic effect allows the egg to develop without treatment with hypertonic sea-water.

Since the entrance of the spermatozoon causes that degree of cytolysis which leads to membrane-formation, it is probable that, in addition to the cytolytic or membrane-forming substance (presumably a higher fatty acid), it carries another substance into the egg which counteracts the deleterious cytolytic effects underlying membrane-formation.

The question may be raised whether the larvae produced by artificial parthenogenesis can reach the mature stage. This question may be answered in the affirmative, since Delage has succeeded in raising several parthenogenetic sea-urchin larvae beyond the metamorphosis into the adult stage and since in all the experiments made by the writer the parthenogenetic plutei lived as long as the plutei produced from fertilised eggs.

(c). ON THE PRODUCTION OF TWINS FROM ONE EGG THROUGH A CHANGE IN THE CHEMICAL CONSTITUTION OF THE SEA-WATER.

The reader is probably familiar with the fact that there exist two different types of human twins. In the one type the twins differ as much as two children of the same parents born at different periods; they may or may not have the same sex. In the second type the twins have invariably the same sex and resemble each other most closely. Twins of the latter type are produced from the same egg, while twins of the former type are produced from two different eggs.

The experiments of Driesch and others have taught us that twins originate from one egg in this manner, namely, that the first two cells into which the egg divides after fertilisation become separated from each other. This separation can be brought about by a change in the chemical constitution of the sea-water. Herbst observed that if the fertilised eggs of the sea- urchin are put into sea-water which is freed from calcium, the cells into which the egg divides have a tendency to fall apart. Driesch afterwards noticed that eggs of the sea-urchin treated with sea-water which is free from lime have a tendency to give rise to twins. The writer has recently found that twins can be produced not only by the absence of lime, but also through the absence of sodium or of potassium; in other words, through the absence of one or two of the three important metals in the sea-water. There is, however, a second condition, namely, that the solution used for the production of twins must have a neutral or at least not an alkaline reaction.

The procedure for the production of twins in the sea-urchin egg consists simply in this:–the eggs are fertilised as usual in normal sea-water and then, after repeated washing in a neutral solution of sodium chloride (of the concentration of the sea-water), are placed in a neutral mixture of potassium chloride and calcium chloride, or of sodium chloride and potassium chloride, or of sodium chloride and calcium chloride, or of sodium chloride and magnesium chloride. The eggs must remain in this solution until half an hour or an hour after they have reached the two-cell stage. They are then transferred into normal sea-water and allowed to develop. From 50 to 90 per cent of the eggs of Strongylocentrotus purpuratus treated in this manner may develop into twins. These twins may remain separate or grow partially together and form double monsters, or heal together so completely that only slight or even no imperfections indicate that the individual started its career as a pair of twins. It is also possible to control the tendency of such twins to grow together by a change in the constitution of the sea-water. If we use as a twin-producing solution a mixture of sodium, magnesium and potassium chlorides (in the proportion in which these salts exist in the sea-water) the tendency of the twins to grow together is much more pronounced than if we use simply a mixture of sodium chloride and magnesium chloride.

The mechanism of the origin of twins, as the result of altering the composition of the sea-water, is revealed by observation of the first segmentation of the egg in these solutions. This cell-division is modified in a way which leads to a separation of the first two cells. If the egg is afterwards transferred back into normal sea-water, each of these two cells develops into an independent embryo. Since normal sea-water contains all three metals, sodium, calcium, and potassium, and since it has besides an alkaline reaction, we perceive the reason why twins are not normally produced from one egg. These experiments suggest the possibility of a chemical cause for the origin of twins from one egg or of double monstrosities in mammals. If, for some reason, the liquids which surround the human egg a short time before and after the first cell-division are slightly acid, and at the same time lacking in one of the three important metals, the conditions for the separation of the first two cells and the formation of identical twins are provided.

In conclusion it may be pointed out that the reverse result, namely, the fusion of normally double organs, can also be brought about experimentally through a change in the chemical constitution of the sea-water. Stockard succeeded in causing the eyes of fish embryos (Fundulus heteroclitus) to fuse into a single cyclopean eye through the addition of magnesium chloride to the sea-water. When he added about 6 grams of magnesium chloride to 100 cubic centimetres of sea-water and placed the fertilised eggs in the mixture, about 50 per cent of the eggs gave rise to one-eyed embryos. “When the embryos were studied the one-eyed condition was found to result from the union or fusion of the ‘anlagen’ of the two eyes. Cases were observed which showed various degrees in this fusion; it appeared as though the optic vessels were formed too far forward and ventral, so that their antero-ventro-median surfaces fused. This produces one large optic cup, which in all cases gives more or less evidence of its double nature.” (Stockard, “Archiv f. Entwickelungsmechanik”, Vol. 23, page 249, 1907.)

We have confined ourselves to a discussion of rather simple effects of the change in the constitution of the sea-water upon development. It is a priori obvious, however, that an unlimited number of pathological variations might be produced by a variation in the concentration and constitution of the sea-water, and experience confirms this statement. As an example we may mention the abnormalities observed by Herbst in the development of sea-urchins through the addition of lithium to sea-water. It is, however, as yet impossible to connect in a rational way the effects produced in this and similar cases with the cause which produced them; and it is also impossible to define in a simple way the character of the change produced.

III. THE INFLUENCE OF TEMPERATURE.

(a) THE INFLUENCE OF TEMPERATURE UPON THE DENSITY OF PELAGIC ORGANISMS AND THE DURATION OF LIFE.

It has often been noticed by explorers who have had a chance to compare the faunas in different climates that in polar seas such species as thrive at all in those regions occur, as a rule, in much greater density than they do in the moderate or warmer regions of the ocean. This refers to those members of the fauna which live at or near the surface, since they alone lend themselves to a statistical comparison. In his account of the Valdivia expedition, Chun (Chun, “Aus den Tiefen des Weltmeeres”, page 225, Jena, 1903.) calls especial attention to this quantitative difference in the surface fauna and flora of different regions. “In the icy water of the Antarctic, the temperature of which is below 0 deg C., we find an astonishingly rich animal and plant life. The same condition with which we are familiar in the Arctic seas is repeated here, namely, that the quantity of plankton material exceeds that of the temperate and warm seas.” And again, in regard to the pelagic fauna in the region of the Kerguelen Islands, he states: “The ocean is alive with transparent jelly fish, Ctenophores (Bolina and Callianira) and of Siphonophore colonies of the genus Agalma.”

The paradoxical character of this general observation lies in the fact that a low temperature retards development, and hence should be expected to have the opposite effect from that mentioned by Chun. Recent investigations have led to the result that life-phenomena are affected by temperature in the same sense as the velocity of chemical reactions. In the case of the latter van’t Hoff had shown that a decrease in temperature by 10 degrees reduces their velocity to one half or less, and the same has been found for the influence of temperature on the velocity of physiological processes. Thus Snyder and T.B. Robertson found that the rate of heartbeat in the tortoise and in Daphnia is reduced to about one-half if the temperature is lowered 10 deg C., and Maxwell, Keith Lucas, and Snyder found the same influence of temperature for the rate with which an impulse travels in the nerve. Peter observed that the rate of development in a sea-urchin’s egg is reduced to less than one-half if the temperature (within certain limits) is reduced by 10 degrees. The same effect of temperature upon the rate of development holds for the egg of the frog, as Cohen and Peter calculated from the experiments of O. Hertwig. The writer found the same temperature- coefficient for the rate of maturation of the egg of a mollusc (Lottia).

All these facts prove that the velocity of development of animal life in Arctic regions, where the temperature is near the freezing point of water, must be from two to three times smaller than in regions where the temperature of the ocean is about 10 deg C. and from four to nine times smaller than in seas the temperature of which is about 20 deg C. It is, therefore, exactly the reverse of what we should expect when authors state that the density of organisms at or near the surface of the ocean in polar regions is greater than in more temperate regions.

The writer believes that this paradox finds its explanation in experiments which he has recently made on the influence of temperature on the duration of life of cold-blooded marine animals. The experiments were made on the fertilised and unfertilised eggs of the sea-urchin, and yielded the result that for the lowering of temperature by 1 deg C. the duration of life was about doubled. Lowering the temperature by 10 degrees therefore prolongs the life of the organism 2 to the power 10, i.e. over a thousand times, and a lowering by 20 degrees prolongs it about one million times. Since this prolongation of life is far in excess of the retardation of development through a lowering of temperature, it is obvious that, in spite of the retardation of development in Arctic seas, animal life must be denser there than in temperate or tropical seas. The excessive increase of the duration of life at the poles will necessitate the simultaneous existence of more successive generations of the same species in these regions than in the temperate or tropical regions.

The writer is inclined to believe that these results have some bearing upon a problem which plays an important role in theories of evolution, namely, the cause of natural death. It has been stated that the processes of differentiation and development lead also to the natural death of the individual. If we express this in chemical terms it means that the chemical processes which underlie development also determine natural death. Physical chemistry has taught us to identify two chemical processes even if only certain of their features are known. One of these means of identification is the temperature coefficient. When two chemical processes are identical, their velocity must be reduced by the same amount if the temperature is lowered to the same extent. The temperature coefficient for the duration of life of cold-blooded organisms seems, however, to differ enormously from the temperature coefficient for their rate of development. For a difference in temperature of 10 deg C. the duration of life is altered five hundred times as much as the rate of development; and, for a change of 20 deg C., it is altered more than a hundred thousand times as much. From this we may conclude that, at least for the sea-urchin eggs and embryo, the chemical processes which determine natural death are certainly not identical with the processes which underlie their development. T.B. Robertson has also arrived at the conclusion, for quite different reasons, that the process of senile decay is essentially different from that of growth and development.

(b) CHANGES IN THE COLOUR OF BUTTERFLIES PRODUCED THROUGH THE INFLUENCE OF TEMPERATURE.

The experiments of Dorfmeister, Weismann, Merrifield, Standfuss, and Fischer, on seasonal dimorphism and the aberration of colour in butterflies have so often been discussed in biological literature that a short reference to them will suffice. By seasonal dimorphism is meant the fact that species may appear at different seasons of the year in a somewhat different form or colour. Vanessa prorsa is the summer form, Vanessa levana the winter form of the same species. By keeping the pupae of Vanessa prorsa several weeks at a temperature of from 0 deg to 1 deg Weismann succeeded in obtaining from the summer chrysalids specimens which resembled the winter variety, Vanessa levana.

If we wish to get a clear understanding of the causes of variation in the colour and pattern of butterflies, we must direct our attention to the experiments of Fischer, who worked with more extreme temperatures than his predecessors, and found that almost identical aberrations of colour could be produced by both extremely high and extremely low temperatures. This can be clearly seen from the following tabulated results of his observations. At the head of each column the reader finds the temperature to which Fischer submitted the pupae, and in the vertical column below are found the varieties that were produced. In the vertical column A are given the normal forms:

(Temperatures in deg C.)
0 to -20 0 to +10 A. +35 to +37 +36 to +41 +42 to +46 (Normal forms)

ichnusoides polaris urticae ichnusa polaris ichnusoides (nigrita) (nigrita)

antigone fischeri io – fischeri antigone (iokaste) (iokaste)

testudo dixeyi polychloros erythromelas dixeyi testudo

hygiaea artemis antiopa epione artemis hygiaea

elymi wiskotti cardui – wiskotti elymi

klymene merrifieldi atalanta – merrifieldi klymene

weismanni porima prorsa – porima weismanni

The reader will notice that the aberrations produced at a very low temperature (from 0 to -20 deg C.) are absolutely identical with the aberrations produced by exposing the pupae to extremely high temperatures (42 to 46 deg C.). Moreover the aberrations produced by a moderately low temperature (from 0 to 10 deg C.) are identical with the aberrations produced by a moderately high temperature (36 to 41 deg C.)

From these observations Fischer concludes that it is erroneous to speak of a specific effect of high and of low temperatures, but that there must be a common cause for the aberration found at the high as well as at the low temperature limits. This cause he seems to find in the inhibiting effects of extreme temperatures upon development.

If we try to analyse such results as Fischer’s from a physico-chemical point of view, we must realise that what we call life consists of a series of chemical reactions, which are connected in a catenary way; inasmuch as one reaction or group of reactions (a) (e.g. hydrolyses) causes or furnishes the material for a second reaction or group of reactions (b) (e.g. oxydations). We know that the temperature coefficient for physiological processes varies slightly at various parts of the scale; as a rule it is higher near 0 and lower near 30 deg. But we know also that the temperature coefficients do not vary equally from the various physiological processes. It is, therefore, to be expected that the temperature coefficients for the group of reactions of the type (a) will not be identical through the whole scale with the temperature coefficients for the reactions of the type (b). If therefore a certain substance is formed at the normal temperature of the animal in such quantities as are needed for the catenary reaction (b), it is not to be expected that this same perfect balance will be maintained for extremely high or extremely low temperatures; it is more probable that one group of reactions will exceed the other and thus produce aberrant chemical effects, which may underlie the colour aberrations observed by Fischer and other experimenters.

It is important to notice that Fischer was also able to produce aberrations through the application of narcotics. Wolfgang Ostwald has produced experimentally, through variation of temperature, dimorphism of form in Daphnia. Lack of space precludes an account of these important experiments, as of so many others.

IV. THE EFFECTS OF LIGHT.

At the present day nobody seriously questions the statement that the action of light upon organisms is primarily one of a chemical character. While this chemical action is of the utmost importance for organisms, the nutrition of which depends upon the action of chlorophyll, it becomes of less importance for organisms devoid of chlorophyll. Nevertheless, we find animals in which the formation of organs by regeneration is not possible unless they are exposed to light. An observation made by the writer on the regeneration of polyps in a hydroid, Eudendrium racemosum, at Woods Hole, may be mentioned as an instance of this. If the stem of this hydroid, which is usually covered with polyps, is put into an aquarium the polyps soon fall off. If the stems are kept in an aquarium where light strikes them during the day, a regeneration of numerous polyps takes place in a few days. If, however, the stems of Eudendrium are kept permanently in the dark, no polyps are formed even after an interval of some weeks; but they are formed in a few days after the same stems have been transferred from the dark to the light. Diffused daylight suffices for this effect. Goldfarb, who repeated these experiments, states that an exposure of comparatively short duration is sufficient for this effect, it is possible that the light favours the formation of substances which are a prerequisite for the origin of polyps and their growth.

Of much greater significance than this observation are the facts which show that a large number of animals assume, to some extent, the colour of the ground on which they are placed. Pouchet found through experiments upon crustaceans and fish that this influence of the ground on the colour of animals is produced through the medium of the eyes. If the eyes are removed or the animals made blind in another way these phenomena cease. The second general fact found by Pouchet was that the variation in the colour of the animal is brought about through an action of the nerves on the pigment-cells of the skin; the nerve-action being induced through the agency of the eye.

The mechanism and the conditions for the change in colouration were made clear through the beautiful investigations of Keeble and Gamble, on the colour-change in crustaceans. According to these authors the pigment-cells can, as a rule, be considered as consisting of a central body from which a system of more or less complicated ramifications or processes spreads out in all directions. As a rule, the centre of the cell contains one or more different pigments which under the influence of nerves can spread out separately or together into the ramifications. These phenomena of spreading and retraction of the pigments into or from the ramifications of the pigment-cells form on the whole the basis for the colour changes under the influence of environment. Thus Keeble and Gamble observed that Macromysis flexuosa appears transparent and colourless or grey on sandy ground. On a dark ground their colour becomes darker. These animals have two pigments in their chromatophores, a brown pigment and a whitish or yellow pigment; the former is much more plentiful than the latter. When the animal appears transparent all the pigment is contained in the centre of the cells, while the ramifications are free from pigment. When the animal appears brown both pigments are spread out into the ramifications. In the condition of maximal spreading the animals appear black.

This is a comparatively simple case. Much more complicated conditions were found by Keeble and Gamble in other crustaceans, e.g. in Hippolyte cranchii, but the influence of the surroundings upon the colouration of this form was also satisfactorily analysed by these authors.

While many animals show transitory changes in colour under the influence of their surroundings, in a few cases permanent changes can be produced. The best examples of this are those which were observed by Poulton in the chrysalids of various butterflies, especially the small tortoise-shell. These experiments are so well known that a short reference to them will suffice. Poulton (Poulton, E.B., “Colours of Animals” (The International Scientific Series), London, 1890, page 121.) found that in gilt or white surroundings the pupae became light coloured and there was often an immense development of the golden spots, “so that in many cases the whole surface of the pupae glittered with an apparent metallic lustre. So remarkable was the appearance that a physicist to whom I showed the chrysalids, suggested that I had played a trick and had covered them with goldleaf.” When black surroundings were used “the pupae were as a rule extremely dark, with only the smallest trace, and often no trace at all, of the golden spots which are so conspicuous in the lighter form.” The susceptibility of the animal to this influence of its surroundings was found to be greatest during a definite period when the caterpillar undergoes the metamorphosis into the chrysalis stage. As far as the writer is aware, no physico-chemical explanation, except possibly Wiener’s suggestion of colour-photography by mechanical colour adaptation, has ever been offered for the results of the type of those observed by Poulton.

V. EFFECTS OF GRAVITATION.

(a) EXPERIMENTS ON THE EGG OF THE FROG.

Gravitation can only indirectly affect life-phenomena; namely, when we have in a cell two different non-miscible liquids (or a liquid and a solid) of different specific gravity, so that a change in the position of the cell or the organ may give results which can be traced to a change in the position of the two substances. This is very nicely illustrated by the frog’s egg, which has two layers of very viscous protoplasm one of which is black and one white. The dark one occupies normally the upper position in the egg and may therefore be assumed to possess a smaller specific gravity than the white substance. When the egg is turned with the white pole upwards a tendency of the white protoplasm to flow down again manifests itself. It is, however, possible to prevent or retard this rotation of the highly viscous protoplasm, by compressing the eggs between horizontal glass plates. Such compression experiments may lead to rather interesting results, as O. Schultze first pointed out. Pflueger had already shown that the first plane of division in a fertilised frog’s egg is vertical and Roux established the fact that the first plane of division is identical with the plane of symmetry of the later embryo. Schultze found that if the frog’s egg is turned upside down at the time of its first division and kept in this abnormal position, through compression between two glass plates for about 20 hours, a small number of eggs may give rise to twins. It is possible, in this case, that the tendency of the black part of the egg to rotate upwards along the surface of the egg leads to a separation of its first cells, such a separation leading to the formation of twins.

T.H. Morgan made an interesting additional observation. He destroyed one half of the egg after the first segmentation and found that the half which remained alive gave rise to only one half of an embryo, thus confirming an older observation of Roux. When, however, Morgan put the egg upside down after the destruction of one of the first two cells, and compressed the eggs between two glass plates, the surviving half of the egg gave rise to a perfect embryo of half size (and not to a half embryo of normal size as before.) Obviously in this case the tendency of the protoplasm to flow back to its normal position was partially successful and led to a partial or complete separation of the living from the dead half; whereby the former was enabled to form a whole embryo, which, of course, possessed only half the size of an embryo originating from a whole egg.

(b) EXPERIMENTS ON HYDROIDS.

A striking influence of gravitation can be observed in a hydroid, Antennularia antennina, from the bay of Naples. This hydroid consists of a long straight main stem which grows vertically upwards and which has at regular intervals very fine and short bristle-like lateral branches, on the upper side of which the polyps grow. The main stem is negatively geotropic, i.e. its apex continues to grow vertically upwards when we put it obliquely into the aquarium, while the roots grow vertically downwards. The writer observed that when the stem is put horizontally into the water the short lateral branches on the lower side give rise to an altogether different kind of organ, namely, to roots, and these roots grow indefinitely in length and attach themselves to solid bodies; while if the stem had remained in its normal position no further growth would have occurred in the lateral branches. From the upper side of the horizontal stem new stems grow out, mostly directly from the original stem, occasionally also from the short lateral branches. It is thus possible to force upon this hydroid an arrangement of organs which is altogether different from the hereditary arrangement. The writer had called the change in the hereditary arrangement of organs or the transformation of organs by external forces HETEROMORPHOSIS. We cannot now go any further into this subject, which should, however, prove of interest in relation to the problem of heredity.

If it is correct to apply inferences drawn from the observation on the frog’s egg to the behaviour of Antennularia, one might conclude that the cells of Antennularia also contain non-miscible substances of different specific gravity, and that wherever the specifically lighter substance comes in contact with the sea-water (or gets near the surface of the cell) the growth of a stem is favoured; while contact with the sea-water of the specifically heavier of the substances, will favour the formation of roots.

VI. THE EXPERIMENTAL CONTROL OF ANIMAL INSTINCTS.

(a) EXPERIMENTS ON THE MECHANISM OF HELIOTROPIC REACTIONS IN ANIMALS.

Since the instinctive reactions of animals are as hereditary as their morphological character, a discussion of experiments on the physico- chemical character of the instinctive reactions of animals should not be entirely omitted from this sketch. It is obvious that such experiments must begin with the simplest type of instincts, if they are expected to lead to any results; and it is also obvious that only such animals must be selected for this purpose, the reactions of which are not complicated by associative memory, or, as it may preferably be termed, associative hysteresis.

The simplest type of instincts is represented by the purposeful motions of animals to or from a source of energy, e.g. light; and it is with some of these that we intend to deal here. When we expose winged aphides (after they have flown away from the plant), or young caterpillars of Porthesia chrysorrhoea (when they are aroused from their winter sleep) or marine or freshwater copepods and many other animals, to diffused daylight falling in from a window, we notice a tendency among these animals to move towards the source of light. If the animals are naturally sensitive, or if they are rendered sensitive through the agencies which we shall mention later, and if the light is strong enough, they move towards the source of light in as straight a line as the imperfections and peculiarities of their locomotor apparatus will permit. It is also obvious that we are here dealing with a forced reaction in which the animals have no more choice in the direction of their motion than have the iron filings in their arrangement in a magnetic field. This can be proved very nicely in the case of starving caterpillars of Porthesia. The writer put such caterpillars into a glass tube the axis of which was at right angles to the plane of the window: the caterpillars went to the window side of the tube and remained there, even if leaves of their food-plant were put into the tube directly behind them. Under such conditions the animals actually died from starvation, the light preventing them from turning to the food, which they eagerly ate when the light allowed them to do so. One cannot say that these animals, which we call positively helioptropic, are attracted by the light, since it can be shown that they go towards the source of the light even if in so doing they move from places of a higher to places of a lower degree of illumination.

The writer has advanced the following theory of these instinctive reactions. Animals of the type of those mentioned are automatically orientated by the light in such a way that symmetrical elements of their retina (or skin) are struck by the rays of light at the same angle. In this case the intensity of light is the same for both retinae or symmetrical parts of the skin.

This automatic orientation is determined by two factors, first a peculiar photo-sensitiveness of the retina (or skin), and second a peculiar nervous connection between the retina and the muscular apparatus. In symmetrically built heliotropic animals in which the symmetrical muscles participate equally in locomotion, the symmetrical muscles work with equal energy as long as the photo-chemical processes in both eyes are identical. If, however, one eye is struck by stronger light than the other, the symmetrical muscles will work unequally and in positively heliotropic animals those muscles will work with greater energy which bring the plane of symmetry back into the direction of the rays of light and the head towards the source of light. As soon as both eyes are struck by the rays of light at the same angle, there is no more reason for the animal to deviate from this direction and it will move in a straight line. All this holds good on the supposition that the animals are exposed to only one source of light and are very sensitive to light.

Additional proof for the correctness of this theory was furnished through the experiments of G.H. Parker and S.J. Holmes. The former worked on a butterfly, Vanessa antiope, the latter on other arthropods. All the animals were in a marked degree positively heliotropic. These authors found that if one cornea is blackened in such an animal, it moves continually in a circle when it is exposed to a source of light, and in these motions the eye which is not covered with paint is directed towards the centre of the circle. The animal behaves, therefore, as if the darkened eye were in the shade.

(b) THE PRODUCTION OF POSITIVE HELIOTROPISM BY ACIDS AND OTHER MEANS AND THE PERIODIC DEPTH-MIGRATIONS OF PELAGIC ANIMALS.

When we observe a dense mass of copepods collected from a freshwater pond, we notice that some have a tendency to go to the light while others go in the opposite direction and many, if not the majority, are indifferent to light. It is an easy matter to make the negatively heliotropic or the indifferent copepods almost instantly positively heliotropic by adding a small but definite amount of carbon-dioxide in the form of carbonated water to the water in which the animals are contained. If the animals are contained in 50 cubic centimetres of water it suffices to add from three to six cubic centimetres of carbonated water to make all the copepods energetically positively heliotropic. This heliotropism lasts about half an hour (probably until all the carbon-dioxide has again diffused into the air.) Similar results may be obtained with any other acid.

The same experiments may be made with another freshwater crustacean, namely Daphnia, with this difference, however, that it is as a rule necessary to lower the temperature of the water also. If the water containing the Daphniae is cooled and at the same time carbon-dioxide added, the animals which were before indifferent to light now become most strikingly positively heliotropic. Marine copepods can be made positively heliotropic by the lowering of the temperature alone, or by a sudden increase in the concentration of the sea-water.

These data have a bearing upon the depth-migrations of pelagic animals, as was pointed out years ago by Theo. T. Groom and the writer. It is well known that many animals living near the surface of the ocean or freshwater lakes, have a tendency to migrate upwards towards evening and downwards in the morning and during the day. These periodic motions are determined to a large extent, if not exclusively, by the heliotropism of these animals. Since the consumption of carbon-dioxide by the green plants ceases towards evening, the tension of this gas in the water must rise and this must have the effect of inducing positive heliotropism or increasing its intensity. At the same time the temperature of the water near the surface is lowered and this also increases the positive heliotropism in the organisms.

The faint light from the sky is sufficient to cause animals which are in a high degree positively heliotropic to move vertically upwards towards the light, as experiments with such pelagic animals, e.g. copepods, have shown. When, in the morning, the absorption of carbon-dioxide by the green algae begins again and the temperature of the water rises, the animals lose their positive heliotropism, and slowly sink down or become negatively heliotropic and migrate actively downwards.

These experiments have also a bearing upon the problem of the inheritance of instincts. The character which is transmitted in this case is not the tendency to migrate periodically upwards and downwards, but the positive heliotropism. The tendency to migrate is the outcome of the fact that periodically varying external conditions induce a periodic change in the sense and intensity of the heliotropism of these animals. It is of course immaterial for the result, whether the carbon-dioxide or any other acid diffuse into the animal from the outside or whether they are produced inside in the tissue cells of the animals. Davenport and Cannon found that Daphniae, which at the beginning of the experiment, react sluggishly to light react much more quickly after they have been made to go to the light a few times. The writer is inclined to attribute this result to the effect of acids, e.g. carbon-dioxide, produced in the animals themselves in consequence of their motion. A similar effect of the acids was shown by A.D. Waller in the case of the response of nerve to stimuli.

The writer observed many years ago that winged male and female ants are positively helioptropic and that their heliotropic sensitiveness increases and reaches its maximum towards the period of nuptial flight. Since the workers show no heliotropism it looks as if an internal secretion from the sexual glands were the cause of their heliotropic sensitiveness. V. Kellogg has observed that bees also become intensely positively heliotropic at the period of their wedding flight, in fact so much so that by letting light fall into the observation hive from above, the bees are prevented from leaving the hive through the exit at the lower end.

We notice also the reverse phenomenon, namely, that chemical changes produced in the animal destroy its heliotropism. The caterpillars of Porthesia chrysorrhoea are very strongly positively heliotropic when they are first aroused from their winter sleep. This heliotropic sensitiveness lasts only as long as they are not fed. If they are kept permanently without food they remain permanently positively heliotropic until they die from starvation. It is to be inferred that as soon as these animals take up food, a substance or substances are formed in their bodies which diminish or annihilate their heliotropic sensitiveness.

The heliotropism of animals is identical with the heliotropism of plants. The writer has shown that the experiments on the effect of acids on the heliotropism of copepods can be repeated with the same result in Volvox. It is therefore erroneous to try to explain these heliotropic reactions of animals on the basis of peculiarities (e.g. vision) which are not found in plants.

We may briefly discuss the question of the transmission through the sex cells of such instincts as are based upon heliotropism. This problem reduces itself simply to that of the method whereby the gametes transmit heliotropism to the larvae or to the adult. The writer has expressed the idea that all that is necessary for this transmission is the presence in the eyes (or in the skin) of the animal of a photo-sensitive substance. For the transmission of this the gametes need not contain anything more than a catalyser or ferment for the synthesis of the photo-sensitive substance in the body of the animal. What has been said in regard to animal heliotropism might, if space permitted, be extended, mutatis mutandis, to geotropism and stereotropism.

(c) THE TROPIC REACTIONS OF CERTAIN TISSUE-CELLS AND THE MORPHOGENETIC EFFECTS OF THESE REACTIONS.

Since plant-cells show heliotropic reactions identical with those of animals, it is not surprising that certain tissue-cells also show reactions which belong to the class of tropisms. These reactions of tissue-cells are of special interest by reason of their bearing upon the inheritance of morphological characters. An example of this is found in the tiger-like marking of the yolk-sac of the embryo of Fundulus and in the marking of the young fish itself. The writer found that the former is entirely, and the latter at least in part, due to the creeping of the chromatophores upon the blood-vessels. The chromatophores are at first scattered irregularly over the yolk-sac and show their characteristic ramifications. There is at that time no definite relation between blood-vessels and chromatophores. As soon as a ramification of a chromatophore comes in contact with a blood- vessel the whole mass of the chromatophore creeps gradually on the blood- vessel and forms a complete sheath around the vessel, until finally all the chromatophores form a sheath around the vessels and no more pigment cells are found in the meshes between the vessels. Nobody who has not actually watched the process of the creeping of the chromatophores upon the blood- vessels would anticipate that the tiger-like colouration of the yolk-sac in the later stages of the development was brought about in this way. Similar facts can be observed in regard to the first marking of the embryo itself. The writer is inclined to believe that we are here dealing with a case of chemotropism, and that the oxygen of the blood may be the cause of the spreading of the chromatophores around the blood-vessels. Certain observations seem to indicate the possibility that in the adult the chromatophores have, in some forms at least, a more rigid structure and are prevented from acting in the way indicated. It seems to the writer that such observations as those made on Fundulus might simplify the problem of the hereditary transmission of certain markings.

Driesch has found that a tropism underlies the arrangement of the skeleton in the pluteus larvae of the sea-urchin. The position of this skeleton is predetermined by the arrangement of the mesenchyme cells, and Driesch has shown that these cells migrate actively to the place of their destination, possibly led there under the influence of certain chemical substances. When Driesch scattered these cells mechanically before their migration, they nevertheless reached their destination.

In the developing eggs of insects the nuclei, together with some cytoplasm, migrate to the periphery of the egg. Herbst pointed out that this might be a case of chemotropism, caused by the oxygen surrounding the egg. The writer has expressed the opinion that the formation of the blastula may be caused generally by a tropic reaction of the blastomeres, the latter being forced by an outside influence to creep to the surface of the egg.

These examples may suffice to indicate that the arrangement of definite groups of cells and the morphological effects resulting therefrom may be determined by forces lying outside the cells. Since these forces are ubiquitous and constant it appears as if we were dealing exclusively with the influence of a gamete; while in reality all that it is necessary for the gamete to transmit is a certain form of irritability.

(d) FACTORS WHICH DETERMINE PLACE AND TIME FOR THE DEPOSITION OF EGGS.

For the preservation of species the instinct of animals to lay their eggs in places in which the young larvae find their food and can develop is of paramount importance. A simple example of this instinct is the fact that the common fly lays its eggs on putrid material which serves as food for the young larvae. When a piece of meat and of fat of the same animal are placed side by side, the fly will deposit its eggs upon the meat on which the larvae can grow, and not upon the fat, on which they would starve. Here we are dealing with the effect of a volatile nitrogenous substance which reflexly causes the peristaltic motions for the laying of the egg in the female fly.

Kammerer has investigated the conditions for the laying of eggs in two forms of salamanders, e.g. Salamandra atra and S. maculosa. In both forms