The Mechanistic Conception of Life

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 fertilized eggs. c) On the production of twins from one egg through a charige in the chemical constitution of the sea-water. — The reader is probably familiar with the fact that there exist two different t>T)es 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. T^\'ins 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 fertilization 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 fertilized 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 afterward noticed that eggs of the sea-urchin treated \\dth 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 fertiUzed as usual in normal sea-water and then, after repeated washing in a neutral solution of sodium chloride (of the concentration of the seawater), 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 seawater 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 (see Figs. 55 to 57). If the egg is afterward 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 Figs. 51-54. — Cell-division in a sea-urchin egg, Strongylocentrotus purpuratus, in normal sea-water. This type of cell-division leads to the formation of one embryo from an egg. M is the fertilization membrane, P a layer of colloidal substance which seems to serve the purpose of keeping all the cells of an egg together. the first two cells and the formation of identical twins are provided.

Figs. 55-58. — Cell-division in the egg of Strongylocentrotus purpuratus which leads to the formation of twins. This cell-division can be observed if the egg is put after fertilization into a neutral mixture of salts in which either KCl, or CaClj, or NaCl is lacking. In such a neutral solution the substance which forms the elastic layer (PM, Fig. 51) is dissolved. During the segmentation the protoplasm of the egg spreads imtil its long axis touches the fertilization membrane. The two daughter-cells formed (Fig. 57) are separated from each other, instead of remaining connected as in the normal cell-division (Fig. 53). If about one hour later the eggs are put back into normal sea-water each of the two ceUs develops into an embryo (Fig. 58), and the egg thus gives rise to two instead of to one embryo.

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 two eyes of a fish embryo {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 c.c. of sea-water and placed the fertilized 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 vesicles 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.^ 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.

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 the 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^ 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° C, we find an astonishingly rich animal and plant life. The same condition ^\^th 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 conclusion 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 heart beat in the tortoise and in Daphnia is reduced to about one-half if the temperature is lowered 10° 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 0. Hertwig.

The writer found the same temperature coefficient for the rate of maturation of the egg of a mollusk (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° C, and from four to nine times smaller than in seas the temperature of which is about 20° C. It is, therefore, exactly the reverse of what we should expect when authors state that the density of organisms at or near the surfac e 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 fertilized and unfertilized eggs of the sea-urchin, and yielded the result that for the lowering of temperature by 1° C, the duration of life was about doubled. Lowering the temperature by 10 degrees therefore prolongs the life of the organism 2^^, 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. 1 The high coeflEicient of temperature of the duration of life may possibly only be found near the upper temperature limit for the life of organisms. But this is suflScient for our theory. 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 knoA\Ti. 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° C, the duration of life is altered five hundred times as much as the rate of development; and, for a change of 20° 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.

1 Weismann showed that infusorians or Tinicellular organisms m general are immortal, while he assiimed that all the other organisms with the exception of their germ-plasm are mortal. Leo Loeb first called attention to the fact that the transplantation of a cancer can be repeated to an milimited series of generations, and since it is the originally transplanted cancer-cell and the cells derived from it by multiplication that survive, he pointed out that this proved that the principle of immortality must also be granted to cancer-cells (1901). Later he generalized this idea and stated that other cells may be considered immortal in the same sense in which AVeismann claimed this for the unicellular organisms. One can indeed well imagine that the same piece of skin might be transplanted through an indefinite series of generations of mice and that such a transplanted piece might outlive an indefinite number of generations of mice in exactly the same way as a cancer cell does.

The natiiral death of the metazoa is perhaps a secondary phenomenon due to the cessation of respiratory motions or of the heart beat. This leads to the death of the cells through lack of oxygen. If respiratory motions and circulation could be maintained indefinitely even the metazoa might be foimd to be immortal. h) Changes in the color of butterflies produced through the influence of temperature. — The experiments of Dorfmeister, Weismann, Merrifield, Standfuss, and Fischer on seasonal dimorphism and the aberration of color 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 color. 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° to 1° 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 color and pattern of butterflies, we must direct our attention to the experiments of Fischer, who worked with more extreme temperatures than his predecessors, and foimd that almost identical aberrations of color could be produced by both extremely high and extremely low temperatures. This can be seen clearly from the following tabulated results of his observations. At the head of each column the temperature to which Fischer submitted the pupae is given, and in the vertical column below are found the varieties that were produced. In the vertical column A are given the normal forms:

The reader will notice that the aberrations produced at a very low temperature (from 0° to —20° C.) are absolutely identical with the aberrations produced by exposing the pupae to extremely high temperatures (from 42° to 46° C.) • Moreover the aberrations produced by a moderately low temperature (from 0° to 10° C.) are dentical with the aberrations produced by a moderately high temperature (from 36° to 41° 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 analyze such results as Fischer's from a physicochemical point of view, we must realize 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, (6) (e.g., oxidations). 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°. But we know also that the temperature coefficients do not vary equally for 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 (6). If, therefore, a certain substance is formed at the normal temperature of the animal in such quantities as are needed for the catenary reaction (6), 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 miderlie the color 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. 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 pol^'ps 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 dayhght suffices for this effect. Goldfarb, who repeated these experiments, states that an exposure of comparatively short duration is sufficient to produce this effect. It is possible that the light favors the formation of substances which are a prerequisite for the origin of polyps and their growth.

facts which show that a large number of animals assume, to some extent, the color of the ground on which they are placed. Pouchet found through experiments upon crustaceans and fish that this influence of the ground on the color 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 color 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 coloration were made clear through the beautiful investigations of Keeble and Gamble, on the color change in crustaceans. According to these authors the pigment cells can, as a rule, be considered as consisting of a central bodj^ from which a system of more or less complicated ramifications or processes spreads out in all directions. As a rule, the center of the cell contains one or more different pigments w^hich 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 color changes under the influence of environment. Thus Keeble and Gamble observed that Macromysis flexuosa appears transparent and colorless or gray on sandy ground. On a dark ground their color 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 center of the cells, while the ramifications are free from pigment. When the animal appears brown both pigments are spread out into 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 coloration of this form was also satisfactorily analyzed by these authors. While many animals show transitory changes in color 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 suflfice. Poulton^ found that in gilt or white surroundings the pupae became light colored 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 luster. 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 Wieners' suggestion of color photography by mechanical color adaptation, has ever been offered for the results of the type of those observed by Poulton.

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 upward a tendency of the white protoplasm to flow dowTi 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 0. Schultze first pointed out. Pfliiger had already shown that the first plane of division in a fertilized 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 do\Mi at the time of its first division and kept in this abnormal position, through compression between two glass plates for about twenty 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 upward 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 do^\^l 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 upward and which has at regular intervals very fine and short bristlelike 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 upward when we put it obliquely into the aquarium, while the roots grow vertically downward. 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 farther 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 behavior 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 favored; while contact with the sea-water of the specifically heavier of the substances, will favor the formation of roots.

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 toward the source of light. If the animals are naturallv sensitive, or if thev are rendered sensitive through the agencies which we shall mention later, and if the light is strong enough, they move toward 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 heliotropic, are attracted by the light, since it can be showTi that they go toward the source of 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 oriented 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 photosensitiveness of the retina (or skin), and second a peculiar nervous connection between the retina and the muscular apparatus. In symmetrically built heliotropic animals in Avhich the symmetrical muscles participate equally in locomotion, the symmetrical muscles work with equal energy as long as the photochemical processes in both eyes are identical. If, however, one eye is struck by stronger light than the other, the symmetrical muscles will work unequall}' and in positively heliotropic animals those muscles will work with greater energy which brings the plane of symmetry back into the direction of the rays of light and the head toward the source of light. As soon as both eyes are struck by the rays of light