Darwin and Modern Science Part 21

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 pota.s.sium, 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 const.i.tution of the sea-water. Stockard succeeded in causing the eyes of fish embryos (Fundulus heteroc.l.i.tus) 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 const.i.tution 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 const.i.tution 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 quant.i.tative 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 astonis.h.i.+ngly rich animal and plant life. The same condition with which we are familiar in the Arctic seas is repeated here, namely, that the quant.i.ty 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, Ctenoph.o.r.es (Bolina and Callianira) and of Siphonoph.o.r.e colonies of the genus Agalma."

The paradoxical character of this general observation lies in the fact that a low temperature r.e.t.a.r.ds 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 r.e.t.a.r.dation of development through a lowering of temperature, it is obvious that, in spite of the r.e.t.a.r.dation 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 b.u.t.tERFLIES PRODUCED THROUGH THE INFLUENCE OF TEMPERATURE.

The experiments of Dorfmeister, Weismann, Merrifield, Standfuss, and Fischer, on seasonal dimorphism and the aberration of colour in b.u.t.terflies 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 b.u.t.terflies, 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 a.n.a.lyse 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 quant.i.ties 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 s.p.a.ce precludes an account of these important experiments, as of so many others.

IV. THE EFFECTS OF LIGHT.

At the present day n.o.body 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 a.s.sume, 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 chromatoph.o.r.es, 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 a.n.a.lysed 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 b.u.t.terflies, especially the small tortoise-sh.e.l.l. 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 l.u.s.tre. 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 ill.u.s.trated 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 a.s.sumed 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 r.e.t.a.r.d this rotation of the highly viscous protoplasm, by compressing the eggs between horizontal gla.s.s 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 gla.s.s 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 gla.s.s 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 a.s.sociative memory, or, as it may preferably be termed, a.s.sociative 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 gla.s.s 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.