General Physiology: An Outline of the Science of Life

More recently, Kochs ('90) performed very detailed experiments upon frogs and water-beetles. He froze these animals in glasses containing water. If the temperature was not very low, there remained around the animals, surrounded by ice, a liquid mass of water, the temperature of which was 2° below the zero-point, as was shown by boring through the mass of ice. If, after boring, this last layer of water was frozen, the animals could still be revived by warming, provided that they had not been frozen longer than five to six hours. By sawing through such a preparation it was shown that the animals were not frozen solid internally. But, if the experiment was extended so that the animals were thus frozen, which was the case when they were brought into cold air of 4° C., all attempts at resuscitation were in vain.

In the light of these experiments the assumption that organisms always perish when the living substance of the tissue-cells itself is frozen solid, appeared very highly probable. But in opposition to them recently Raoul Pictet ('93) has established facts in accordance with which our ideas must apparently be wholly changed. This well-known investigator, who has made a number of surprising and extraordinarily valuable discoveries concerning the chemical effects of very low temperatures, recently carried out in his laboratory experiments upon the physiological effects of such temperatures. The objects of his experiments were protected by wood from contact with the metal walls of the cold vessel in which they were placed, so that they were exposed to the low temperature of the air only. It was thus shown that different animals behave very differently. Fishes that were cooled down to -15° C. in a block of ice remained living after careful warming, although others in the same experiment could be ground to powder like ice. But upon cooling to -20° C. the fishes died. Frogs endured without dying a temperature of -28° C., myriopods -50° C., snails -120° C., and bacteria even less than -200° C. In view of these surprising experiments it can hardly be doubted that in individual cases the living substance of cells can be frozen to ice without losing its capacity of life.

These phenomena suggest the question whether in frozen organisms there is really a complete standstill of the vital processes— a question that Preyer believes must be answered in the affirmative. Theoretically, there is nothing opposed to this idea ; for, when it is seen how with falling temperature the energy of the vital processes constantly decreases, it must be believed that in time a point may be reached where they cease altogether. The possibility that the cell-liquid itself can freeze without abolishing the vital capacity of the cell, would support this view ; for, as has been seen, life cannot exist without water in the liquid state. It would be expected therefore, that, as soon as the water in the living substance has passed over into the solid state, the chemical transformations in the cell would be at a standstill. Bub conclusive experiments for the decision of this question are thus far wanting. If it should be established that living substance in the frozen condition can be maintained for years capable of life, just as certain dried organisms can be so maintained for years, decades, and even centuries, then the probability would approximate to certainty that life in frozen organisms is really at a standstill. At present this is not settled. One fact that is opposed to this idea is the observation made by Pictet, that frozen organisms cannot endure a farther fall of temperature beyond a certain point. Upon thawing they cannot be revived. If life were really at a complete standstill, it would be difficult to understand why

a farther sinking of the temperature should still be of influence. For the present, therefore, we must forgo a definitive solution of this question. The establishment of the maximum of external temperature meets with difficulties similar to those surrounding that of the minimum. In every case the maximum is represented by the point where the proteids in the living substance of the cell coagulate. The proteids play in the life of the cell the most essential role, and it is conceivable that, when the dissolved albumin passes over into the solid state, metabolism, in other words life, must cease. Accordingly, it might appear very simple to determine the maximum of temperature at which life can still exist. But the temperature of coagulation is very different for different proteids, and, moreover, there are kinds of organisms that still live even at temperatures at which all proteids must long since have coagulated.

In a similar manner as with the minimum, Kiihne ('64) performed experiments upon Amoeba regarding the maximum of temperature, and found that, when creeping actively at ordinary temperatures, it contracted at 35° C., but still remained capable of life ; after being heated to 40° — 45° C. it could not be revived by cooling. Thus, Kiihne was able to establish that one proteid of the amoeba-cell, which he regarded as contractile substance, coagulated at 40° C., another at 45° C. For plant-cells Max Schultze ('63) found the death-point to be at 47° C. In contrast to these, various other authors have given accounts of remarkable cases in which organisms exist at much higher temperatures. The most remarkable testimony was the observation of Ehrenberg ('58), who found living ciliate Infusoria and Rotifera between the threads of Oscillaria in the hot springs of Ischia at a temperature of 81° — 85° C. Hoppe-Seyler ('77), who tested this statement of Ehrenberg at Casamicciola, Ischia, found considerably lower temperatures. Algce, when exposed to hot vapours, were living at 64'7° C., but, when in water, the highest temperature in which they existed was only 53° C. Hence it is certain that organisms are still able to live in water of 53° C.

Some time ago very detailed investigations were undertaken in the hot springs of the Yellowstone Park in North America, and living algoe were found at much higher temperatures. The older statement of Ehrenberg does not appear therefore, to have been incorrect. Although these statements are surprising, a well-authenticated and easily observed fact is known that is much more remarkable. This is the behaviour of the spores of certain bacteria to high temperatures. Koch, Brefeld and others, have shown that the spores of the bacillus of splenic fever (Bacillus anthracis) and the hay-bacillus (Bacillus subtilis) can endure

temperatures of more than 100° C. without losing their capacity of life. For the present an explanation of these puzzling facts is wanting. It can only be assumed that the proteids in these organisms occur in a condition in which they cannot be made to coagulate by high temperatures, even, as in the case of the spores of the hay-bacillus, by a boiling temperature. The two assumptions, that, in spite of the temperature of the surrounding medium, the living substance is not heated to the coagulation-point of the proteid, and that the vital capacity is maintained in spite of the coagulation of the proteids in them, are equally improbable. It is not yet known upon what molecular changes the process of coagulation is based, and by what conditions apart from the known factors its appearance is influenced. When more is known upon these questions, some light will be thrown also upon the puzzling phenomena mentioned above.

Like temperature, the pressure surrounding bodies has an influence upon their chemical constitution. This is especially noticeable in cases where the chemical body exists in a medium with the constituents of which it is in chemical relation. If this condition is fulfilled, if a chemical body exists in a gaseous or liquid medium containing substances that have a chemical affinity for it, then, by an increase of the pressure, a chemical combination between the body and the substances in the medium can take place, and by a subsequent decrease of the pressure a decomposition into the previous constituents can occur. This phenomenon depends upon an antagonism between the vibrations of the atoms and the pressure. With a greater pressure the atoms become crowded together, hence more atoms of the medium are able to come into contact with atoms of the body ; with a less pressure the vibrations become again so great that the atoms are disengaged from the loose combination.

Living substance exists in such a condition. It lives in a medium, either air or water, with which it can undergo chemical exchange. It is clear, therefore, that the pressure, either of the air or of the water, will have a great significance for life, and that a pressure within definite limits must belong to the general vital conditions. Unfortunately this condition has been very little investigated thus far, and at present it is possible to state only in part under what pressure of air or water life in general is still possible, and between what limits of pressure it is confined in its present form upon the earth's surface. The experimental investigation of this

problem will require specialised methods, and the values for the individual constituents of the air and the water, such as oxygen, carbonic acid, etc., must be separately determined manometrically. In discussing oxygen as a general condition of life, we became acquainted with the importance of the partial pressure of this gas,1 and learned that pure oxygen at a pressure of more than three atmospheres is fatal to homothermal animals, while with ordinary air the same result appears at a pressure of 15 — 20 atmospheres. Death likewise follows when the partial pressure of the oxygen falls too low.

The venturesome method of balloon-travel has been employed to collect facts regarding the height at which the pressure of the air becomes so small that danger to human life results. The balloon trip that was made out of Paris in the year 1875, by Spinelli, Sivel and Tissandier, has become famous. They rose with considerable rapidity, and without any disturbance reached a height of 7,000 metres. At about 7,500 metres, Tissandier relates, they felt constantly increasing weakness and apathy, which soon increased to complete absence of the power of motion, although their minds still remained clear. They could no longer perform voluntary movements, nor could they even use their tongues for speaking. After Tissandier had made the observation that the balloon had passed a height of 8,000 metres, and after vain efforts to communicate this fact to his two companions, he lost consciousness. When he awoke, they had descended to 7,059 metres. Then Spinelli, who also had awaked, threw out sand in order that they should not fall too rapidly. As a result of this the balloon again rose, and the aeronauts again lost consciousness. When Tissandier awoke a second time they had sunk to 6,000 metres, and the barometer showed that the balloon had reached a height of about 8,500 metres. Spinelli and Sivel never regained consciousness.

The minimum of air-pressure under which plants and animals can still remain alive can be determined by the air-pump. In such an experiment the most important thing for animals is the partial pressure of oxygen, for plants that of carbonic acid. As regards the water-pressure under which life can exist, far fewer facts are known than as regards the pressure of the air. The interesting deep-sea investigations of the last ten years have shown, in opposition to earlier ideas, that living organisms exist even in the greatest depths of the sea, where darkness always prevails and bodies are subject to a pressure of several hundred atmospheres. This pressure is so great that upon its sudden withdrawal, as when the animals are drawn to the surface, they burst. Fishes come up swollen, with their scales standing out and their intestines protruding from their mouths (Fig. 132) ; this is observed

even in the fishes that live in the depths of the Lake of Constance. The height to which the pressure can rise before all life ceases has thus far not been investigated. The diminution of the waterpressure to the pressure of the atmosphere resting upon the water, by means of an air-pump, appears to be without influence upon organisms living in the water. But a great diminution of the FIG. 132.— Ntoscopelus macrolepidotus, brought to the surface from a depth of 1500m. The eye and the intestines are swollen out and the scales are falling off, owing to the great tension of the skin over the body. (After Keller.)

water-pressure is not possible without altering the liquid state of the water. Here the question of the minimum of water-pressure passes over into that of the minimum of air-pressure, and the partial pressure of the contained gases, watervapour, oxygen, etc., and becomes connected with the questions of the importance of moisture in the atmosphere, oxygen, etc., as general conditions of life. The conditions, thus far spoken of, namely, a supply of food and other substances, a definite degree of temperature, and a certain pressure, comprise all the general conditions of life that must be afforded by the medium. Others, such as light, are likewise external but not general conditions, and pertain only to certain organisms or groups of organisms.

But along with the general external conditions there are associated others that must be fulfilled also in order that life can continue. These lie within the organism itself, and constitute the general internal conditions of life. Obviously the chief requisite for the existence of life through the fulfilment of all external conditions is the presence of a substance, capable of life, in which vital phenomena can take place. Hence, if a tiny drop of living substance be imagined in a medium in which all the external conditions of life are fulfilled, it must be assumed that it will remain living so long as disturbing influences do not enter from without. But experiments contradict this.

A small mass of living substance can be easily obtained by cutting off with a fine scalpel under the microscope a piece of hyaline protoplasm from a living cell, e.g., Amoeba. The piece cut off is living ; this is recognised from the fact that after the opera- PIG. 133. — Stentor roeselii, a ciliate-infusorian cell. The clear, extended, rod-shaped mass in the interior is the nucleus. A, Cut into two nucleated pieces at * ; at B and Cthe nucleated pieces have become regenerated into whole Stentors and continue to live.

tion it still performs such movements as the whole Amoeba performs. The external vital conditions, moreover, are all fulfilled, for the part exists in the same medium and has the same external relations as the whole Amasba. Nevertheless, it lasts for a short time only, it soon dies and cannot be restored to life by any agency. Every like experiment without exception upon any other cell yields the same result (Fig. 133). In all such cases a certain mass of living substance exists in a medium in which all external vital conditions are fulfilled, and yet the mass cannot continue living. Hence some factor among the general conditions of life is wanting.

Inasmuch as there exists upon the earth at present no living substance that is homogeneous throughout, the absent factor, as shown by the experiment, is the natural coherence and correlation of the essential parts of the organism. This is true equally of the cell-community and the individual cell. But the objection may be raised that in many cases parts and even whole organs can be separated from an organism without endangering its existence. This is true, but in all such cases the parts are such as are not absolutely necessary to the maintenance of the individual, whether it be because they are present in abundance and can be replaced in function by others, or because they are not closely related to the other parts, and, therefore, when separated, represent complete individuals. A polyp can be cut into two parts, both of which continue to live, and from a polypstalk a single polyp can be cut off without dying. In the above experiment upon Amoeba the nucleated cell-body continues living even after the separation of a portion of the protoplasm, because it still possesses a quantity of protoplasmic particles of the same kind as were removed. But the piece of protoplasm that is cut off perishes, because its connection and correlation with the nuclear mass have ceased.

The living substance that now exists upon the earth's surface is recognised only in the form of cells, either alone or bound together into cell-communities. The cell contains as its essential constituents two different substances, the protoplasm and the nucleus.1 Wherever a little protoplasm and a little nuclear substance exist in union; there is a cell ; and only such is capable of life when the external vital conditions are fulfilled. A large cell can be divided into many pieces capable of life, so long as the requirements are complied with that every piece shall possess some protoplasm and a little nuclear substance, and that the disproportion between the two masses shall not exceed a certain limit.2 With some skill it is not difficult to perform the experiment upon large unicellular organisms. But, if a cell be so divided that the nucleus is separated from the protoplasm, both parts invariably perish.

Since the cell is the general elementary constituent of organisms, the individual of the lowest order, the association of nucleus and protoplasm in the cell may be established as a general internal condition of life. Only where these two are united can life continue to exist. material substratum is present in which it can take place, and, on the other, certain external conditions are fulfilled. The same the same necessity that characterises the appearance of physical phenomena, when matter capable of life is present, and when the external general and special conditions of life are fulfilled. In other words vital phenomena are an expression of the correlation of living substance and the surrounding medium, or, as Claude Bernard ('79) says: "Vital manifestations result from a conflict between two factors — the organised living substance and the medium."

In considering this correlation the question comes up : How was it with life at a time when conditions wholly different from present ones prevailed upon the globe ? Was life then able to exist ? When and how did it arise ? As is well known, the earth was once in a fiery condition, like the sun from which it came. The hard rocks and solid metals that now compose its solidified crust were then in a molten state ; its liquid nucleus was surrounded by an atmosphere of incandescent gases ; its particles were in violent motion, and its temperature measured thousands of degrees.

The idea that the earth in its evolution once passed through such a condition is now an accepted generality of all branches of natural science. Astronomy, physics, geology, geogony, mineralogy and chemistry, all agree in this. Moreover, modern science, with the help of the telescope and the spectroscope, has brought the fact directly before us, that even now, everywhere in the universe, the same process of evolution that the earth once passed through is being repeated, and that there exist upon other heavenly bodies conditions analogous to each stage of the earth's evolution. There now exist in space gaseous nebulae, molten spheres, and solid, ice-cold masses, the last representing the present condition of the moon and the future fate of the earth.

The fact that the earth was once in a condition in which its temperature was enormous and not a drop of water existed upon it, in short, a condition in which the vital conditions that are^liow regarded as indispensable to the existence of organisms were wanting — this fact will always be an important factor with whiqh all speculations upon the origin of life upon the earth must deal. In the light of this we will consider the various views upon the origin of life that have been founded upon a scientific basis by various men of science, and will endeavour to form some idea respecting it, even though the idea be only a general one.

The modern doctrine of spontaneous generation (archegony, abiogenesis, generatio spontanea or cequivoca, etc.) in its general form is as follows. Since there was a time in the evolution of the earth when the existence of the living substance that now inhabits the cool surface of the latter was absolutely impossible, living substance must have arisen from lifeless substance at some later period. The question accordingly arises, how and under what conditions were the first organisms created ?

To the ancients, even to a mind having so comprehensive a knowledge of nature as that of Aristotle, the idea presented no especial difficulties that animals, such as worms, insects and even fishes, could come into existence out of mud. Only at a relatively late time and particularly in connection with the researches of Redi and Svvammerdamm upon the development of insects, were these crude ideas laid aside as incompatible with established scientific knowledge.

But the doctrine of spontaneous generation obtained a new point of support, when the invention of the microscope led to the discovery of a world hitherto wholly unknown and excessively rich in forms, when it was found that whenever an aqueous infusion of dead organic substance was prepared, after a short time an abundance of minute living beings developed in it, which even yet are termed Infusoria. It was fully believed that in Infusoria organisms had been found that were produced by spontaneous generation out of the dead substances in the infusion. This view necessarily seemed all the more probable because the Infusoria were the lowest and simplest beings that had been known up to that time. But in this case also it was established later that the organisms did not originate spontaneously, but were developed from germs that were previously contained in the substances or came into the vessel through the air. Milne Edwards, Schwann, Max Schultze, Helmholtz and others showed that if the substances had previously been freed from germs by boiling, and if germs were prevented from entering through the air, the development of Infusoria never took place, however long the infusion was allowed to stand.

When, later, the smallest of all micro-organisms, the Bacteria, began to attract strongly the attention of the scientific world, and when it was found by refined methods of investigation that these minute beings or their germs are present everywhere in the air, the earth and the water, the doctrine of spontaneous generation seized upon them and claimed that they as the lowest organisms are continually arising at the present time from lifeless matter. But modern bacteriology, with its admirable and delicate methods, for which it is indebted to its founders, especially Pasteur and Robert Koch, has refuted this doctrine again. It has shown that by the exclusion of all germs that can come to the preparation from the outside, even the richest nutrient medium, containing all the substances required for the nutrition of bacteria in the most favourable mixture, remains free from micro-organisms ; and, on the other hand, that a whole world of diverse forms develop in the medium as soon as it is left standing, for a brief time, open to the air.

Along with this continual strife over the doctrine of spontaneous generation, attempts have been made, even down to very recent times, to manufacture living organisms artificially in the laboratory. The latest of these attempts is associated especially with the name of Pouchet, who was the last active adherent of the view that it is possible to produce artificially from lifeless matter unicellular organisms, such as bacteria, yeast, and similar microbes, simply by mixing the necessary constituents and putting them under favourable external conditions. Even when at times these experiments have seemed to lead to positive results, the bacteriologists have always appeared with their critical methods, and have shown that in every case there was a development of germs that had come in from the outside or were already present in the vessels used for the experiment. These attempts are really not different from the undertaking of the famulus Wagner to compound man himself from chemical mixtures in a retort. How can one hope to produce chemically even the simplest organism when the chemical composition of living proteids, the most important substances of which all living substance consists, is at present completely unknown ?

To Haeckel ('66) belongs the credit of having removed from the early absurd ideas of spontaneous generation their sound kernel and of having transferred it to a purely scientific soil. For him the question is indifferent, whether at the present day living substance arises anywhere spontaneously or not. To-day, more than thirty years after Haeckel wrote, and after our knowledge of the lowest organisms and their reproduction has made so enormous a development, the great majority of investigators are inclined to answer this question negatively. Nevertheless, Haeckel was the first to draw sharply the conclusion that because there was a time when the earth was in a condition that excluded all organic life, living substance must have originated at some time in the earth's development from lifeless substances. According to him this time cannot be dated earlier than when the watervapour, suspended throughout the atmosphere, had been precipi-

tated in the form of liquid. Further, he justly lays the greatest value upon the principle that the organisms that arose by spontaneous generation must have been, not cells but the lowest and simplest organisms that can be imagined, " completely homogeneous, structureless, formless lumps of proteid." It is conceivable that these living proteid lumps arose from the mutual action of substances dissolved in the primitive sea. But Haeckel expressly refuses to discuss in detail the " how " of their origin : " Every detailed portrayal of autogony is for the present inadmissible, for the reason that we can form absolutely no satisfactory idea of the peculiar condition presented by the earth's surface at the time of the first appearance of organisms." From the very simple and low organisms that arose spontaneously, which on account of their simplicity Haeckel termed Monera, there have been derived by continuous descent the cells and all forms of organisms that to-day inhabit the earth's surface.