General Physiology: An Outline of the Science of Life

This in its essentials is the doctrine of spontaneous generation in its present form. Notwithstanding the fact that its conclusion is so simple and obvious, it has been contradicted on many sides and has led to the establishment of other theories upon the origin of life upon the earth. The theory of germs of lower organisms capable of life moving about in space, or, as Preyer has termed it in brief, the theory of cosmozoa, was the first to appear in recent times in opposition to the doctrine of spontaneous generation. Its founder was H. E. Richter ('65, 70, and 71). Starting from the idea that small solid particles are moving about everywhere in space and in the rapid flight of the heavenly bodies are continually being stripped off from them, Richter assumes that, at the same time and attached to these solid particles, germs of microorganisms capable of life are also continually being thrown off from such heavenly bodies as are inhabited and carried to others. If such germs come to other heavenly bodies whose state of development presents favourable vital conditions, especially moderate heat and moisture, they begin there to develop and become the startingpoint of a host of organisms. Somewhere in space, Richter thinks, there have always been heavenly bodies upon which life exists in the form of cells. The existence of living cells in the universe is eternal. " Omne vivum ab ceternitate e cellula" says Richter, modifying anew, according to the precedent of Virchow, Harvey's old dictum. Organic life, therefore, has never originated but has always been transferred from one world to another. Thus, according to Richter, the problem of the origin of life upon the earth is not : How has life arisen upon the earth ? but : How has

it come to the earth from other worlds ? and this question he answers by the theory of cosmozoa. For the possibility that germs capable of life came from space through the atmosphere to the earth's surface, without perishing from the incandescent heat arising from the enormous friction, Richter believes that he finds a support in the assertion of observers that in many meteoric stones traces of coal and even humus and petroleum-like substances occur. If these can come to the earth without undergoing combustion it is possible that germs capable of life also pass through the atmosphere without losing their vital capacity.

That organic germs can endure a long journey through space from one heavenly body to another without water and food cannot be doubted, if in apparently dead organisms, such as the spores of micro-organisms, there be recognised substance actually capable of life that can continue in its apparently dead condition for a long time without water and food and yet revive as soon as it comes under the required vital conditions. Independently of Richter, some years later Helmholtz and Sir William Thompson discussed the question whether life may not, perhaps, have been transferred from other heavenly bodies to the earth ; and both termed this view not unscientific. Helmholtz ('84) says : " Meteoric stones sometimes contain hydrocarbon compounds ; the intrinsic light of the heads of comets shows a spectrum that is very similar to that of the incandescent electric light in gases containing hydrocarbons. But carbon is the characteristic element of the organic compounds of which livingbodies are composed. Who can say whether these bodies that swarm everywhere through space do not spread also the germs of life wherever a new world has become capable of affording a dwelling-place to organic creatures ? And this life we might, perhaps, have reason to regard as even allied to our own in germ, however various may be the forms in which it might adapt itself to the conditions of its new dwelling-place." That meteors can be the bearers of such germs Helmholtz holds to be entirely possible, since large meteoric stones in passing through the atmosphere of the earth are greatly heated upon their surface only, while in their interior they remain cool. Helmholtz says, further, regarding the theory of cosmozoa : " I cannot contend against one who would regard this hypothesis as highly or wholly improbable. But it appears to me to be a wholly correct scientific procedure, when all our endeavours to produce organisms out of lifeless substance are thwarted, to question whether after all life has ever arisen, whether it may not be even as old as matter, and whether its germs, passed from one world to another, may not have developed where they found favourable soil." " The true alternative is evident ; organic life has either begun to exist at some one time, or has existed from eternity."

By considerations of another kind, Preyer ('80) has arrived at a theory regarding the derivation of life which is opposed to the theories both of spontaneous generation and of cosmozoa. Preyer cannot accept the idea of spontaneous generation for the following reasons. If it be assumed that at some one time in the earth's development living substance has arisen spontaneously from lifeless substance, it must be claimed that this is possible even now. But the failure of the innumerable experiments directed toward this problem has made it in the highest degree improbable. On the other hand, the supposition that spontaneous generation was possible only once in the primaeval past, but now no longer occurs, is likewise improbable ; " for the same conditions that are essential for the maintenance of life and are now realised, must necessarily have been realised also at the time of the supposed origin of living from inorganic bodies ; otherwise, the product of spontaneous generation would not have been able to continue living." In other words, if spontaneous generation were once possible, it is difficult to see why it is not possible now.

Preyer is likewise not able to accept the theory of cosmozoa, because he sees in it not a solution, but only a postponement of the problem, i.e., a shifting of it from the earth to some other world, the problem itself, however, always remaining. Proceeding from the inductive fact that organisms are always derived from other organisms similar to them, that thus far observation has never been able to establish the origin of any organism without a parent, Preyer raises the question whether the problem of spontaneous generation may not rest upon a false conception, when it demands that living substance shall at some time have originated from lifeless substance. Must it not rather be formulated in the reverse order : has lifeless originated from living substance ? Organisms are always derived from other living organisms ; but inorganic, lifeless substance is continually being derived from both lifeless substance and living organisms, either being excreted from the latter as dead matter or remaining after their death. In contrast, therefore, to the doctrine of spontaneous generation, Preyer puts forward the theory that living substance is the primary thing, and that lifeless substance is derived from it secondarily by excretion.1 He thus demands that continuity in the derivation of living substance has never been broken. " Whoever interrupts the series of successive generations of organisms by the introduction of a generation without previous parentage ; in other words, whoever denies the continuity of life is arbitrary."

Omne viviim e vivo — this proposition has never experienced an exception. The consequences following from this idea are very interesting. If life upon the earth has never been derived from lifeless, but always from living substance, life must have existed Avhen the earth was still an incandescent body. In fact, Preyer so concludes. He is, therefore, obliged to give to the conception of life a considerably wider scope than usual ; and to regard as living not only present living substance, but also the incandescent liquid masses as they existed at one time in place of protoplasmic organisms. " If, however, we free ourselves," says Preyer, " from the wholly voluntary and really improbable thought that only protoplasm of the quality existing at present can live, and from the old prejudice, which is sustained simply by convenience, that at first only the inorganic existed, then we will not shrink from the one great step further, we will lay aside spontaneous generation and recognise that vital motion has had no beginning. Omne vivum e vivo ! "

In accordance with these considerations, Preyer sketches somewhat as follows the picture of the derivation of life upon the earth. Originally the whole molten mass of the earth's body was a single giant organism. The powerful movement that its substance possessed was its life. When the earth's body began to cool, the substances that could no longer remain in the liquid state at that temperature, as, e.g., the heavy metals, were separated out as solid masses, and, since they no longer had a share in the vital movements of the whole, formed dead, inorganic substance. Thus arose the first inorganic masses. This process continued. It is remembered that at first hot, molten masses represented the life of the earth. " When in the course of time these compounds became solidified upon the surface of the globe, or, in other words, died, compounds of the elements that thus far had remained still gaseous and liquid appeared, and these became gradually more and more like protoplasm, the basis of the living substance of the present day. With the decrease of temperature and the lessened dissociations there must constantly have appeared more complex compounds, chemical substitutions, denser bodies, and more involved and correlated movements of the parts which were being massed closer together. Thus, the first forms of plants and animals, resembling one another and made possible by advancing differentiation, were able to exist."

" We do not say, therefore, that protoplasm as such existed from the beginning of the earth's formation ; or that without beginning it wandered as such from elsewhere out of space to the cooled earth ; or, still less, that without life it became compounded upon the planets out of inorganic bodies, as spontaneous generation would have it ; but we maintain that the movement that exists in the universe without beginning is life, and that, after the bodies now termed inorganic had been separated out upon the cooling surface of the incandescent planet by its intense vital activity, and were not able to return again into the hot liquids which gradually decreased in quantity because of the progressive decrease in temperature of the earth's crust — we maintain that, after this had occurred, protoplasm must necessarily have remained over. The heavy metals, once organic elements, no longer melted, and did not return into the circulation from which they had been cast out. They are the signs of the rigor mortis of the gigantic, cooling, primaeval organisms, whose breath perchance was luminous iron-vapour, whose blood was liquid metal, and whose food was meteorites."

In one of the most suggestive works in physiological literature Pfliiger (75,1) has discussed very fully the question of the origin of life upon the earth, and has defended the idea of spontaneous generation, that living substance originated upon the earth itself out of lifeless substances. Pfliiger's ideas are especially valuable in that in a strictly scientific manner he discusses the problem in intimate connection with the facts of physiological chemistry, and follows it out far into detail.

The essential point of Pfliiger's investigation is constituted by the chemical characteristics of proteid as that substance with which life in its essentials is inseparably united. There exists a fundamental difference between dead proteid, as it occurs, e.g., in egg-albumin, and living proteid, as it constitutes living substance ; this difference is the self-decomposition of the latter. All living substance is continually being decomposed, in some degree spontaneously and more through outside influences, while dead proteid under favourable conditions remains intact for an unlimited time. The chief condition of this decomposition is intramolecular oxygen, i.e., the oxygen that occurs in the living proteid molecule, and is continually being received by it from the outside through respiration. That this oxygen is the essential condition follows from the facts that during the decomposition carbonic acid is continually being formed, and that carbonic acid does not arise from living proteid by direct oxidation of the carbon and a simple splitting-off of the carbonic acid molecule, but by dissociation, i.e., by an internal rearrangement of the atoms and the separation of new atomic groups from one another. Living substance must contain oxygen already in combination in the living molecule, and in the deconiDosition a rearrangement must take place, otherwise it cannot be conceived how, as Pfliiger has shown upon frogs, animals can exist longer than a day without free oxygen in an

atmosphere of nitrogen and yet constantly expire carbonic acid. Why the addition of oxygen will transform a more stable molecule into a more labile condition becomes clear when it is borne in mind that, as Keklile has shown, in all organic chemistry there is no single molecule that contains enough oxygen to oxidise all the hydrogen-atoms of the molecule to water and all the carbon-atoms to carbonic acid. For this reason molecules are more or less stable and not inclined to dissociation, unless other chemical causes bring in some lability. If, however, sufficient oxygen be introduced to ensure the possibility of the oxidation of the atoms of carbon and of hydrogen into the stable molecules, carbonic acid and water, by intramolecular rearrangement, the power of decomposition must become increased, for the affinity of carbon and hydrogen for oxygen is very great. Thus, the great inclination of living substance to decomposition is conditioned essentially by the intramolecular oxygen.

A comparison of the decomposition-products of living proteid and those obtained by the artificial oxidation of dead proteid is of great importance. The significant fact here appears, that the non-nitrogenous products of the latter agree essentially with those of the former, but that " the great majority of the nitrogenous products [of the latter] have not even a remote similarity to the majority of those arising in the living body." It follows that, as regards its non-nitrogenous groups of atoms, its hydrocarbon radicals, living proteid cannot be essentially different from dead proteid, but that a fundamental difference must exist as regards the nitrogenous radicals. Here a starting-point for further consideration is afforded by the fact that of the nitrogenous decomposition-products of living proteid, such as uric acid, creatin, and, moreover, the nuclein bases, guanin, xanthin, hypoxanthin, and adenin, a part contain cyanogen, CN, as a radical, and a part, like urea, the most important of all the nitrogenous decompositionproducts of living proteid, can be produced artificially from cyanogen compounds by a rearrangement of the atoms. This points strongly to the probability that living proteid contains the radical cyanogen, and thus differs fundamentally from dead or food-proteid. Pfliiger thereupon says: "In the formation of cell-substance, i.e., of living proteid, out of food-proteid, a change of the latter takes place, the atoms of nitrogen going into a cyanogen-like relation with the atoms of carbon, probably with the absorption of considerable heat." That considerable heat is absorbed in the formation of cyanogen follows from the fact that, as calorimetric investigations show, cyanogen is a radical possessing a great quantity of internal energy.

By the addition of cyanogen to the living molecule, therefore, there is " introduced into the living matter energetic internal motion." Accordingly, the great property of decomposition possessed by living proteid is explained as the result of the absorption of oxygen ; for. since the atoms of cyanogen are in active vibration, the carbon-atom of the cyanogen at the approach of two oxygenatoms will pass out of the sphere of influence of the nitrogenatom into that of the oxygen, and will unite with the latter into carbonic acid. Thus the cause of the formation of carbonic acid, i.e., of the decomposition of living substance, lies in the cyanogen, and the condition is the intramolecular introduction of oxygen.

The idea that it is the cyanogen especially that confers upon the living proteid molecule its characteristic properties is supported especially by many analogies that exist between living proteid and the compounds of cyanogen. In the first place, a product of the oxidation of cyanogen, cyanic acid, HCNO, possesses great similarity to living proteid. Pfliiger calls attention to the following interesting points of comparison. Both bodies grow by polymerisation, by chemically combining similar molecules like chains into masses ; the growth of living substance takes place thus, and in this way also the polymeric cyamelid, HwCnN?lOn, comes from cyanic acid, HCNO. Further, both bodies in the presence of water are spontaneously decomposed into carbonic acid and ammonia. Both afford urea by dissociation, i.e., by intramolecular rearrangement, not by direct oxidation. Finally, both are liquid and transparent at low temperatures, and coagulate at higher ones, cyanic acid earlier, living proteid later. " This similarity," says Pfliiger, " is so great that I might term cyanic acid a half-living molecule."

These points of view yield most important suggestions concerning the question how life may have arisen upon the earth. " When we think of the beginning of organic life, we must not think primarily of carbonic acid and ammonia ; for they are the end of life, not the beginning." " The beginning lies rather in cyanogen." Hence the problem of the origin of living substance culminates in the question: How does cyanogen arise? Here, organic chemistry presents the highly significant fact, that cyanogen and its compounds, such as potassium cyanide, ammonium cyanide, hydrocyanic acid, cyanic acid, etc., arise only in an incandescent heat, e.g., when the necessary nitrogenous compounds are brought in contact with burning coal, or when the mass is heated to a white heat. " Accordingly, nothing is clearer than the possibility of the formation of cyanogen-compounds when the earth was wholly or partially in a fiery or heated state." Moreover, chemistry shows how the other essential constituents of proteid, such as the hydrocarbons, the alcohol radicals, etc., can likewise arise synthetically in heat.

" It is seen how strongly and remarkably all facts of chemistry point to fire as the force that has produced by synthesis the constituents of proteid. In other words, life is derived from fire, and its fundamental conditions were laid down at a time when the earth was still an incandescent ball." " If now we consider the immeasurably long time during which the cooling of the earth's surface dragged itself slowly along, cyanogen and the compounds that contain cyanogen- and hydrocarbon-substances had time and opportunity to indulge extensively their great tendency toward transformation and polymerisation and to pass over with the aid of oxygen, and later of water and salts, into that self-destructive proteid, living matter."

Pfluger thereupon summarises his ideas in the following sentences : " Accordingly, I would say that the first proteid to arise was living matter, endowed in all its radicals with the property of vigorously attracting similar constituents, adding them chemically to its molecule, and thus growing ad infinitum. According to this idea, living proteid does not need to have a constant molecular weight ; it is a huge molecule undergoing constant, never-ending formation and constant decomposition, and probably behaves toward the usual chemical molecules as the sun behaves toward small meteors."

" In the plant, living proteid simply continues to do what it has always done since its origin, i.e., regenerate or grow ; wherefore I believe that all proteid existing in the world to-day was derived directly from the first proteid. Therefore, I am doubtful about the occurrence of spontaneous generation at the present time. Comparative biology also points unmistakably to the idea that all living substance has taken its origin from a single root only."

Among the ideas regarding the derivation of life upon the earth that are contained in the theories just presented, two notions stand in sharp contrast to one another. This contrast finds expression in the alternative already set forth by Helmholtz * : " Organic life either has begun to exist at some one time or has existed from eternity" The former notion lies at the foundation of the doctrine of spontaneous generation, the latter at that of the theory of cosmozoa, and in a certain sense at the basis of Preyer's theory also. Evidently the two notions are mutually exclusive. If one is accepted, the other must be rejected. To which of the two shall we adhere ?

We will test first the theory of cosmozoa. According to it life has never originated, but has existed in the universe from eternity, and has simply been transferred from one world to another. In the present condition of our knowledge it is scarcely possible to obtain a direct contradiction of this doctrine and conclusive proof of itsimpossibility. This will be true so long as experience does not suffice to enable us to recognise as wholly impossible the transfer of protoplasmic germs capable of life from one world to another. But; although direct contradiction of the doctrine is at present impossible, the thought that living substance has existed from eternity and has never originated from inorganic substance appears in the highest degree improbable.

As a comparison of organisms and inorganic bodies has shown,1 organisms originate only from those chemical elements that occur in inorganic matter also, and differ from the latter only in the chemical compounds of which they are composed. The essential compounds of living substance, proteids, do not stand, therefore,. in fundamental contrast to inorganic compounds, and differ from the latter no more than these differ from one another. Hence any general consideration that is formulated regarding the derivation of living substance, especially of proteid, must be equally applicable in its fundamental points to inorganic compounds, such as minerals, feldspar, quartz, etc. But it is shown more clearly in non-living than in living substance to what untenable consequences the idea that lies at the foundation of the doctrine of cosmozoa leads ; for, if it be assumed that the complex compounds of living substance, especially proteids, have never originated, but have existed from eternity somewhere in space and have come thence to the earth, with the same logic and the same degree of probability it must be assumed that inorganic compounds also, quartz and feldspar, have always been present as such somewhere in space, and have come to the earth through space from another world. And if this line of thought be carried out to all chemical compounds composing the earth, — and it is as probable of them as of the compounds of living substance — it would lead to the absurd conclusion that all of the earth's compounds have wandered already complete as such from outside into our planetary system.

Scarcely any man of science would be willing to accept this conclusion ; every geologist is acquainted with examples of minerals that demonstrably have originated as such chemically upon the earth, and every chemist manufactures daily in his laboratory chemical compounds out of simpler substances. No thinking chemist, indeed, now doubts that even the so-called chemical elements did not exist originally as such, but that those elements possessing high atomic weight have been derived by condensation from those having less atomic weight. If the final conclusion be deduced from the cosmozoan ideas, all evolution, not only of living substance but of the whole earth, must be denied ; for, if all compounds have existed as such from eternity

and have never originated from simple substances, evolution has not taken place. This is the inexorable consequence of a full acceptance of the cosmozoan doctrine. We repeat that one has no right to assume for feldspar a principle of derivation different from that assumed for albumin. Both are compounds of chemical elements. One fundamental fact in plant physiology practically contradicts the assumption that life has never originated from inorganic substances ; namely, at the present time living substance is continually being formed in the plant-cell from simple inorganic compounds, carbonic acid, water, sulphates, nitrates, etc. Between the small seed put into the earth in the spring and the huge plant that grows from it during the summer, an enormous quantity of living substance has been formed out of the purely inorganic substances of the environment, and when winter comes, almost the whole quantity of this living substance returns again to simpler inorganic compounds. It is here seen how inseparably related are inorganic and organic nature, how living substance is originating continually from lifeless substance, and is continually being decomposed again into lifeless substance. Nageli ('84), one of the most talented botanists, says rightly : " One fact — that in organisms inorganic substance becomes organic substance, and that the organic returns completely to the inorganic — is sufficient to enable us to deduce by means of the law of causation the spontaneous origin of organic nature from inorganic." " If in the physical world all things stand in causal connection with one another, if all phenomena proceed along natural paths, then organisms, which build themselves up from and finally disintegrate into the substances of which inorganic nature consists, must have originated primitively from inorganic compounds. To deny spontaneous generation is to proclaim a miracle."

In a sense entirely different from that of the doctrine of cosmozoa, which has met with little acceptance, Preyer, in his theory, interprets life as without beginning and eternal. He says : The living substance now inhabiting the earth's surface is derived by continuous descent from the substances that once in a melted condition constituted the earth's mass. Not to term the latter substances living would be arbitrary, since no .sharp limit can be established. Since, however, these substances are derived from the sun's mass, and the latter forms simply a portion of the matter of the universe, which is in eternal motion, so life, which itself is only a complex process of motion, is as old as matter.

It is evident that the essential difference between Preyer's theory and the doctrine of spontaneous generation consists in a different understanding of the conception of life. Following the usage of language, the doctrine of spontaneous generation terms living only living substance as it is now recognised, in contradistinction from lifeless substance; while Preyer extends the conception much farther, even to incandescent mixtures, which have not the slightest similarity to present living substance except that they are in energetic motion. If this wide extension of the conception of life be accepted, no objection can be raised against the other consequences of Preyer's theory. It is, however, questionable whether it is judicious or allowable to carry the vital conception so far.

The conception of living substance, as scientifically established at the present time, has arisen from an exact comparison of existing living organisms and existing inorganic bodies. As has been seen,1 there is but one absolute difference between these two, and this consists in the metabolism of proteids. No inorganic body possesses proteid. On the other hand, proteid is not wanting in any organism ; and that which constitutes the life of an organism, wherein the living differs from the dead organism, is the metabolism of the proteid. This difference between living organisms and dead inorganic bodies, although not fundamental or elementary, is, nevertheless, profound and affords the sole means of sharply characterising living substance. If it be nullified by terming " living " bodies that cannot contain proteid, such as the incandescent masses of the once fiery globe, the advantage afforded by a sharp definition is wholly lost, and the conception of living substance is dissipated.

But here, from the standpoint of Preyer's theory, the question may be raised : If the living substance of to-day is derived in uninterrupted descent from molten mixtures, where is the limitbeyond which the substance may be termed living ? This question assumes a postulate that is wholly unsupported, viz., that there was a gradual, uninterrupted transition between the molten mixtures and the proteids. Hitherto we have laid great stress upon the idea that no fundamental difference exists between lifeless substances and organisms ; but it cannot be proved that an uninterrupted transition between molten substances and organisms existed. It is known, moreover, that when two chemical compounds act upon one another, the resulting substances are not necessarily joined with the original substances by transitionstages, however different they may be from them. Regarding the relations that may have prevailed upon the earth's surface when water was precipitated as liquid, we cannot form even an approximate idea. The idea that living proteid originated without transition by the action of bodies wholly different from it chemically, when the proper conditions existed, would be at least as probable as the idea of a gradual descent associated with uninterrupted transitions.

Further, Preyer implies that the incandescent masses to which he extends the conception of life have had a metabolism. This cannot be substantiated. It cannot be doubted, indeed, that these masses possessed an extremely energetic internal motion ; and, life is nothing but a complex motion, to which every other molecular motion is allied in principle. Nevertheless, vital motion, metabolism, is a complex motion very sharply characterising the living organism ; it consists in the continual self-decomposition of living substance, the giving-off to the outside of the decomposition-products, and, in return, the taking-in from the outside of certain substances, which give to the organism the material with which to regenerate itself and grow by the formation of similar groups of atoms, i.e., by polymerisation. This is characteristic of all living substance. But that this peculiar complex motion existed in the incandescent mixtures of the earth and has suffered no interruption from that time down to the present living substance, is in a high degree doubtful. Mixtures of this kind, which, as lava, can be observed at the present day in volcanoes, and there are still at so high a temperature that in flowing from a cleft of the crater over a precipice they present the wonderfully fascinating spectacle of an incandescent waterfall — these extremely liquid mixtures, however mobile they may be, show no metabolism in the real sense, and hence should not be termed living. Nor can the original incandescent mass of the earth be so termed deliberately, however impressive and suggestive Preyer's theory is.

There then remains as the sole difference between Preyer's doctrine and that of spontaneous generation the point involved in the very unessential question, whether living substance has come from lifeless substance gradually and by imperceptible transitions ; or whether it has been formed more directly, as is the product of two bodies in a chemical reaction in a test-tube, and has taken on its characteristic properties. In neither case will the conclusion be avoided that living substance once came from substances that are customarily termed lifeless.

Upon the basis of the ideas developed by Pfliiger we are now in a position to form in gross outline an approximate idea of the origin of life upon the earth. The beginnings of living substance reach down into the time when the earth's surface was still incandescent. The compounds of cyanogen then present constitute the essential material from which living substance took its origin. With their property of ready decomposition they were forced into correlation with various kinds of compounds of carbon, whose origin was due likewise to the great heat. When water was precipitated in the form of liquid upon the earth's surface, these compounds entered