Popular Science Monthly/Volume 80/January 1912/The Mechanistic Conception of Life
THE
POPULAR SCIENCE
MONTHLY
JANUARY, 1912
| THE MECHANISTIC CONCEPTION OF LIFE[1] |
By JACQUES LOEB
ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH
1. Introductory
THE reader is aware that two conflicting conceptions are held in regard to the nature of life, namely, a vitalistic and a mechanistic. The vitalists deny the possibility of a complete explanation of life in terms of physics and chemistry. The mechanists proceed as though a complete and unequivocal physico-chemical analysis of life were the attainable goal of biology. It should also be stated that whenever a vitalist desires to make a contribution to science which is more substantial and lasting than mere argument or metaphor, he forgets or lays aside his vitalism and proceeds on the premises and methods of the mechanist. It is thus obvious that as far as the progress of biology is concerned the difference of viewpoint between vitalists and mechanists is of no consequence.
The difference between the two opposite views becomes only of importance when the results of biology are applied to ethical and sociological problems. Since applications of this kind present themselves constantly, the biologist may be pardoned if he raises the question whether or not our present state of knowledge justifies the expectation that life phenomena may ultimately be completely explained in terms of physics and chemistry. I intend to put before you a brief survey of some results, in the main recent, of scientific inquiry which I think may be utilized for an answer to this question.
Before going into these data, it may be necessary to allude briefly to a not uncommon misapprehension in regard to the nature of biological "truth" and methods. It is seemingly often taken for granted by laymen that "truth" in biology or science in general is of the same order as "truth" in certain of the mental sciences; that is to say, that everything rests on argument or rhetoric and that what is regarded as true to-day may be expected with some probability to be considered untrue to-morrow. It happens in science, especially in the descriptive sciences like paleontology or zoology, that hypotheses are forwarded, discussed and then abandoned. It should, however, be remembered that modern biology is fundamentally an experimental and not a descriptive science; and that its results are not rhetorical, but always assume one of two forms: it is either possible to control a life phenomenon to such an extent that we can produce it at desire at any time (as, e. g., the contraction of an excised muscle); or we succeed in finding the numerical relation between the conditions of the experiment and the biological result (e. g., Mendel's law of heredity). Biology as far as it is based on these two principles can not retrogress, but must advance.
2. The Beginning of Scientific Biology
Scientific biology, defined in this sense, begins with the attempt made by Lavoisier and Laplace (1780) to show that the quantity of heat which is formed in the body of a warm-blooded animal is equal to that formed in a candle, provided that the quantities of carbon dioxide formed in both cases are identical. This was the first attempt to reduce a life-phenomenon, namely, the formation of animal heat, completely to physico-chemical terms. What these two investigators began with primitive means has been completed by more recent investigators—Pettenkofer and Voit, Rubner and Zuntz. The oxidation of a foodstuff always furnishes the same amount of heat, no matter whether it takes place in the living body or outside.
These investigations left a gap. The substances which undergo oxidations in the animal body—starch, fat and proteins—are substances which at ordinary temperature are not easily oxidized. They require the temperature of the flame in order to undergo rapid oxidation through the oxygen of the air. This discrepancy between the oxidations in the living body and those in the laboratory manifests itself also in other chemical processes, e. g., digestion or hydrolytic reactions, which were at first found to occur outside the living body rapidly only under conditions incompatible with life. This discrepancy was done away with by the physical chemists, who demonstrated that the same acceleration of chemical reactions which is brought about by a high temperature can also be accomplished at a low temperature with the aid of certain specific substances, the so-called catalyzers. This progress is connected preeminently with the names of Berzelius and Wilhelm Ostwald. The specific substances which accelerate the oxidations at body temperature sufficiently to allow the maintenance of life are the so-called ferments of oxidation.
The work of Lavoisier and Laplace not only marks the beginning of scientific biology, it also touches the core of the problem of life; for it seems that oxidations form a part, if not the basis, of all life phenomena in higher organisms.
3. The "Riddle of Life"
By the "riddle of life" not everybody will understand the same thing. We all, however, desire to know how life originates and what death is, since our ethics must be influenced to a large extent through the answer to this question. We are not yet able to give an answer to the question as to how life originated on the earth. We know that every living being is able to transform food-stuffs into living matter; and we also know that not only the compounds which are formed in the animal body can be produced artificially, but that chemical reactions which take place in living organisms can also be repeated at the same rate and temperature in the laboratory. The gap in our knowledge which we feel most keenly is the fact that the chemical character of the catalyzers (the enzymes or ferments) is still unknown. Nothing indicates, however, at present that the artificial production of living matter is beyond the possibilities of science.
This view does not stand in opposition to the idea of Arrhenius that germs of sufficiently small dimensions are driven by radiation pressure through space; and that these germs if they fall upon new cosmic bodies possessing water, salts and oxygen and the proper temperature, give rise to a new evolution of organisms. Biology will certainly retain this idea, but I believe that we must also follow out the other problem: namely, either succeed in producing living matter artificially, or find the reasons why this should be impossible.
4. The Activation of the Ego
Although we are not yet able to state how life originated in general, another, more modest problem has been solved, that is, how the egg is caused by the sperm to develop into a new individual. Every animal originates from an egg and in the majority of animals a new individual can only then develop if a male sex-cell, a spermatozoon, enters into the egg. The question as to how a spermatozoon can cause an egg to develop into a new individual was twelve years ago still shrouded in that mystery which to-day surrounds the origin of life in general. But to-day we are able to state that the problem of the activation of the egg is for the most part reduced to physico-chemical terms. The egg is in the unfertilized condition a single cell with only one nucleus. If no spermatozoon enters into it, it perishes after a comparatively short time, in some animals in a few hours, in others in a few days or weeks. If, however, a spermatozoon enters into the egg, the latter begins to develop, i. e., the nucleus begins to divide into two nuclei and the egg which heretofore consisted of one cell is divided into two cells. Subsequently each nucleus and each cell divides again into two, and so on. These cells have in many eggs the tendency to remain at the surface of the egg or to creep to the surface and later such an egg forms a hollow sphere whose shell consists of a large number of cells. On the outer surface of this hollow sphere cilia are formed and the egg is now transformed into a free-swimming larva. Then an intestine develops through the growing in of cells in one region of the blastula and gradually the other organs, skeleton, vascular system, etc., originate. Embryologists had noticed that occasionally the unfertilized eggs of certain animals, e. g., sea-urchins, worms, or even birds, show a tendency to a nuclear or even a cell division; and R. Hertwig, Mead and Morgan had succeeded in inducing one or more cell divisions artificially in such eggs. But the cell divisions in these cases never led to the development of a larva, but at the best to the formation of an abnormal mass of cells which soon perished.
I succeeded twelve years ago in causing the unfertilized eggs of the sea-urchin to develop into swimming larvæ by treating them with seawater, the concentration of which was raised through the addition of a small but definite quantity of a salt or sugar. The eggs were put for two hours into a solution the osmotic pressure of which had been raised to a certain height. When the eggs were put back into normal seawater they developed into larvæ and a part of these larvæ formed an intestine and a skeleton. The same result was obtained in the eggs of other animals, starfish, worms and mollusks. These experiments proved the possibility of substituting physico-chemical agencies for the action of the living spermatozoon, but did not yet explain how the spermatozoon causes the development of the egg, since in these experiments the action of the spermatozoon upon the egg was very incompletely imitated. When a spermatozoon enters into the egg it causes primarily a change in the surface of the egg which results in the formation of the so-called membrane of fertilization. This phenomenon of membrane formation which had always been considered as a phenomenon of minor importance did not occur in my original method of treating the egg with hypertonic sea-water. Six years ago while experimenting on the Calif ornian sea-urchin, Strongylocentrotus purpuratus, I succeeded in finding a method of causing the unfertilized egg to form a membrane without injuring the egg. This method consists in treating the eggs for from one to two minutes with sea-water to which a definite amount of butyric acid (or some other monobasic fatty acid) has been added. If after that time the eggs are brought back into normal sea-water, all form a fertilization membrane in exactly the same way as if a spermatozoon had entered. This membrane formation or rather the modification of the surface of the egg which underlies the membrane formation starts the development. It does not allow it, however, to go very far at room temperature. In order to allow the development to go further it is necessary to submit the eggs after the butyric acid treatment to a second operation. Here we have a choice between two methods. We can either put the eggs for about one half hour into a hypertonic solution (which contains free oxygen); or we can put them for about three hours into sea-water deprived of oxygen. If the eggs are then returned to normal seawater containing oxygen they all develop; and in a large number the development is as normal as if a spermatozoon had entered.
The essential feature is therefore the fact that the development is caused by two different treatments of the egg; and that among these the treatment resulting in the formation of the membrane is the more important one. This is proved by the fact that in certain forms, as for instance the star-fish, the causation of the artificial membrane formation may suffice for the development of normal larvæ; although here too the second treatment increases not only the number of larvæ, but also improves the appearance of the larvæ, as R. Lillie found.
The question now arises, how the membrane formation can start the development of the egg. An analysis of the process and of the nature of the agencies which cause it yielded the result that the unfertilized egg possesses a superficial cortical layer, which must be destroyed before the egg can develop. It is immaterial by what means this superficial cortical layer is destroyed. All agencies, which cause a definite type of cell destruction—the so-called cytolysis—cause also the egg to develop, as long as their action is limited to the surface layer of the cell. The butyric acid treatment of the egg mentioned above only serves to induce the destruction of this cortical layer. In the eggs of some animals this cortical layer can be destroyed mechanically by shaking the egg, as A. P. Mathews found in the case of star-fish eggs and I in the case of the eggs of certain worms. In the case of the eggs of the frog it suffices to pierce the cortical layer with a needle, as Bataillon found in his beautiful experiments a year ago.[2] The mechanism by which development is caused is apparently the same in all these cases, namely, the destruction of the cortical layer of the eggs. This can be caused generally by certain chemical means which play a rôle also in bacteriology; but it can also be caused in special cases by mechanical means, such as agitation or piercing of the cortical layer. It may be mentioned parenthetically that foreign blood sera have also a cytolytic effect, and I succeeded in causing membrane formation and in consequence the development of the sea-urchin egg by treating it with the blood of various animals, e. g., of cattle, or the rabbit.
Recently Shearer has succeeded in Plymouth in causing a number of parthenogenetic plutei produced by my method to develop beyond the stage of metamorphosis, and Delage has reported that he raised two larvæ of the sea-urchin produced by artificial parthenogenesis to the stage of sexual maturity. We may, therefore, state that the complete imitation of the developmental effect of the spermatozoon by certain physico-chemical agencies has been accomplished.
I succeeded in showing that the spermatozoon causes the development of the sea-urchin egg in a way similar to that in my method of artificial parthenogenesis; namely, by carrying two substances into the egg, one of which acts like the butyric acid and induces the membrane formation, while the other acts like the treatment with a hypertonic solution and enables the full development of the larvæ. In order to prove this for the sea-urchin egg foreign sperm, e. g., that of the starfish, must be used. The sperm of the sea-urchin penetrates so rapidly into the sea-urchin egg that almost always both substances get into the egg. If, however, star-fish sperm is used for the fertilization of the sea-urchin egg, in a large number of cases, membrane formation occurs before the spermatozoon has found time to entirely penetrate into the egg. In consequence of the membrane formation the spermatozoon is thrown out. Such eggs behave as if only the membrane formation had been caused by some artificial agency, e. g., butyric acid. They begin to develop, but soon show signs of disintegration. If treated with a hypertonic solution they develop into larvæ. In touching the egg contents the spermatozoon had a chance to give off a substance which liquefied the cortical layer and thereby caused the membrane formation by which the further entrance of the spermatozoon into the egg was prevented. If, however, the starfish sperm enters completely into the egg before the membrane formation begins, the spermatozoon carries also the second substance into the egg, the action of which corresponds to the treatment of the egg with the hypertonic solution. In this case the egg can undergo complete development into a larva.
F. Lillie has recently confirmed the same fact in the egg of a worm, Nereis. He mixed the sperm and eggs of Nereis and centrifuged the mass. In many cases the spermatozoa which had begun to penetrate into the egg were thrown off again. The consequence was that only a membrane formation resulted without the spermatozoon penetrating into the egg. This membrane formation led only to a beginning but not to a complete development. We may, therefore, conclude that the spermatozoon causes the development of the egg in a way similar to that which takes place in the case of artificial parthenogenesis. It carries first a substance into the egg which destroys the cortical layer of the egg in the same way as butyric acid does; and secondly a substance which corresponds in its effect to the influence of the hypertonic solution in the sea-urchin egg after the membrane formation.
The question arises as to how the destruction of the cortical layer can cause the beginning of the development of the egg. This question leads us to the process of oxidation. Years ago I had found that the fertilized sea-urchin egg can only develop in the presence of free oxygen; if the oxygen is completely withdrawn the development stops, but begins again promptly as soon as oxygen is again admitted. From this and similar experiments I concluded that the spermatozoon causes the development by accelerating the oxidations in the egg. This conclusion was confirmed by experiments by O. Warburg and by Wasteneys and myself in which it was found that through the process of fertilization the velocity of oxidations in the egg is increased to four or six times its original value. Warburg was able to show that the mere causation of the membrane formation by the butyric acid treatment has the same accelerating effect upon the oxidations as fertilization.
What remains unknown at present is the way in which the destruction of the cortical layer of the egg accelerates the oxidations. It is possible that the cortical layer acts like a solid crust and thus prevents the oxygen from reaching the surface of the egg or from penetrating into the latter sufficiently rapidly. The solution of these problems must be reserved for further investigation.
We, therefore, see that the process of the activation of the egg by the spermatozoon, which twelve years ago was shrouded in complete darkness, to-day is practically completely reduced to a physico-chemical explanation. Considering the youth of experimental biology we have a right to hope that what has been accomplished in this problem will occur in rapid succession in those problems which to-day still appear as riddles.
5. Nature of Life and Death
The nature of life and of death are questions which occupy the interest of the layman to a greater extent than possibly any other purely theoretical problem; and we can well understand that humanity did not wait for experimental biology to furnish an answer. The answer assumed the anthropomorphic form characteristic of all explanations of nature in the prescientific period. Life was assumed to begin with the entrance of a "life principle" into the body; that individual life begins with the egg was of course unknown to primitive or pre-scientific man. Death was assumed to be due to the departure of this "life principle" from the body.
Scientifically, however, individual life begins (in the case of the sea-urchin and possibly in general) with the acceleration of the rate of oxidation in the egg, and this acceleration begins after the destruction of its cortical layer. Life of warm blooded animals—man included—ends with the cessation of oxidation in the body. As soon as oxidations have ceased for some time the surface films of the cells, if they contain enough water and if the temperature is sufficiently high, become permeable for bacteria, and the body is destroyed by microorganisms. The problem of the beginning and end of individual life is physico-chemically clear. It is, therefore, unwarranted to continue the statement that in addition to the acceleration of oxidations the beginning of individual life is determined by the entrance of a metaphysical "life principle" into the egg; and that death is determined, aside from the cessation of oxidations, by the departure of this "principle" from the body. In the case of the evaporation of water we are satisfied with the explanation given by the kinetic theory of gases and do not demand that—to repeat a well-known jest of Huxley—the disappearance of the "aquosity" be also taken into consideration.
6. Heredity
It may be stated that the egg is the essential bearer of heredity. We can cause an egg to develop into a larva without sperm, but we can not cause a spermatozoon to develop into a larva without an egg. The spermatozoon can influence the form of the offspring only when the two forms are rather closely related. If the egg of a sea-urchin is fertilized with the sperm from a different species of sea-urchin the larval form has distinct paternal characters. If, however, the eggs of a sea-urchin are fertilized with the sperm of a more remote species, e. g., a star-fish, the result is a sea-urchin larva which possesses no paternal characters, as I found and as Godlewski, Kupelwieser, Hagedoorn and Baltzer were able to confirm. This fact has some bearing upon the further investigation of heredity, inasmuch as it shows that the egg is the main instrument of heredity, while apparently the spermatozoon is restricted in the transmission of characters to the offspring. If the difference between spermatozoon and egg exceeds a certain limit the hereditary effects of the spermatozoon cease and it acts merely as an activator to the egg.
As far as the transmission of paternal characters is concerned, we can say to-day that the view of those authors was correct who, with Boveri, localized this transmission not only in the cell nucleus, but in a special constituent of the nucleus, the chromosomes. The proof for this was given by facts found along the lines of Mendelian investigations. The essential law of Mendel, the law of segregation, can in its simplest form be expressed in the following way. If we cross two forms which differ in only one character every hybrid resulting from this union forms two kinds of sex-cells in equal numbers; two kinds of eggs if it is a female, two kinds of spermatozoa if it is a male. The one kind corresponds to the pure paternal, the other to the pure maternal type. The investigation of the structure and behavior of the nucleus showed that the possibility for such a segregation of the sex-cells in a hybrid can easily be recognized during a given stage in the formation of the sex-cells, if the assumption is made that the chromosomes are the bearers of the paternal characters. The proof for the correctness of this view was furnished through the investigation of the heredity of those qualities which occur mainly in one sex; e. g., color blindness which occurs preeminently in the male members of a family.
Nine years ago McClung published a paper which solved the problem of sex determination, at least in its essential feature. Each animal has a definite number of chromosomes in its cell nucleus. Henking had found that in a certain form of insects (Pyrrhocoris) two kinds of spermatozoa exist which differ in the fact that the one possesses a nucleolus while the other does not. Montgomery afterwards showed that Henking's nucleolus was an accessory chromosome. McClung first expressed the idea that this accessory chromosome was connected with the determination of sex. Considering the importance of this idea we may render it in his own words:
A most significant fact, and one upon which almost all investigators are united in opinion, is that the element is apportioned to but one half of the spermatozoa. Assuming it to be true that the chromatin is the important part of the cell in the matter of heredity, then it follows that we have two kinds of spermatozoa that differ from each other in a vital matter. We expect, therefore, to find in the offspring two sorts of individuals in approximately equal numbers, under normal conditions, that exhibit marked differences in structure. A careful consideration will suggest that nothing but sexual characters thus divides the members of a species into two well-defined groups, and we are logically forced to the conclusion that the peculiar chromosome has some bearing upon the arrangement.
I must here also point out a fact that does not seem to have the recognition it deserves; viz., that if there is a cross division of the chromosomes in the maturation mitoses, there must be two kinds of spermatozoa regardless of the presence of the accessory chromosome. It is thus possible that even in the absence of any specialized element a preponderant maleness would attach to one half of the spermatozoa, due to the "qualitative division of the tetrads."
The researches of the following years, especially the brilliant work of E. B. Wilson, Miss Stevens, T. H. Morgan and others, have amply confirmed the correctness of this ingenious idea and cleared up the problem of sex determination in its main features.
According to McClung each animal forms two kinds of spermatozoa in equal numbers, which differ by one chromosome. One kind of spermatozoa produces male animals, the other female animals. The eggs are all equal in these animals. More recent investigations, especially by E. B. Wilson, have shown that this view is correct for many animals.
While in many animals there are two kinds of spermatozoa and only one kind of eggs, in other animals two kinds of eggs and only one kind of spermatozoa are formed, e. g., sea-urchins and certain species of birds and of butterflies (Abraxas). In these animals the sex is predetermined in the egg and not in the spermatozoon. It is of interest that, according to Guyer, in the human being two kinds of spermatozoa exist and only one kind of eggs; in man, therefore, sex is determined by the spermatozoon.
How is sex determination accomplished? Let us take the case which according to Wilson is true for many insects and according to Guyer for human beings, namely, that there are two kinds of spermatozoa and one kind of egg. According to Wilson all unfertilized eggs contain in this case one so-called sex chromosome, the X-chromosome. There are two kinds of spermatozoa, one with and one without an X-chromosome. Given a sufficiently large number of eggs and of spermatozoa, one half of the egs will be fertilized by spermatozoa with and one half by spermatozoa without an X-chromosome. Hence one half of the eggs will contain after fertilization two X-chromosomes each and one half only one X-chromosome each. The eggs containing only one X-chromosome give rise to males, those containing two X-chromosomes give rise to females—as Wilson and others have proved. This seems to be a general law for those cases in which there are two kinds of spermatozoa and one kind of eggs.
These observations show why it is impossible to influence the sex of a developing embryo by external influences. If, for example, in the human a spermatozoon without an X-chromosome enters into an egg, the egg will give rise to a boy, but if a spermatozoon with an X-chromosome gets into the egg the latter will give rise to a girl. Since always both kinds of spermatozoa are given off by the male it is a mere matter of chance whether a boy or a girl originates; and it agrees with the law of probability that in a large population the number of boys and girls borne within a year is approximately the same.
These discoveries solved also a series of other difficulties. Certain types of twins originate from one egg after fertilization. Such twins have always the same sex, as we should expect since the cells of both twins have the same number of X-chromosomes,
In plant lice, bees and ants, the eggs may develop with and without fertilization. It was known that from fertilized eggs in these animals only females develop, males never. It was found that in these animals the eggs contain only one sex-chromosome; while in the male are found two kinds of spermatozoa, one with and one without a sex-chromosome. For Phylloxera and Aphides it has been proved with certainty by Morgan and others that the spermatozoa which contain no sex-chromosome can not live, and the same is probably true for bees and ants. If, therefore, in these animals an egg is fertilized it is always done by a spermatozoon which contains an X-chromosome. The Qgg has, therefore, after fertilization in these animals always two Z-chromosomes and from such eggs only females can arise.
It had been known for a long time that in bees and ants the unfertilized eggs can also develop, but such eggs give rise to males only. This is due to the fact that the eggs of these animals contain only one X-chromosome and from eggs with only one chromosome only males can arise (at least in the case of animals in which the male is heterozygous for sex).
The problem of sex determination has, therefore, found a simple solution, and simultaneously Mendel's law of segregation finds also its solution.
In many insects and in man the cells of the female have two sex-chromosomes. In a certain stage of the history of the egg one half of the chromosomes leaves the egg (in the form of the "polar-body") and the egg keeps only half the number of chromosomes. Each egg, therefore, retains only one X or sex-chromosome. In the male the cells have from the beginning only one X-chromosome and each primordial spermatozoon divides into two new (in reality into two pairs of) spermatozoa, one of which contains an X-chromosome while the other is without such a chromosome. What can be observed here directly in the male animal takes place in every hybrid: during the critical, so-called maturation division of the sexual cell in the hybrid a division of the chromosomes occurs whereby only one half of the sex cells receive the hereditary substance in regard to which the two original pure forms differ.
That this is not a mere assumption can be shown in those cases in which the hereditary character appears only, or preeminently, in one sex as, e. g., color blindness which appears mostly in the male. If a color-blind individual is mated with an individual with normal color vision the heredity of color blindness in the next two generations corresponds quantitatively with what we must expect on the assumption that the chemical substances determining color vision are contained in the sex-chromosomes. In the color-blind individual something is lacking which can be found in the individual with normal color perception. The factor for color vision is obviously transmitted through the sex-chromosome. In the next generation color blindness can not appear since each fertilized egg contains the factor for color perception. In the second generation, however, the theory demands that one half of the males should be color blind. In man these conditions can not always be verified numerically since the number of children is too small to yield the conditions to be expected according to the calculus of probability. T. H. Morgan has found in a fly (Drosophila) a number of similar sex-limited characters, which behave like color blindness, e. g., lack of pigment in the eyes. These flies have normally red eyes. Morgan has observed a mutation with white eyes, which occurs in the male. When he crossed a white-eyed with a red-eyed female all flies of the first generation were red-eyed; since all flies had the factor for pigment in their sex-cells; in the second generation all females and exactly one half of the males had red eyes, the other half of the males, however, white eyes, as the theory demands.
From these and numerous similar breeding experiments of Correns, Doncaster, and especially of Morgan, we may conclude with certainty that the sex-chromosomes are the bearers of those hereditary characters which appear preeminently in one sex. We say preeminently since theoretically we can predict cases in which color blindness or white eyes must appear also in the female. Breeding experiments have shown that this theoretical prediction is justified. The riddle of Mendel's law of segregation finds its solution by these experiments and incidentally also the problem of the determination of sex which is only a special case of the law of segregation, as Mendel already intimated.
The main task which is left here for science to accomplish is the determination of the chemical substances in the chromosomes which are responsible for the hereditary transmission of a quality, and the determination of the mechanism by which these substances give rise to the hereditary character. Here the ground has already been broken. It is known that for the formation of a certain black pigment the cooperation of a substance—tyrosin—and of a ferment of oxidation—tyrosinase—is required. The hereditary transmission of the black color through the male animal must occur by substances carried in the chromosome which determine the formation of tyrosin or tyrosinase or of both. We may, therefore, say that the solution of the riddle of heredity has succeeded to the extent that all further development will take place purely in cytological and physico-chemical terms.
While until twelve years ago the field of heredity was the stamping ground for the rhetorician and metaphysician it is to-day perhaps the most exact and rationalistic part of biology, where facts can not only be predicted qualitatively, but also quantitatively.
7. The Harmonious Character of the Organisms
It is not possible to prove in a short address that all life phenomena will yield to a physico-chemical analysis. We have selected only the phenomena of fertilization and heredity, since these phenomena are specific for living organisms and without analogues in inanimate nature; and if we can convince ourselves that these processes can be explained physico-chemically we may safely expect the same of such processes for which there exist a priori analogies in inanimate nature, as, e. g., for absorption and secretion.
We must, however, settle a question which offers itself not only to the layman but also to every biologist, namely, how we shall conceive that wonderful "adaptation of each part to the whole" by which an organism becomes possible. In the answer of this question the metaphysician finds an opportunity to put above the purely chemical and physical processes something specific which is characteristic of life only: the "Zielstrebigkeit," the "harmony" of the phenomena, or the "dominants" of Reinke and similar things.
With all due personal respect for the authors of such terms I am of the opinion that we are dealing here, as in all cases of metaphysics, with a play on words. That a part is so constructed that it serves the "whole" is only an unclear expression for the fact that a species is only able to live—or to use Roux's expression—is only durable, if it is provided with the automatic mechanism for self-preservation and reproduction. If, for instance, warm-blooded animals should originate without a circulation they could not remain alive, and this is the reason why we never find such forms. The phenomena of "adaptation" cause only apparent difficulties since we rarely or never become aware of the numerous faultily constructed organisms which appear in nature. I will illustrate by a concrete example that the number of species which we observe is only an infinitely small fraction of those which can originate and possibly not rarely do originate, but which we never see since their organization does not allow them to exist long. Moenkhaus found ten years ago that it is possible to fertilize the egg of each marine bony fish with the sperm of practically any other marine bony fish. His embryos apparently lived only a very short time. This year I succeeded in keeping such hybrid embryos between distantly related bony fish alive for over a month. It is, therefore, clear that it is possible to cross practically any marine teleost with any other.
The number of teleosts at present in existence is about 10,000. If we accomplish all possible hybridization 100,000,000 different crosses will result. Of these teleosts only a very small proportion, namely about one one-hundredth of one per cent., can live. It turned out in my experiments that the heterogeneous hybrids between bony fishes formed eyes, brains, ears, fins and pulsating hearts, blood and blood vessels, but could live only a limited time because no blood circulation was established at all—in spite of the fact that the heart beat for weeks—or that the circulation, if it was established at all, did not last long.
What prevented these heterogeneous fish embryos from reaching the adult stage? The lack of the proper "dominants"? Scarcely. I succeeded in producing the same type of faulty embryos in the pure breeds of a bony fish (Fundulus heteroclitus) by raising the eggs in 50 c.c. of sea-water to which was added 2 c.c. one one-hundredth per cent. NaCN", The latter substance retards the velocity of oxidations and I obtained embryos which were in all details identical with the embryos produced by crossing the eggs of the same fish with the sperm of remote teleosts, e. g., Ctenolabrus or Menidia. These embryos, which lived about a month, showed the peculiarity of possessing a beating heart and blood, but no circulation. This suggests the idea that heterogeneous embryos show a lack of "adaptation" and durability for the reason that in consequence of the chemical difference between heterogeneous sperm and egg the chemical processes in the fertilized egg are abnormal.
The possibility of hybridization goes much further than we have thus far assumed. We can cause the eggs of echinoderms to develop with the sperm of very distant forms, even mollusks and worms (Kupelwieser); but such hybridizations never lead to the formation of durable organisms.
It is, therefore, no exaggeration to state that the number of species existing to-day is only an infinitely small fraction of those which can and possibly occasionally do originate, but which escape our notice because they can not live and reproduce. Only that limited fraction of species can exist which possesses no coarse disharmonies in its automatic mechanism of preservation and reproduction. Disharmonies and faulty attempts in nature are the rule, the harmonically developed systems the rare exception. But since we only perceive the latter we gain the erroneous impression that the "adaptation of the parts to the plan of the whole" is a general and specific characteristic of animate nature, whereby the latter differs from inanimate nature.
If the structure and the mechanism of the atoms were known to us we should probably also get an insight into a world of wonderful harmonies and apparent adaptations of the parts to the whole. But in this case we should quickly understand that the chemical elements are only the few durable systems among a large number of possible but not durable combinations. Nobody doubts that the durable chemical elements are only a product of blind forces. There is no reason for conceiving otherwise the durable systems in living nature.
8. The Contents of Life
The contents of life from the cradle to the bier are wishes and hopes, efforts and struggles and unfortunately also disappointments and suffering. And this inner life should be amenable to a physico-chemical analysis? In spite of the gap which separates us to-day from such an aim I believe that it is attainable. As long as a life phenomenon has not yet found a physico-chemical explanation it usually appears inexplicable. If the veil is once lifted we are always surprised that we did not guess from the first what was behind it.
That in the case of our inner life a physico-chemical explanation is not beyond the realm of possibility is proved by the fact that it is already possible for us to explain cases of simple manifestations of animal instinct and will on a physico-chemical basis; namely, the phenomena which I have discussed in former papers under the name of animal tropisms. As the most simple example we may mention the tendency of certain animals to fly or creep to the light. We are dealing in this case with the manifestation of an instinct or impulse which the animals can not resist. It appears as if this blind instinct which these animals must follow, although it may cost them their life might be explained by the same law of Bunsen and Roscoe, which explains the photo-chemical effects in inanimate nature. This law states that within wide limits the photo-chemical effect equals the product of the intensity of light into the duration of illumination. It is not possible to enter here into all the details of the reactions of these animals to light, we only wish to point out in which way the light instinct of the animals may possibly be connected with the Bunsen-Roscoe law.
The positively heliotropic animals—i. e., the animals which go instinctively to a source of light—have in their eyes (and occasionally also in their skin) photosensitive substances which undergo chemical alterations by light. The products formed in this process influence the contraction of the muscles—mostly indirectly, through the central nervous system. If the animal is illuminated on one side only the mass of photochemical reaction products formed on that side in the unit of time is greater than on the opposite side. Consequently the development of energy in the symmetrical muscles on both sides of the body becomes unequal. As soon as the difference in the masses of the photochemical reaction products on both sides of the animal reaches a certain value the animal, as soon as it moves, is automatically forced to turn towards one side. As soon as it has turned so far that its plane of symmetry is in the direction of the rays, the symmetrical spots of its surface are struck by the light at the same angle and in this case the intensity of light and consequently the velocity of reaction of the photochemical processes on both sides of the animal become equal. There is no more reason for the animal to deviate from the motion in a straight line and the positively heliotropic animal will move in a straight line to the source of light. (It was assumed that in these experiments the animal is under the influence of only one source of light and positively heliotropic.)
In a series of experiments I have shown that the heliotropic reactions of animals are identical with the heliotropic reactions of plants. It was known that sessile heliotropic plants bend their stems to the source of light until the axis of symmetry of their tip is in the direction of the rays of light. I found the same phenomenon in sessile animals, e. g., certain hydroids and worms. Motile plant organs, e. g., the swarm spores of plants, move to the source of light (or if they are negatively heliotropic away from it) and the same is observed in motile animals. In plants only the more refrangible rays from green to blue have these heliotropic effects, while the red and yellow rays are little or less effective; and the same is true for the heliotropic reactions of animals.
It has been shown by Blaauw for the heliotropic curvatures of plants that the product of the intensity of a source of light into the time required to induce a heliotropic curvature is a constant; and the same result was obtained simultaneously by another botanist, Froschl, It is thus proved that the Bunsen-Roscoe law controls the heliotropic reactions of plants. The same fact had already been proved for the action of light on our retina.
The direct measurements in regard to the applicability of Bunsen's law to the phenomena of animal heliotropism have not yet been made. But a number of data point to the probability that the law holds good here also. The first of these facts is the identity of the light reactions of plants and animals. The second is at least a rough observation which harmonizes with the Bunsen-Roscoe law. As long as the intensity of light or the mass of photochemical substances at the surface of the animal is small, according to the law of Bunsen, it must take a comparatively long time until the animal is automatically oriented by the light, since according to this law the photochemical effect is equal to the product of the intensity of the light into the duration of illumination. If, however, the intensity of the light is strong or the active mass of the photochemical substance great, it will require only a very shoi't time until the difference in the mass of photochemical reaction products on both sides of the animal reaches the value which is necessary for the automatic turning to (or from) the light. The behavior of the animals agrees with this assumption. If the light is sufficiently strong the animals go in an almost straight line to the source of light; if the intensity of light (or the mass of photosensitive substances on the surface of the animal) is small the animals go in irregular lines, but at last they also land at the source of light, since the directing force is not entirely abolished. It will, however, be necessary to ascertain by direct measurements to what extent these phenomena in animals are the expression of Bunsen-Eoscoe's law. But we may already safely state that the apparent will or instinct of these animals resolves itself into a modification of the action of the muscles through the influence of light; and for the metaphysical term "will" we may in these instances safely substitute the chemical term "photochemical action of light."
Our wishes and hopes, disappointments and sufferings have their source in instincts which are comparable to the light instinct of the heliotropic animals. The need of and the struggle for food, the sexual instinct with its poetry and its chain of consequences, the maternal instincts with the felicity and the suffering caused by them, the instinct of workmanship and some other instincts are the roots from which our inner life develops. For some of these instincts the chemical basis is at least sufficiently indicated to arouse the hope that their analysis, from the mechanistic point of view, is only a question of time.
9. Ethics
If our existence is based on the play of blind forces and only a matter of chance; if we ourselves are only chemical mechanisms—how can there be an ethics for us? The answer is, that our instincts are the root of our ethics and that the instincts are just as hereditary as is the form of our body. We eat, drink and reproduce not because mankind has reached an agreement that this is desirable, but because, machine-like, we are compelled to do so. We are active, because we are compelled to be so by processes in our central nervous system; and as long as human beings are not economic slaves the instinct of successful work or of workmanship determines the direction of their action. The mother loves and cares for her children not because metaphysicians had the idea that this was desirable, but because the instinct of taking care of the young is inherited just as distinctly as the morphological characters of the female body. We seek and enjoy the fellowship of human beings because hereditary conditions compel us to do so. We struggle for justice and truth since we are instinctively compelled to see our fellow beings happy. Economic, social and political conditions or ignorance and superstition may warp and inhibit the inherited instincts and thus create a civilization with a faulty or low development of ethics. Individual mutants may arise in which one or the other desirable instinct is lost, just as individual mutants without pigment may arise in animals; and the offspring of such mutants may, if numerous enough, lower the ethical status of a community. Not only is the mechanistic conception of life compatible with ethics; it seems the only conception of life which can lead to an understanding of the source of ethic