The Mechanistic Conception of Life

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 those of 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 eggs. According to Wilson all unfertilized eggs contain in this case one so-called Figs. 11-16 (after E. B. Wilson). — Diagrammatic presentation of sex determination in an insect (Protenor). a a are the nuclei of im^fertUized eggs. Each contains one sex cliromosome marked X; the other six dark spots are the chromosomes which are supposed to transmit hiereditary characters not connected with sex. b and c represent the two different types of sperm; b containing a sex chromosome X, c being without such a chromosome.

d represents the constitution of the egg nucleus after it is fertilized by a spermatozoon of the type b containing a sex chromosome. This egg now has two sex chromosomes and therefore wiU give rise to a female, e represents a fertilized egg after a spermatozoon of the type c (without a sex chromosome) has entered it. This egg contains after fertilization only one sex chromosome X and hence will give rise to a male. 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 eggs 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 being 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 born 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 cannot live, and the same is probably true for bees and ants. If, therefore, in these animals an egg is

1 It is stated that the number of males bom exceeds that of the females by a slight percentage. If this statement is correct it must be due to a secondarycause, e.g., a greater motility or greater duration of life of the male spermatozoon. Further researches will be needed to clear up this point. fertilized it is always done by a spermatozoon which contains an X-chromosome. The egg has, therefore, after fertilization in these animals always two X-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 also finds 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 leave the egg (in the form of the '^ polar-body") and it 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 sho"v\Ti 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 ^vith 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 cannot 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 cannot be verified. 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 pre-eminently in one sex. We say pre-eminently, since theoretically we can predict cases in which color blindness or white eyes must appear also in the female. Breeding experiments have sho^\TL that this theoretical prediction is justified. The riddle of Mendel's law of segregation finds its solution through 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 knowTi 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 today perhaps the most exact and rationalistic part of biology, where facts cannot only be predicted qualitatively, but also quantitatively. 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 to this question the metaphysician finds an opportunity to put above the purely chemical and physical processes somethingspecific 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 continue 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 nmnber of teleosts at present in existence is about 10,000. If we accomplish all possible hybridizations 100,000,000 different crosses will result. Of these teleosts only a very small proportion, namely about one one-hundredth of 1 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 — in spite of the fact that the heart beat for weeks — or that the circulation, if it was estabhshed 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. 1/100 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., Ctenolahrus 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 farther 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 today is only an infinitely small fraction of those which can and possibly occasionally do originate, but which escape our notice because they cannot five 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 knowTi 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 the product of blind forces. There is no reason for conceiving otherwise the durable systems in living nature.

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 gulf which separates us today 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 cannot 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 photochemical effects in inanimate nature. This law states that within wide limits the photochemical 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 hehotropic 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 toward 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 this 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 knowTti that sessile heliotropic plants bend Figs. 18 and 19. — Positive heliotropism of the polyps of Eudendrium. The new polyp-bearing stems all grow in the direction of the rays of light which is indicated by an arrow in each figure. (From nature.) These animals bend in the same way to the light as the stems of positively heliotropic plants kept under similar conditions.

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 Fig. 20. — Positive heliotropism of a marine worm (Spirographis). (From nature.) The light fell into the aquarium from one side only and the worms all bent their heads toward the source of light, as the stems of positively heliotropic plants would do under the same conditions.

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 mth the Bunsen-Roscoe law. As long as the intensity of light or the mass of photochemical substances at the surfaces 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 short 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-Roscoe'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 'Svill" 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.

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 \vith ethics : it seems the only conception of life which can lead to an understanding of the source of ethics.