Читать книгу The Organism as a Whole, from a Physicochemical Viewpoint - Jacques Loeb - Страница 5

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1. The physical researches of the last ten years have put the atomistic theory of matter and electricity on a definite and in all probability permanent basis. We know the exact number of molecules in a given mass of any substance whose molecular weight is known to us, and we know the exact charge of a single electron. This permits us to state as the ultimate aim of the physical sciences the visualiza­tion of all phenomena in terms of groupings and displacements of ultimate particles, and since there is no discontinuity between the matter constituting the living and non-living world the goal of biology can be expressed in the same way.

This idea has more or less consciously prevailed for some time in the explana­tion of the single processes occurring in the animal body or in the explana­tion of the func­tions of the individual organs. Nobody, not even a scientific vitalist, would think of treating the process of diges­tion, metabolism, produc­tion of heat, and electricity or even secre­tion or muscular contrac­tion in any other than a purely chemical or physico­chemical way; nor would anybody think of explaining the func­tions of the eye or the ear from any other standpoint than that of physics.

When the actions of the organism as a whole are concerned, we find a totally different situa­tion. The same physiologists who in the explana­tion of the individual processes would follow the strictly physico­chemical viewpoint and method would consider the reac­tions of the organism as a whole as the expression of non-physical agencies. Thus Claude Bernard,1 who in the investiga­tion of the individual life processes was a strict mechanist, declares that the making of a harmonious organism from the egg cannot be explained on a mechanistic basis but only on the assump­tion of a “directive force.” Bernard assumes, as Bichat and others had done before him, that there are two opposite processes going on in the living organism: (1) the phenomena of vital crea­tion or organizing synthesis; (2) the phenomena of death or organic destruc­tion. It is only the destructive processes which give rise to the physical manifesta­tions by which we judge life, such as respira­tion and circula­tion or the activity of glands, and so on. The work of crea­tion takes place unseen by us in the egg when the embryo or organism is formed. This vital crea­tion occurs always according to a definite plan, and in the opinion of Bernard it is impossible to account for this plan on a purely physico­chemical basis.

There is so to speak a pre-established design of each being and of each organ of such a kind that each phenomenon by itself depends upon the general forces of nature, but when taken in connec­tion with the others it seems directed by some invisible guide on the road it follows and led to the place it occupies. …

We admit that the life phenomena are attached to physico­chemical manifesta­tions, but it is true that the essential is not explained thereby; for no fortuitous coming together of physico­chemical phenomena constructs each organism after a plan and a fixed design (which are foreseen in advance) and arouses the admirable subordina­tion and harmonious agreement of the acts of life. …

We can only know the material conditions and not the intimate nature of life phenomena. We have therefore only to deal with matter and not with the first causes or the vital force derived therefrom. These causes are inaccessible to us, and if we believe anything else we commit an error and become the dupes of metaphors and take figurative language as real. … Determinism can never be but physico­chemical determinism. The vital force and life belong to the metaphysical world.

In other words, Bernard thinks it his task to account for individual life phenomena on a purely physico­chemical basis—but the harmonious character of the organism as a whole is in his opinion not produced by the same forces and he considers it impossible and hopeless to investigate the “design.” This attitude of Bernard would be incomprehensible were it not for the fact that, when he made these statements, the phenomena of specificity, the physi­ology of development and regenera­tion, the Mendelian laws of heredity, the animal tropisms and their bearing on the theory of adapta­tion were unknown.

This explanation of Bernard’s attitude is apparently contradicted by the fact that Driesch2 and v. Uexküll,3 both brilliant biologists, occupy today a standpoint not very different from that of Claude Bernard. Driesch assumes that there is an Aristotelian “entelechy” acting as directing guide in each organism; and v. Uexküll suggests a kind of Platonic “idea” as a peculiar characteristic of life which accounts for the purposeful character of the organism.

v. Uexküll supposes as did Claude Bernard and as does Driesch that in an organism or an egg the ultimate processes are purely physico­chemical. In an egg these processes are guided into definite parts of the future embryo by the Mendelian factors of heredity—the so-called genes. These genes he compares to the foremen for the different types of work to be done in a building. But there must be something that makes of the work of the single genes a harmonious whole, and for this purpose he assumes the existence of “supergenes.”4 v. Uexküll’s ideas concerning the nature of a Mendelian factor and of the “supergenes” are expressed in metaphorical terms and the assump­tion of the “supergenes” begs the ques­tion. The writer is under the impression that this author was led to his views by the belief that the egg is entirely undifferentiated. But the unfertilized egg is not homogeneous, on the contrary, it has a simple but definite physico­chemical structure which suffices to determine the first steps in the differentia­tion of the organism. Of course, if we suppose as do v. Uexküll and Driesch that the egg has no structure, the development of structure becomes a difficult problem—but this is not the real situa­tion.

2. Claude Bernard does not mention the possibility of explaining the harmony or apparent design in the organism on the basis of the theory of evolu­tion, he simply considers the problem as outside of biology. It was probably clear to him as it must be to everyone with an adequate training in physics that natural selec­tion does not explain the origin of varia­tion. Driesch and v. Uexküll consider the Darwinian theory a failure. We may admit that the theory of a forma­tion of new species by the cumulative effect of aimless fluctuating varia­tions is not tenable because fluctuating varia­tion is not hereditary; but this would only demand a slight change in the theory; namely a replacement of the influence of fluctuating varia­tion by that of equally aimless muta­tions. With this slight modifica­tion which is proposed by de Vries,5 Darwin’s theory still serves the purpose of explaining how without any pre-established plan only purposeful and harmonious organisms should have survived. It must be said, however, that any theory of life phenomena must be based on our knowledge of the physico­chemical constitu­tion of living matter, and neither Darwin nor Lamarck was concerned with this. Moreover, we cannot consider any theory of evolu­tion as proved unless it permits us to trans­form at desire one species into another, and this has not yet been accomplished.

It may be of some interest to point out that we do not need to make any definite assump­tion concerning the mechanism of evolu­tion and that we may yet be able to account for the fact that the surviving organisms are to all appearances harmonious. The writer pointed out that of all the 100,000,000 conceivable crosses of teleost fish (many of which are possible) not many more than 10,000, i.e., about one-hundredth of one per cent., are able to live and propagate. Those that live and develop are free from the grosser type of disharmonies, the rest are doomed on account of a gross lack of harmony of the parts. These latter we never see and this gives us the erroneous concep­tion that harmony or “design” is a general character of living matter. If anybody wishes to call the non-viability of 9999/100 per cent. of possible teleosts a process of weeding out by “natural selec­tion” we shall raise no objec­tion, but only wish to point out that our way of explaining the lack of design in living nature would be valid even if there were no theory of evolu­tion or if there had never been any evolu­tion.

3. v. Uexküll is perfectly right in connecting the problem of design in an organism with Mendelian heredity. The work on Mendelian heredity has shown that an extremely large number of independently transmissible Mendelian factors help to shape the individual. It is not yet proven that the organism is nothing but a mosaic of Mendelian factors, but no writer can be blamed for considering such a possibility. If we assume that the organism is nothing but a mosaic of Mendelian characters it is difficult indeed to understand how they can force each other into a harmonious whole6; even if we make ample allowance for the law of chance and the corresponding wastefulness in the world of the living. But it is doubtful whether this idea of the rôle of Mendelian factors is correct. The facts of experi­mental embryology strongly indicate the possibility that the cytoplasm of the egg is the future embryo (in the rough) and that the Mendelian factors only impress the individual (and variety) characters upon this rough block. This idea is supported by the fact that the first development—in the sea urchin to the gastrula stage inclusive—is independent of the nucleus, which is the bearer of the Mendelian factors. Not before the skeleton or mesenchyme is formed in the sea urchin egg is the influence of the nucleus noticeable. This has been shown in the experi­ments of Boveri in which an enucleated fragment of an egg was fertilized with a spermato­zoön of a foreign species. If this is generally true, it is conceivable that the generic and possibly also the species characters of organisms are determined by the cytoplasm of the egg and not by the Mendelian factors.

In any case, we can state today that the cytoplasm contains the rough preforma­tion of the future embryo. This would show then that the idea of the organism being a mosaic of Mendelian characters which have to be put into place by “supergenes” is unnecessary. If the egg is already the embryo in the rough we can imagine the Mendelian factors as giving rise to specific substances which go into the circula­tion and start or accelerate different chemical reac­tions in different parts of the embryo, and thereby call forth the finer details characteristic of the variety and the individual. The idea that the egg is the future embryo is supported by the fact that we can call forth a normal organism from an unfertilized egg by artificial means; while it is apparently impossible to cause the spermato­zoön to develop into an organism outside the egg.

4. The influence of the whole on the parts is nowhere shown more strikingly than in the field of regenera­tion. It is known that pieces cut from the plant or animal may give rise to new growth which in many cases will restore somewhat the original organism. Instead of asking what is the cause of this so-called regenera­tion we may ask, why the same pieces do not regenerate as long as they are parts of the whole. In this form the mysterious influence of the whole over its parts is put into the foreground. We shall see that growth takes place in certain cells when certain substances in the circula­tion can collect there. The mysterious influence of the whole on these parts consists often merely of the fact that the circulating specific or non-specific substances—we cannot yet decide which—will in the whole be attracted by certain spots and that this will prevent them from acting on other parts of the organism. If such parts are isolated the substances can no longer flow away from these parts and the parts will begin to grow. It thus becomes utterly unnecessary to endow such organisms with a “directing force” which has to elaborate the isolated parts into a whole.

5. The same difficulty which we have discussed in regard to morphogenesis exists also in connec­tion with those instincts which preserve the life of the organism and of the race. The reader need only be reminded of all the complicated instincts of mating by which sperm and eggs are brought together; or those by which the young are prevented from starva­tion to realize the apparently desperate problems in store for a mechanist, to whom the assump­tion of design is meaningless. And yet we are better off in regard to our knowledge of the instincts than we are in regard to morphogenesis, as in the former we can show that the apparent instincts in some cases obey simple physico­chemical laws with almost mathematical accuracy. Since the validity of the law of gravita­tion has been proved for the solar system the idea of design in the motion of the planets has lost its usefulness, and this fact must serve us as a guide wherever we attempt to put science beyond the possibility of mysticism. As soon as we can show that a life phenomenon obeys a simple physical law there is no longer any need for assuming the action of non-physical agencies. We shall see that this has been accomplished for one group of animal instincts; namely those which determine the rela­tion of animals to light, since these are being gradually reduced to the law of Bunsen and Roscoe. This law states that the chemical effect of light equals the product of intensity into dura­tion of illumina­tion. Some authors object to the tendency toward reducing everything in biology to mathematical laws or figures; but where would the theory of heredity be without figures? Figures have been responsible for showing that the laws of chance and not of design rule in heredity. Biology will be scientific only to the extent that it succeeds in reducing life phenomena to quantitative laws.

Those familiar with the theories of evolution know the extensive rôle ascribed to the adapta­tions of organisms. The writer in 1889 called atten­tion to the fact that reac­tions to light—e.g., positive helio­tropism—are found in organisms that never by any chance make use of them; and later that a great many organisms show definite instinctive reac­tions towards a galvanic current—galvano­tropism—although no organism has ever had or ever will have a chance to be exposed to such a current except in laboratory experi­ments. This throws a different light upon the seemingly purposeful character of animal reac­tions. Heliotropism depends primarily upon the presence of photo­sensitive substances in the eye or the epidermis of the organism, and these substances are inherited regardless of whether they are useful or not. It is only a metaphor to call reac­tions resulting from the presence of photo­sensitive substances “adapta­tion.” In this book other examples are given which show that authors have too often spoken of adapta­tion to environ­ment where the environ­ment was not responsible for the phenomena. The blindness of cave animals and the resistance of certain marine animals to higher concentra­tions of sea water are such cases. Cuénot speaks of “preadapta­tion” to express this rela­tion. The fact is that the “adapta­tions” often existed before the animal was exposed to surroundings where they were of use. This relieves us also of the necessity of postulating the existence of the inheritance of acquired characters, although it is quite possible that the future may furnish proof that such a mode of inheritance exists.

6. We have mentioned that according to Claude Bernard two groups of phenomena occur in the living organism: (1) the phenomena of vital crea­tion or organizing synthesis (especially in the egg and during development); (2) the phenomena of death or organic destruc­tion. These two processes are briefly discussed in the first and last chapters.

These introductory remarks may perhaps make it easier for the reader to retain the thread of the main ideas in the details of experi­ments and tables given in this book.