Imagini ale paginilor
PDF
ePub

practice, habit, and the ever-increasing use of an organ, to bring it to a degree of perfection which we should at the beginning have considered to be impossible. If we compare the uncivilized savages with civilized nations, we find among the former a development of the organs of sense sight, smell, and hearing-such as civilized nations can hardly conceive of. On the other hand, the brain, that is, mental activity, among more civilized nations is developed to a degree of which the wild savages have no idea.

There appears indeed to be a limit given to the adaptability of every organism, by the "type" of its tribe or phylum; that is, by the essential fundamental qualities of this tribe, which have been inherited from a common ancestor, and transmitted by conservative inheritance to all its descendants. Thus, for example, no vertebrate animal can acquire the ventral nerve-chord of articulate animals, instead of the characteristic spinal marrow of the vertebrate animals. However, within this hereditary primary form, within this inalienable type, the degree of adaptability is unlimited. The elasticity and fluidity of the organic form manifests itself, within the type, freely in all directions, and to an unlimited extent. But there are some animals, as, for example, the parasitically degenerate crabs and worms, which seem to pass even the limit of type, and have forfeited all the essential characteristics of their tribe by an astonishing degree of degeneration.

As to the adaptability of man, it is, as in all other animals, also unlimited, and since it is manifested in him above all other animals, in the modifications of the brain, there can be absolutely no limit to the knowledge which man in a further progress of mental cultivation may not be

able to attain to. Even the human mind, according to the law of unlimited adaptation, enjoys an infinite perspective of becoming ever more and more perfect. It is this consideration which proves the worthlessness of the muchtalked-of "Ignorabimus Speech," which Du Bois Reymond, the Berlin physiologist, in 1873, most unjustifiably directed against the advance of science in his discourse "On the Limits to our Knowledge of Nature" (" Über die Grenzen des Naturkennens "). I have entered my protest against this infamous "Ignorabimus Speech"-which clerical obscurantism has made its watchword-in the preface to my "Anthropogeny" (1874), and again in my treatise on "Freedom in Science and Teaching" ("Freie Wissenschaft und freie Lehre ").

These remarks are sufficient to show the extent of the phenomena of Adaptation, and the great importance to be attached to them. The laws of Adaptation, or the facts of Variation, are just as important as the laws of Inheritance. All phenomena of Adaptation can, in the end, be traced to conditions of nutrition of the organism, in the same way as the phenomena of Inheritance are referable to conditions of reproduction; but the latter, as well as the former, may further be traced to chemical and physical, that is, to mechanical causes. According to Darwin's Theory of Selection, the new forms of organisms, the transformations which artificial selection produces in the state of cultivation, and which natural selection produces in the state of nature, arise solely by the interaction of such causes.

CHAPTER XI.

NATURAL SELECTION BY THE STRUGGLE FOR EXISTCELLULAR SELECTION AND

ENCE.
SELECTION.

PERSONAL

Interaction of the Two Organic Formative Causes, Inheritance and Adaptation.-Natural and Artificial Selection.-Struggle for Existence, or Competition for the Necessaries of Life.-Disproportion between the Number of Possible or Potential, and the Number of Real or Actual Individuals. Complicated Correlations of all Neighbouring Organisms. -Mode of Action in Natural Selection.-Homochromic Selection as the Cause of Sympathetic Colourings.-Sexual Selection as the Cause of the Secondary Sexual Characters.-The Struggle of Parts in the Organism (Roux).—Functional'Self-Formation of Suitable Structures. -Teleological Mechanism.-Cellular Selection (Protista) and Personal Selection (Histona).-Selection of the Cells and of the Tissues.The Principle of Selection in Empedocles.-Mechanical Origin of what is Suitable for a Purpose from what is Unsuitable.-Philosophical Range of Darwinism.

In order to arrive at a right understanding of Darwinism, it is, above all, necessary that the two organic functions of Inheritance and Adaptation, which we examined in our last chapters, should be more closely examined. If we do not, on the one hand, examine the purely mechanical nature of these two physiological activities, and the various action of their different laws, and if, on the other hand, we do not consider how complicated the interaction of these

VOL I.

T

different laws of Inheritance and Adaptation must be, we shall not be able to understand how these two functions, by themselves, have been able to produce all the variety of animal and vegetable forms, which, in fact, they have. We have, at least, hitherto been unable to discover any other formative causes besides these two, and if we rightly understand the necessary and infinitely complicated interaction of Inheritance and Adaptation, we do not require to look for other unknown causes for the change of organic forms. These two fundamental causes are, as far as we can see, completely sufficient.

Even long before Darwin had published his Theory of Selection, some naturalists, and especially Goethe, had assumed the interaction of two distinct formative tendencies -a conservative or preserving, and a progressive or changing formative tendency—as the causes of the variety of organic forms. The former was called by Goethe the centripetal or specifying tendency, the latter the centrifugal tendency, or the tendency to metamorphosis. These two tendencies completely correspond with the two processes of Inheritance and Adaptation. Inheritance is the centripetal or internal formative tendency which strives to keep the organic form in its species, to form the descendants like the parents, and always to produce identical things from generation to generation. Adaptation, on the other hand, which counteracts inheritance, is the centrifugal or external formative tendency, which constantly strives to change the organic forms through the influence of the varying agencies of the outer world, to create new forms out of those existing, and entirely to destroy the constancy or permanency of species. Accordingly as Inheritance or Adaptation pre

dominates in the struggle, the specific form either remains constant or changes into a new species. The degree of constancy of form in the different species of animals and plants, which obtains at any moment, is simply the necessary result of the momentary predominance which either of these two formative powers (or physiological activities) has acquired over the other.

If we now return to the consideration of the process of selection or choice, the outlines of which we have already examined, we shall be in a position to see clearly and distinctly that both artificial and natural selection rest solely upon the interaction of these two formative tendencies. If we carefully watch the proceedings of an artificial selectora farmer or a gardener-we find that only these two constructive forces are used by him for the production of new forms. The whole art of artificial selection rests solely upon a thoughtful and wise application of the laws of Inheritance and Adaptation, and upon their being applied and regulated in a skilful and systematic manner. Here the will of man constitutes the selecting force.

The case of natural selection is quite similar, for it also employs merely these two organic constructive forces, the physiological functions of Adaptation and Heredity, in order to produce the different species. But the selecting principle or force, which in artificial selection is represented by the conscious will of man acting for a definite purpose, consists in natural selection of the unconscious struggle for existence acting without a definite plan. What we mean by "struggle for existence" has already been explained in the seventh chapter. It is the recognition of its exceeding importance which constitutes one of the greatest of Darwin's

« ÎnapoiContinuă »