Illustrations of positivism : a selection of articles from the "Positivist review" in science, philosophy, religion, and politics / by John Henry Bridges.

  • Bridges, John Henry, 1832-1906.
Date:
1915
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Licence: Public Domain Mark

Credit: Illustrations of positivism : a selection of articles from the "Positivist review" in science, philosophy, religion, and politics / by John Henry Bridges. Source: Wellcome Collection.

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    inheritance of characters acquired by each individual during its brief period of existence. Habitual exertion of any muscles will cause these muscles to grow. Lamarck maintained that the increased size was transmitted to descendants, and that here lay a source of evolutionary change. By many students of evolution, notably by Mr. Spencer, such transmission is accepted; by others, and foremost among them by Weismann, it is not merely denied, but asserted to be impossible. The grounds of Weismann’s denial are to be looked for in his Theory of Heredity. The germ-plasm of each fertilized ovum con- sists, in his view, of two distinct portions, one of which, by a series of successive divisions, differentiates into various tissues and organs of which the body of the mature animal or plant is composed, while the other is transmitted unchanged to form the germ-cell of the succeeding generation. While so transmitted, Weismann maintains that it is entirely cut off from all connection with the tissues of the body containing it. Those tissues, therefore, can exercise no influence upon it. Ultimately this germ-plasm is mixed with that of another individual of the same species. In the offspring that ensues varia- tions from the parental forms arise, and from these variations those which make for success in the struggle for existence survive and are perpetuated. This selection of favourable variations is, in Weismann’s opinion, the sole source of organic evolution. On this the first remark to be made is that the foundation of Weismann’s theory—the doctrine, namely, that the germ-plasm of a species is handed on from one generation to another unmodified by the tissues of the body containing it—is a pure supposition, resting on but few facts, and against the probability of which there is much to be urged. The cells of an organism are not so cut off from one another as this theory would imply. “It is becoming more and more clear,” says Mr. Adam Sedgwick, “that the cells composing the tissues of an organism are not isolated units, but that they are connected with one another.”’ ‘The protoplasm of the whole body being continuous, change in the molecular constitution of any part of it would naturally be expected to spread in time through the whole mass.’’* In the second place, it is most important to recognize the fallacy that so often arises in comparing natural selection with the artificial selection practised by the breeder and the florist. These persons fix on any variation whatever that strikes their fancy in an animal or 1 Quoted from Spencer’s Inadequacy of Natural Selection, p. 40. C