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JAMA 100 Years Ago
September 24, 2008

THE MNEMIC THEORY OF DEVELOPMENT.

JAMA. 2008;300(12):1471. doi:10.1001/jama.300.12.1471-a

President Darwin’s1 inaugural address at the meeting of the British Association for the Advancement of Science, September 2, with its avowed adherence to the anti-Weismann doctrine of the inheritance of acquired characters, must have come as a shock to many of his hearers to whom this view had been only an exploded scientific heresy. It is evidence, however, that it will not down, and Darwin's adherence to it will be welcomed by a respectable quota of biologists, medical men and pathologists. If the study of plants has led Darwin to the belief in an inherited organic memory of the cells as an explanation of heredity and development, the idea ought, it would seem, to fit still better in the animal kingdom, where we are accustomed to expect psychic activities such as memory, even in their most rudimentary form, more than in plants. According to this mnemic hypothesis of Semon, as adopted by Darwin, somatic inheritance lies at the root of all evolution and involves the acceptance of the inheritance of acquired characters. He does not find that the idea of a special germ plasm, independent of the body cells generally, with its hypothetic determinants and biophores, accounts for the facts of ontogeny, that is, “the capacity of the ovum of developing into a more or less predetermined form”; it does not explain its automatic character. Development is to him an actual instance of habit. This mnemic theory, he holds, “makes the positive action of natural selection more obvious; if evolution is a process of drilling organisms into habits, the elimination of those that can not learn is an integral part of the process.” The only basis for acceptance of any theory is that it best explains facts, and up to this time Weismann's theory has been so accepted by many. Darwin's attack on it will undoubtedly excite general comment and reopen the question with some who had considered it settled. Of those who have considered it settled there will perhaps be fewer among physicians than among biologists, for the clinical evidence of the inheritance of acquired characters is not always so easily disposed of as some writers on this subject have claimed.

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