Heredity / by J. Arthur Thomson.

  • John Arthur Thomson
Date:
1920
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Heredity / by J. Arthur Thomson. Source: Wellcome Collection.

    33/692 page 13
    A QUESTION OF WORDS the secondary or directive factors which operate upon the raw materials of progress which variabihty supphes—notably Selection and Isolation—is not relevant at present. § 4. Л Question of Words In every discussion with a serious purpose it is important that there should be clearness as to the terms used. We must, therefore, ask the reader to notice our definition of the chief terms. Thus by heredity we do not mean the general fact of observation that like tends to beget like, nor a power making for continuity or persistence of characters—to be opposed to the power of varying—nor anything but the organic or genetic relation between successive generations ; and by inheritance we mean organic inheritance —all that the organism is or has to start with in virtue of its hereditary relation to parents and ancestors. We do not forget that for man in particular there is an external heritage—a social inheritance—which counts for much. By innate or inborn we mean all that is potentially implied in the fertilised egg-cell ; by the expression of the inheritance we mean the realisation of inborn potentialities in the course of development in an appropriate environment ; by a congenital character {pace many medical writers) we mean one demonstrable at birth, which is not necessarily germinal, being often due to peculiarities—e.g. infection or poisoning or mechanical injury during pre-natal development. Thus, tubercle may be con¬ genital, but it is never inherited. By modifications or acquired characters we mean structural changes in the body induced by changes in the environment or in the function, and such that they transcend the limit of organic elasticity, and therefore persist after the inducing conditions have ceased to operate. By a variation we mean not any observed difference between offspring and parent, between an individual and the mean of
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