What is eugenics? / by Leonard Darwin.

  • Leonard Darwin
Date:
1932
Catalogue details

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

Credit: What is eugenics? / by Leonard Darwin. Source: Wellcome Collection.

    27/100 page 17
    HEREDITARY QUALITIES 17 If a more scientific proof that men are not all born alike is demanded, it can be obtained by a study of identical twins. The great likeness often existing between twins is not to be explained by a Ukeness between their past sur¬ roundings. This we know to be the case because twins who were separated soon after birth, and who lived very different lives, have often remained extraordinarily alike. The only satisfactory explanation which can be given of the great similarity between identical twins is that they sprang from one and the same germ. Whatever small differences there are between what are called identical twins may, no doubt, be explained by differences in their past surroundings. But ordinary brothers are exposed to nearly the same differences in surroundings as are twins. Brothers who are not twins cannot, therefore, differ from each other much more than do twins because of the effects of their past surroundings. To whatever extent ordinary brothers differ more than twins, to that extent some other explanation has to be found for these extra differences between them. And no explanation can be suggested for these differences except that they are due to the differ¬ ences which existed between the germs from which the brothers sprang. We thus get a rough measure of those differences existing amongst the members of any one family which can be traced back to their originating germs ; differ¬ ences which may be called hereditary differences. Here a point may be mentioned which has puzzled many persons. How is it possible for persons belonging to the same family to show hereditary differences ? If brothers do differ widely, does not this prove that heredity counts for little ? Now the qualities of both a father and a mother are to some extent, as it were, passed on to and then carried by the germ out of which their son originated. But the son will pass on more of his father's qualities and less of his mother's to one grandchild than to another. The grandchildren wiU in consequence differ amongst themselves. And this they wiU do although the whole of their qualities may have been derived without change
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