Heredity : a study with special reference to disease / by R.A. Douglas Lithgow.

  • Robert Alexander Douglas-Lithgow
Date:
1889
Catalogue details

Licence: Public Domain Mark

Credit: Heredity : a study with special reference to disease / by R.A. Douglas Lithgow. Source: Wellcome Collection.

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    like is not only modified by the influence of external cir cumstances, but to a still greater extent by that tendency to variation which is manifested in all animals, and which Darwin regarded as the greatest argument in favour of his theory of evolution. There are in fact no two beings pre cisely alike; brothers and sisters present many differences; and even were they formed according to the undisturbed force of hereditary transmission, endless diversifications would result from inevitable diversities of proportion of transmission, parental, as well as ancestral. The operation of hereditary tendencies is perpetually disturbed by innu merable circumstances too subtle for us to appreciate, and yet capable of producing varieties infinite alike in extent and degree. How these spontaneous differences occur we cannot tell; but that individual varieties are transmissible, and capable of producing new races in domesticated animals by artificial selection is indisputable; and in the human family the tendency to spontaneous variation is even still greater. I now propose to view heredity in its physiological, psy chological, and pathological aspects, and in the space at my disposal to give at least some general idea of this most important biological law. There are few natural phenomena more interesting and marvellous than those affecting inherited peculiarities of feature, figure, character, constitution, habits, etc., and their perpetuation, and the numerous facts of hereditary trans mission are all the more wonderful when we contrast them with other and better-known phenomena, which experience and scientific progress have enabled us to group under cer tain definite laws. The laws of planetary motion are recog nised, and to a great extent understood; the systematic
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