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.

    32/692 page 12
    12 HEREDITY AND INHERITANCE being transmissible, among multicellular organisms reproducing sexually, is extremely doubtful. Evolution.—Briefly and concretely stated, the general doc¬ trine of organic evolution suggests, as we all know, that the plants and animals now around us are the results of natural processes of growth and change working throughout unthinkably long ages ; that the forms we see are the lineal descendants of ancestors on the whole somewhat simpler ; that these are de¬ scended from yet simpler forms, and so on, backwards, till we lose our clue in the unknown, but doubtless momentous, vital events of pre-Cambrian ages, or, in other words, in the thick mist of life's beginnings. The essentially simple idea is that the present is the child of the past, and the parent of the future. It is a way of looking at organic history, a genetic description, a modal formulation. A process of Becoming leads to a new phase of Being ; the study of evolution is a study of Werden und Vergehen und W eiter-wer den. But we have to pass from a modal interpretation to a causal one. We have to try to discover the factors in the age-long process, and this leads us into a region where at present uncer¬ tainties abound. As biologists we start with the postulate of simple living organisms—feeding, working, growing, wasting, reproducing in an appropriate environment. And we try to discover the possible factors in the long evolution-process, the outcome of which is the present-day world of life. Amid all the uncertainties, this is certain, that the fundamental condition of evolution is that genetic relation which we call heredity,—a relation such that it admits, on the one hand, of a continuity of hereditary resemblance from generation to generation ; and, on the other hand, of an organic changefulness which we call variability. Without the hereditary relation there could have been no succession of generations at all. Without hereditary resemblance on the one hand, and hereditary variation on the other, there could have been no evolution. Any discussion oí
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