Outlines of zoology / by J. Arthur Thomson.

Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of zoology / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    109/748 (page 83)
    ator of biologist and physicist alike is as inexpressibly marvellous as the philosopher's greatest common measure. There are at least two great problems in evolution :— (A) How do changes or variations arise ? (B) How do these variations become new, well- adapted, progressive species ? A. The Primary Factors in Evolution—YtYach will account for the origin of variations. It is well to recognise that this is the fundamental prob- lem. Unless we can give some theory of the origin of variations, we have no material for further construction. Unfortunately we are very ignorant about the whole matter. But, in the first place, it is quite certain that the environ- ment in the widest sense must be the primary cause of changes in animals. So far almost all naturalists are agreed. Thus Herbert Spencer says, The action of the environment is the primordial factor in organic evolution, and Weismann traces the origin of variations in the long run to the action of the environment upon the simplest organ- isms. By the environment we mean all external influences —mechanical, chemical, physical ; pressure, moisture, aeration, heat, light, and also food. [It is indeed possible that there are environmental influences of which our senses are unaware,—the spiritual influxes of which Wallace speaks, but these are obviously beyond the scope of scientific analysis.] * That all variations are ultimately due to the action of the environment in the widest sense, we regard as an axiom. But in regard to the direct conditions of organic change, naturalists are or ought to be very uncertain. I shall state the possibilities. * It .seems to me desiralile to avoid talking about liie principles of evolution, e.g., as variability, heredity, and natural selection, or as variability, heredity, isolation. Not only are words like princi])lc used in the loosest possible way, but they suggest a conclusiveness which certainly docs not exist, and they are apt to become intellectual fetishes. Variability is only a term for the tendency organisms have to vary, the conditions of variation being numerous and obscure. Similarly, heredity is only a term for the relation of organic continuity between successive generations.