The principles of heredity with some applications / by G. Archdall Reid.

  • Reid, G. Archdall (George Archdall O'Brien), Sir, 1860-1929.
Date:
1906
Catalogue details

Licence: In copyright

Credit: The principles of heredity with some applications / by G. Archdall Reid. Source: Wellcome Collection.

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    sumably the phenomena of heredity and evolution are alike in man and in other species. The Mendelian-mutation theory, therefore, is obviously erroneous. There is no escape from the conclusion that it is founded on an amazingly restricted survey of the facts;—unless indeed we adopt the hypothesis that human racial differences are not innate, not founded on real evolution, but are merely due to “conditions of the environment, to nutrition, correlation of organs, and the like.” In that case we must sup- pose that the mulatto, for example, appears to be a blend of his parents, not because he really blends their innate qualities, but because his environment is a blend of the African and the Euro- pean. In other words, we must suppose that all human racial differences are mere acquirements. If then a mutation is not a character, large or small, the inheritance of which is Mendelian, what is it ? Presumably it is a considerable change in a single character—a change which separates it more or less widely from the ordinary type of that character (Bateson),—or it is a simultaneous change, large or small, in a considerable number of characters (De Vries). Which- ever view we adopt, the difficulties in the way of a mutation theory of evolution seem insuperable. It must be remembered that every species under pain of extinction must be closely adapted to its environment, that all its ancestral types must always have been so adapted, and that evolution is nothing other than a pro- cess of continuous adaptation to a changing environment. The exceedingly delicate co-adaptation of all the parts of the organism to one another, especially in the case of the higher animals, must also be borne in mind. This co-adaptation is a principal “object,” a principal result, of selection ; for by means of it organisms are fitted to their environments. Lastly, it must be borne in mind that selection is, not only a cause of evolution, but also a cause of specific stability.1 In the absence of selection species tend to change rapidly. Mutations are admittedly rare, and even so are seldom adaptive—so rare and so seldom adaptive that the student of his own species, which unfortunately not every biologist is, sees few mutations and never one that is adaptive. Imagine the effect of a large mutation (e. g. tumour) on the exquisitely co-ordinated animal economy, especially if it be internally situated ! Imagine the effect of a considerable number of smaller mutations ! In either case the chances against increased adaptation of the whole to the environment and of the parts to one another are as a million to one. But suppose the seemingly impossible happened, and a variation of one or other type did prove to be favourable, the question of its persistence would then arise. To persist the mutat ion, since Mendelism is excluded, would have to be, not only favourable, but so prepotent in a Darwinian sense as to be able to maintain itself against all