Correlation and Application of Statistics to Problems of Heredity 61
Galton in his Chapter III deals with the theory of Organic Stability, illustrating it by the model of a polygonal slab, which has positions of stable equilibrium with various degrees of stability, i.e, which may require large or only small displacements to pass from one position of equilibrium to a second. He considers that his model (see Fig. 12) shows how the following conditions
A B C B Fig. 12.
may co-exist: (1) Variability within narrow limits without prejudice to the purity of the breed (i); (2) Partly stable sub-types (ii); (3) Tendency, when much disturbed, to revert from a sub-type to an earlier form; (4) Occasional sports which may give rise to new types (iii) (pp. 27-30). Again the whole argument is one of analogy, and the reader may be pardoned a little vexation when he finds such important topics as the Stability of Sports and Infertility of Mixed Types only discussed (pp. 30-32) by reference to the analogy of hansom cabs and the impossibility of their useful blend with four-wheelers * !
The fact, I think, is that Galton's own ideas at this time were obscured by his belief that the ancestors actually did contribute to the heritage ; he regarded the incipient structure of the new being to be the result of a clash of elements contributed from many ancestral sources, and the resulting building up out of more or less opposing elements of a particulate individual inheritance as the result of chance j'. A further source of difficulty to Galton in his interpretation of hereditary phenomena lay in his mistake as to the nature of regression. This forced on him the conception of positions of stable equilibrium, each with its own centre of regression, and led him to the view that evolution must generally proceed by sports, and not by minute steps. It is true that on p. 32 he draws a distinction between the two views that the steps may be small and that they must be small, but as he has elsewhere applied his view of regression to indicate that small steps cannot be the source of evolution, the distinction is not really much of a concession (see our pp. 31-2). The following words of Galton deserve, however, to be quoted not only
* I find my copy of the Natural Inheritance, read and annotated forty years ago, defaced by many marginal notes expressing anger at Galton's analogies in this Chapter. But these notes were written before I had read and grasped the value of much of the later work in the book.
t Of course the Mendelian appeals to the same doctrine of chance to explain the variation in the members of an individual brood or litter, but be does so on the basis of homogeneous germ cells having a heterogeneous factor formula. I am inclined to believe that the germ cells of the same individual are not always and absolutely homogeneous, at any rate in the higher organisms, and ' that the clash of elements to be determined by chance need not lie in the factors of the formulae of the gametes, but in the fertilising germ cells themselves.