Darwin seems to have thought that the range of variation shown by the offspring of a given individual about that type of individual would be as wide as the range shown by the original population (fig. 79), but Galton's work has made it clear that this is not the case in a general or mixed population. If the offspring of individuals continued to show, as Darwin seems to have thought, as wide a range on each side of their parents' size, so to speak, as did the original population, then it would follow that selection could slide successive generations along in the direction of selection. [Illustration: FIG. 80. Diagram illustrating the results of selection for extra bristles in D. ampelophila. Selection at first produces decided effects which soon slow down and then cease. (MacDowell.)] Darwin himself was extraordinarily careful, however, in the statements he made in this connection and it is rather by implication than by actual reference that one can ascribe this meaning to his views. His contemporaries and many of his followers, however, appear to have accepted this _sliding scale_ interpretation as the cardinal doctrine of evolution. If this is doubted or my statement is challenged then one must explain why de Vries' mutation theory met with so little enthusiasm amongst the older group of zoölogists and botanists; and one must explain why Johannsen's splendid work met with such bitter opposition from the English school--the biometricians--who amongst the post-Darwinian school are assumed to be the lineal descendants of Darwin. And in this connection we should not forget that just this sort of process was supposed to take place in the inheritance of use and disuse. What is gained in one generation forms the basis for further gains in the next generation. Now, Darwin not only believed that acquired characters are inherited but turned more and more to this explanation in his later writings. Let us, however, not make too much of the matter; for it is much less important to find out whether Darwin's ideas were vague, than it is to make sure that our own ideas are clear. If I have made several statements here that appear dogmatic let me now attempt to justify them, or at least give the evidence which seems to me to make them probable.