agencies which cause "sports" are the real causes of the mutability of species, and natural selection can do nothing more than to exterminate disadvantageous sports, and thus favor advantageous ones.
The "organic stability" to which so much is attributed is held to be due to the fact that the child inherits in part from its parents, in part from more remote ancestors; and since the sum of its ancestry, or its "mid-parentage," is on the average nearer than any exceptional parents to the mean of the race, the children of selected parents are on the average more mediocre than their parents.
It is quite possible that Galton's data may be valuable and trustworthy, and that they may yet fail to prove this generalization; and I shall try to show that this is the case, although I am not sure that I fully grasp his point of view.
I assume that he regards a zoölogical type or species as something; something which is due to a "principle of stability," which is not the result of selection. This is assuredly the current interpretation of his statements, and it is from this standpoint that I shall examine his writings.
If this is not his opinion; if he really believes that this "principle" owes its existence to past selection; if he only deduces from his data the generalization that the results of past selection may persist after it has ceased to act; I see no ground for criticism, for his data assuredly prove this much, although I can not reconcile his statement that "the principle of stability is independent of selection" with the belief that it is the result of past selection.
Before entering upon the discussion of the subject it may be well to ask what evidence there is that the child does inherit from any ancestor except its parents, for descent from a long line of ancestors is not necessarily equivalent to inheritance from them, and it is quite possible that the conception of a "mid-parent" may be nothing but a logical abstraction.
Most of its support is derived from the phenomena of reversion or atavism; from the appearance in children of ancestral features which were not exhibited by the parents. While these phenomena are familiar and real, we may well doubt whether any of them are reversions in Galton's sense.
In some cases we can prove that a so-called reversion is simply the manifestation of a feature which is latent in the structure of all the normal individuals of the species. The occurrence of a distinct premaxillary bone in man is an example of this sort of reversion. It is the outcome of the arrest of normal development, and this arrest might have happened to any member of the species. We do not know what causes the
