Page:Popular Science Monthly Volume 48.djvu/525

Rh arrest, but the view that it is due to some adverse circumstance which has kept the individual from completing its development is much more simple and probable than the view that the child inherits its distinct premaxilla from any ancestor except its parents.

When the son of a beardless boy grows up and acquires a beard, we may be permitted to say that he has inherited his grandfather's beard; but this is only a figure of speech, and he actually inherits the beard which his father might have acquired had he lived; nor would the case of a child descended from a series of ten or a hundred beardless boys and beardless women be any different. If we were to propagate a plant by cuttings, for ten or a hundred generations, under conditions which did not permit it to flower, and were finally to put the last of the series where it did flower, we should not be justified in saying that it did not inherit its flower from the preceding cutting; nor would the case be any different if, for some reason, this preceding cutting could not be made to bloom.

The phenomena of polymorphism in insects and in hydroids present illustrations of the normal inheritance of latent characters, but we find in them no ground for the assertion that the ancestral characteristics of the medusa are not inherited from the hydroid which produces it.

The sum of the visible features of the parent, plus the sum of its latent potencies, may be called a "mid-parent" for statistical purposes, if we see fit, but there is no evidence that this midparent is anything else than.the actual parent.

With this introductory note, we may now enter upon the study of Galton's works, the central point of which is as follows:

If we select any one characteristic of a natural group of animals—such a characteristic as the weight of the individuals, or the ratio between the lengths of their arms and legs, or anything else which admits of exact numerical statement—it will be found that while no two members of the group are exactly alike, they nevertheless conform to a type, and show the existence of a standard, the mean or average, to which the majority adhere pretty closely, while other members of the group are more abnormal, and show marked deviation from the mean.

If the cases tabulated are numerous enough, the individuals will conform, so far as this quality is concerned, to what is known in statistical science as the law of frequency of error. This agreement will be so close, when great numbers of instances are examined, that the number of individuals which depart from the mean to any specified degree may be computed mathematically.