Page:Popular Science Monthly Volume 48.djvu/694

622 congenitally deaf and the state of the hearing of the wives is unknown produced congenitally deaf children.

Of the twenty-six families in which both parents are deaf and have congenitally deaf children, there are five families in which one of the parents has one deaf parent, seventeen families in which both parents have deaf relatives of the same generation, four in which one parent has deaf relatives of the same generation, and five in which neither parent has deaf relatives of the same generation.

Of the twenty-six families in which both parents are congenitally deaf and have hearing children only, there is none in which either parent has a deaf parent, so far as reported, twelve families in which both parents have deaf relatives of the same generation, eleven families in which one parent has deaf relatives of the same generation, and three families in which neither parent has deaf relatives of the same generation.

This illustration proves that the origin of an individual peculiarity has much to do with the question of its inheritance, and that we can not be sure that statistical data illustrate inheritance unless we can separate phenomena of ancestry from those of nurture.

Furthermore, in order to prove that children always revert to the mean or type of the race, and are on the average more mediocre than their parents, we must prove that this is the case when both parents have the same inherited peculiarity.

Galton shows that this is true of the stature of children both whose parents were tall or both short, but he has not shown that it is true when the peculiarity in the stature of both parents is the same inherited peculiarity. He points out that stature may be affected by diversity in the thickness of more than one hundred bodily parts, and it is plain that if the extra height of a tall father is due to a long femur for example, the chances are a hundred to one that the femur of the tall mother is normal and that her extra height is due to some other peculiarity—thick intervertebral bodies, for example.

There is statistical evidence from other sources to prove that if both parents have long femurs and have brothers and sisters with long femurs, the children, instead of reverting to mediocrity, may on the average be expected to have femurs very much above the mean, and that some of them may have them longer than either parent.

Many facts in our stock of information regarding domestic animals and cultivated plants show that hereditary peculiarities are often very persistent independently of selection, and the experience of all breeders shows that this tendency is greatly intensified when both parents have the same inherited peculiarity.