Page:Popular Science Monthly Volume 48.djvu/689

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T may be well to remind those who are not familiar with statistical reasoning that a type may exhibit the influence of inheritance and yet be of no value as a basis for generalization on inheritance.

The bullet type shows the influence of aim, but if we use it to test the accuracy of aim or the excellence of the rifle we may be led astray if some other influence, such as the weight of the bullet, act on all or on a majority of the shots and escape detection. In this case the type may seem to prove that the rifle is inaccurate or improperly sighted when it is not, and we can not assume that because a type shows the influence of aim it is a test of aim.

So a characteristic or a group of characteristics of living things may conform to the mathematical law of deviation from a mean, and may thus form a type, and this type may show the influence of inheritance, without being a safe basis for generalization regarding inheritance.

This may be illustrated by an example. If we were to tabulate the prices of all the horses sold within a given period, we should undoubtedly find that they would conform to a type: that there is a mean or average price; that the horses which fetch more than this price are equal in number to those which fetch less, and that the prices group themselves about the mean according to the law of error. If the term be long enough to include several generations, we shall find that inheritance or "blood" has a marked influence on price, and that the children of high or moderate or low priced parents are much more likely than horses selected at random to bring the same price. This type will exhibit the influence of inheritance, but it will be of no value in studying inheritance unless we can in some way separate the influence of blood from the influence of supply and demand which has far more to do with the average price and with the type.

That the price of horses is, on the whole, determined like that of other commodities is obvious, and it is also obvious that the type may be changed by events which have no relation to inheritance, such as the application of electricity to street cars.

A change of this sort, such as took place when steam replaced stage coaches, is a "sport" or sudden and fundamental change of type, but this may also be changed by slight and gradual