LECTURE VII— Part II 



It may be well to remind those who are not familiar with statis- 

 tical 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 im- 

 properly aimed when it is not, and we cannot 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 priced horses are much 

 more likely than horses selected at random to bring the same 

 high prices. The 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. 



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