112 HEREDITY [ch. 



generation the machinery for segregation is inade- 

 quate. Extreme cases of this are possibly the cause of 

 the frequent sterility of hybrids, but it may be that 

 when the parental differences areinsufficient to prevent 

 the formation of fertile germ-cells, they may yet be 

 enough to interfere with normal Mendelian segregation. 

 Certain aspects of inheritance in mankind have 

 now been reviewed, and it remains briefly to indicate 

 the lines on which our knowledge may be of practical 

 importance. One of the things which is especially 

 prominent when the evidence is considered as a 

 whole is the exceeding definiteness or determinancy 

 of the process of heredity. Given parents of certain 

 constitution, it can be said with confidence that on 

 the average a certain proportion of their offspring 

 will have such and such characters. It matters not 

 whether the character considered is regarded from 

 the standpoint of the Biometrician or the Mendelian, 

 both agree that what is present in the germ-cell will 

 be present in the individual, and that external con- 

 ditions as a rule play but a small part in determining 

 its appearance. The Biometrician finds an average 

 value for the intensity of inheritance, and shows that 

 it is sensibly the same whether the character con- 

 sidered is stature, eye-colour, ability, or tendency to 

 congenital disease. When the character in question 

 is a simple case of presence or absence, the Men- 

 delian finds that it is present in a definite proportion 

 of the children of affected parents, so that he can say 



