84 THE FIRST PRINCIPLES OF HEREDITY 



group in the expression of the image of the grandmother 

 herself. In short, in grandmother, father, and grandchild 

 there must take place each time exclusive inheritance ; the 

 image must be expressed by the Idants derived from one 

 parent only, otherwise the Idants added from the other 

 parent would modify the image and prevent complete re- 

 semblance. While such complete resemblance is very un- 

 likely, and also in reality very rare, there being as a rule only 

 more or less of a family likeness, resemblance in a certain set 

 of characters is more easily possible, as a majority of Ids of 

 the grandfather may be carried forward to the grandchild, 

 and thus originate the same set of characteristics once 

 more. 



a h c d e f g h y. i h I m n p q ist generation. 



ah c d X i k I m ci h c f x I n o p 2nd generation. 



ah C I , . . . 3rd generation. 



Fig. 44. — Inheritance from Aunt. (After Weismann.) 



In a similar way one could explain resemblance to an uncle 

 or an aunt. In Fig. 44 each individual has eight Idants, 

 the thick letters representing the four predominant Idants, 

 which in each case determine the image of the individual. 



In the first line we have the grandfather paired with 

 grandmother ; in the second line two daughters (second 

 generation). One of the daughters has a child (third 

 generation), which, as will be seen from the diagram, has 

 the same predominant Idants as its mother's sister, and 

 therefore resembles her. 



[h) Reversion to Distant Ancestors. 



We speak of Reversion in its proper sense when an 

 individual belonging to a recent race harks back in certain 

 characteristics to an ancient race, which lies in the line of 



