124 The Third and Fourth Generation 



four; such a condition is known as poly dactyl- 

 ism. Sometimes a case is recorded in which a 

 person has fingers with two joints instead of 

 three and a thumb with one joint in place of 

 two (brachydactylism). Such human abnor- 

 malities are inherited. There is given on the 

 opposite page a chart (Fig. 8) of a family 

 tree in which brachydactylism is very common; 

 it is based on a study made by Drinkwater. 

 Males in the chart are represented by $, 

 females by 2, ma tings by = . The circles are 

 of solid color # in individuals affected with the 

 deformity, open O in normal individuals. The 

 character seems to behave like a Mendelian 

 dominant, though one could make no very posi- 

 tive assertion on this point from so few indi- 

 viduals. But it is very evident that such a 

 physical character once in the stock is trans- 

 mitted generation after generation, reappearing 

 continually in the offspring. 



There is presented on p. 127 a chart (Fig. 9) 

 of the transmission of cataract. This disease is 

 characterized by the appearance of an opaque 

 area in the usually transparent parts of the 



