INHERITANCE OF DISEASE 369 



cause developmental defects. For example, the offspring of a 

 tubercular mother is likely to be delicate, lack resistance to dis- 

 ease, or may show definite defects, because of faulty nutrition. 

 The toxic and nutritional effects on the offspring are due to its 

 environment. In these cases the environment is the mother's 

 body. The relation is called intramaternal environment in con- 

 trast with the environment of the child after it is born. This 

 is called extramaternal environment. 



The relation of alcoholism to the offspring is not conclusively 

 established. Experiments are constantly being conducted to 

 determine whether alcohol may modify the germ cells or not. It 

 is a fact that children of drunkards are frequently defective. This 

 defectiveness may be (1) due to the fact that alcoholism is merely 

 a symptom of a degenerate stock. In this case the children will 

 be defective, not because their parents drank, but because their 

 parents were defective. The parents' drinking is merely one of 

 the symptoms of their defectiveness. (2) It may be that alcohol 

 directly poisons the germ plasm. In this case parents of sound 

 stock, who become addicted to alcoholism, will have defective off- 

 spring. (3) It may be that the intemperate parents do not take 

 adequate care of their children and this leads to the defects of the 

 children. Whether the tendency for drinking alcoholic liquors 

 is inherited, has not yet been definitely proved. 



Environment versus Heredity. One of the most interesting 

 studies that has been conducted upon the relation of environment 

 to heredity in the life of a child is the investigation of identical 

 twins. Twins are thought to originate in the following way. When 

 a fertilized egg divides into two cells, some mechanical or chemical 

 difficulty may separate one cell from the other. Then each cell 

 develops separately. The development of the two cells is exactly 

 alike in every particular because each cell resulted from the 

 division of the original cell. Such a development results in twins 

 identical in sex, height, coloring, and almost every other particular. 



