532 
BIOLOGY AND HUMAN PROGRESS 
this valuable industry and it was discovered that the Euro¬ 
pean grapevine could be grafted on to the root of any one 
of several of the American grapes. When these vines were 
thus grafted, they were found to possess a resistance to these 
four diseases with the result that the vineyards of Europe 
were saved. 
This success was due to scientific study which discovered 
the difference between the European and American grapes 
in regard to their power to resist disease and the fact that the 
quality of the European grape would not be altered by grafting 
it on the roots of the American varieties. Here we have an 
illustration of variation that is not confined to the shape and 
size of the parts but to their resistance to disease. This may 
be described as an acquired immunity that is transmitted 
by heredity from one generation of grapes to another. The 
word immunity used in this last sentence was explained on 
page 516. If we explain the scientific meaning of heredity, 
this brilliant discovery in regard to grapes will mean more to 
you. 
406. Heredity. — An easy way to understand the technical 
meaning of this term which we use so frequently is to make 
a study of your hand. Normally we all have four fingers 
and one thumb but our hands are quite unlike. This is 
readily seen by comparing the three hands in Figure 439. The 
large one is the hand of father, the middle one of daughter, 
and the right one of mother. The first and the last fingers of 
the father’s hand are about the same length, while the little 
finger of the mother is much shorter than her first finger. 
This difference in the length of these two fingers is repeated 
in the hand of the daughter. We say that the daughter in¬ 
herited from her mother a short little finger and a long fore¬ 
finger. 
Now if we study the position of the thumb in its relation 
to the palm of the hand, it is seen that the distance between 
the thumb and the base of the forefinger is proportionately 
