232 SECRETS OF ANIMAL LIFE 
mak; and more progress has been made with the 
science of heredity since the century began than in 
all previous years. For this we have to thank numer- 
ous investigators—among whom Bateson stands pre- 
eminent. What are the central ideals of Mendel- 
ism? How far does it apply? What practical 
promise does it offer? 
There are three fundamental ideas in the Men- 
delian conception of inheritance: 1. The first is the 
idea of “ unit characters.’’” An inheritance is, in part 
at least, built up of numerous more or less clear-cut, 
crisply defined, non-blending characters, which are 
continued in some of the descendants as discrete 
wholes, neither merging nor dividing. If a man 
has his fingers all thumbs, 7.¢., with two joints 
instead of three, this unit character of “ brachy- 
dactylism” is sure to be continued in a certain 
proportion of his descendants. Night-blindness, or 
the inability to see in faint light, has been traced 
through a lineage since near the beginning of the 
seventeenth century. A definite type of very 
intelligent dwarf has been known to reappear for 
four or five generations. The persistence of the 
Hapsburg lip is a familiar instance of the way in 
which a unit character comes to stay. These unit 
characters behave as if they were discrete entities 
which can be shuffled about and distributed to the 
offspring to some degree independently of one an- 
other. Some suppose that they are represented by 
specific particles in the germinal material; others 
would not go further than saying that they are 
