144 THE BEHAVIOR OF UNIT 



The particular topic which I have heen asked 

 to discuss is the behavior of unit characters in 

 heredity. 



The subject of heredity units is one to which 

 Darwin gave much thought. With characteristic 

 thoroughness and patience he assembled the facts 

 of inheritance, reversion, bud variation, regenera- 

 tion, and related subjects, which in his opinion 

 had a common underlying cause, and with delib- 

 eration framed a tentative hypothesis to explain 

 them. This hypothesis, which he called pangen- 

 esis, was itself short-lived, but has left a numer- 

 ous progeny. The most important are the idio- 

 plasm theories of Weismann and Nageli, and the 

 theory of intracellular pangenesis of De Vries. 

 Darwin's hypothesis was useful because it set 

 people to thinking, observing, and experiment- 

 ing. The theories of Weismann, Nageli, and 

 De Vries were attempts to bring Darwin's fun- 

 damental idea into harmony with facts subse- 

 quently discovered. All these theories were 

 scaffolding, not masonry. 



MENDEL'S LAW 



2- 



A conception of unit characters fundamen- 

 tally different from Darwin's, one which 

 antedates slightly the pangenesis theory, but 

 which suffered total eclipse by it, is to-day 

 known as Mendel's law. It accords so fully 

 with a variety of biological facts discovered 



