146 THE BEHAVIOR OF UNIT 



well-known theories in which the existence of 

 units in heredity was assumed. Such was the 

 fate deservedly of the highly speculative theories 

 of Nageh, and undeservedly of the generalization 

 reached by Gregor Mendel, a scientific protege 

 of Nageh. 



Mendel did not frame any complete theory of 

 heredity, but observed, as the result of experi- 

 ment, that certain characters of plants are, in 

 crosses, inherited by definite proportions of the 

 offspring. He framed in general terms a state- 

 ment of what those proportions are and advanced 

 a simple hypothesis to account for them. Men- 

 del's generalization we know to-day as Mendel's 

 law, and his hypothesis as the theory of unit 

 characters. 



By a unit character in the sense of Mendel's 

 law, we mean any quality or part of an organism, 

 or assemblage of qualities or parts, which can be 

 shown to be transmitted in heredity as a whole 

 and independently of other qualities or parts. 

 Thus Mendel found that the starchy char- 

 acter of the seed of some varieties of garden- 

 peas, which makes the seeds round and smooth 

 when dried, is a quality which may by suitable 

 crosses be replaced by a sugary character, causing 

 wrinkling of the seed on drying. This change of 

 the seed character through crossing may be 

 brought about without essential modification of 

 the other parts of the plant. Round and wrin- 

 kled seed forms in peas, Mendel accordingly con- 



