296 PHYSIOLOGIC GENETICS 



treatments may be assayed by measuring behavior which is comparatively trivial, 

 although it serves well for analysis of the genetic effects. The two methods are not 

 really contradictory but are, rather, supplementary. 



Dr. Scott illustrates examples of genetic populations which diverged and converged 

 in behavior as they developed. Such convergence or divergence of means does not 

 necessarily mean that genetic effects are becoming less or greater as the animals grow 

 older. In some instances we have been able to show that the contribution of heredity 

 to differences as measured by interclass correlation stays relatively constant as animals 

 age or as they practice a skill. Looking only at means may be deceiving. 



Finally, I would like to close by stating that behavioral phenotypes are more 

 difficult to measure than ordinary phenotypes, but a large number of investigators have 

 shown that the problems of behavioral genetics are not insuperable. In fact, the diffi- 

 culties of studying the genetics of behavior are very similar to those encountered with 

 complex physiologic characters. Great progress may be expected from studies of 

 behavioral effects of well-defined genetic entities. Another important area is the 

 determination of the heritability of continuously distributed traits. The concepts of 

 a developmental threshold is familiar to students of growth and also appears important 

 in behavioral genetics. The unique factor of behavioral genetics is its need of 

 extensive and detailed control of the life history of its subjects and its use of special 

 methods for the reliable objective measurements of transitory phenomena. 



Dr. Scott: Dr. Ginsburg argues that behavioral genetics is no different from 

 physiologic genetics, and I agree that one of the basic scientific questions in this field 

 is how heredity affects behavior. However, I think that behavorial genetics goes 

 beyond physiologic genetics in that a higher level of organization is being investigated 

 and consequently phenomena are encountered which are not apparent on the lower 

 level. Dr. Wright has already illustrated the complicated types of interaction 

 involved in the inheritance and production of pigment. Adding another level of 

 organization makes Dr. Wright's system appear relatively simple. 



