BEHAVIORAL DIFFERENCES 285 



are apt to be many branchings and turnings so that a single gene may connect with 

 several patterns, and the paths from a single pattern may lead to a variety of genes. 



The relationship between genetic and behavioral variation. — Behavior is one of the ways 

 by which organisms maintain homeostasis in the face of environmental changes. In 

 its simplest sense, behavior is an attempt to adapt to some sort of change. The adaptive- 

 ness of responses is ordinarily considered to be the outcome of natural selection. 

 Organisms with responses not adaptive are less fit and have fewer survivors. Because 

 of the great variety of possible changes in the external environment, there is little 

 possibility of evolving ready-made behaviors which will adapt an organism to every 

 situation. The course of evolution has been toward an increased capacity for behavioral 

 variability. As Jennings 661 pointed out long ago, some variability of behavior is 

 found even in protozoa. Behavioral variability is, however, most characteristic of 

 the vertebrates and particularly of mammals. Referring again to the fighting behavior 

 of the mouse, we have noted that a caged male attacked by another will at first react 

 by fighting back. If this is unsuccessful, he runs away. If he cannot escape, he assumes 

 a defense posture and as a final resort may become completely passive. A primary 

 characteristic of adaptation through behavior is variation according to the conditions 

 of stimulus. An animal tries first one behavioral pattern, then another, and finally 

 adopts the one which is most successful. (In this account, we are deliberately omitting 

 abnormal or maladaptive behavior which may result from unsuccessful adaptation.) 



The science of genetics is concerned with the study of variation. When the effect 

 of heredity on behavior is analyzed, an apparent paradox is discovered : that one of the 

 characters affected by genes is behavioral variability itself. Consequently, a large 

 proportion of behavioral variation independent of genetic variation must always be 

 expected. Homeostasis of the individual may be best accomplished by changes in 

 behavioral patterns rather than constancy. 894 



Another factor which must be included in the study of behavior of higher animals 

 is the process of learning. Repetition of a particular situation or stimulus tends to 

 reduce behavior to learned habits which are relatively nonvarying. Such increased 

 constancy of individual behavior, however, does not necessarily reduce the proportion 

 of variance assignable to heredity. 410 



The concept of threshold. — For the most part, animals do not respond uniformly 

 over a wide range of stimulus intensity. No reaction may be given until a certain 

 threshold is reached, and beyond this point increased stimulation sometimes has little 

 effect. A behavioral character to which the threshold concept may be applied is the 

 syndrome of audiogenic seizures in mice. Sounds below a certain intensity do not 

 evoke seizures. Once threshold is reached, however, the intensity of the convulsive 

 response may be just as great when the response is made to jingling keys as when it is 

 made to a powerful motor-driven bell. The threshold concept may be applied to the 

 genotype as well as to the stimulus. Temporarily it will be assumed that susceptibility 

 to audiogenic seizure is determined by a single pair of alleles A and a. The AA 

 genotype in the model is highly susceptible to sounds and convulses very easily. The 



