GENETIC TYPE AND THE ENDOCRINES 639 



litters. The differences in behavior are not only greater 

 among the members of the group, but the individual per- 

 formances of each type tend to be more variable than either 

 the bassethound-shepherd or the bulldog-bassethound. 



It may be assumed that there are two basic factors influ- 

 encing the constitutional complex. One is the combination 

 and patterning of the genes which determine bone, glands 

 and the nervous structures of the body, and the other is 

 the action of mutations. The modifications brought about 

 by these breeding experiments may be considered the result 

 of recombinations of the genes involved. Only one of the 

 many behavioral modifications which have occurred can pos- 

 sibly be attributed directly to a mutation. This is the ex- 

 tremely passive defense behavior which places the dogs 

 possessing it in the abnormal group. In each of the three 

 cases that have ocenrred, the animals were offspring of two 

 pure types, a bassethound and a German shepherd, neither 

 of which had the reaction. Since it is not typical of either 

 breed it is either a recessive character from an ancestor not 

 included among the dogs at the experimental station, or its 

 occurrence is due to a mutation. Another possibility is that 

 the behavior is based on a recombination of genetic factors, 

 due to mixing widely different types, resulting in a character 

 not observed in either grandparent type. In fact, one of the 

 dogs, 1301 9 , appeared in a litter witli a highly mixed genetic 

 background. The parents were offspring of a bassethound- 

 shepherd Fj bred back to the bassethound, making them 

 theoretically three-quarters hound and one-quarter German 

 shepherd. The other dog, 1513 $ , appeared in a litter obtained 

 by breeding two second generation hybrids. There have been 

 only two extreme cases out of twenty-two related dogs, not 

 enough to determine its genetic probability. 



A consideration of the dogs which differ in size from the 

 bassethound and English bulldog, yet which show similar 

 behaviors, raises another important question. This involves 

 the number of factors of the genetic complex that may be 

 modified without leading to a different constitution. The 



