DEPARTMENT OF GENETICS. 



133 



parent race and thrive well under all sorts of conditions, with any sort of 

 food. It seems as though some phenomenon of heterosis is involved. Per- 

 haps there has occurred a combination of factors that worked for greater 

 vigor in the hybrids, and to this greater vigor is due the lowered time average. 

 This same vigor probably does not account for the superiority in the number 

 of perfect trials, for perfect trials are not 

 closely correlated with time. This is shown 

 by the pure races, which are alike in the 

 -matter of perfect trials, although the more 

 vigorous albinos make better time. 



Among the hybrids are not only those that 

 make an unusually high number of perfect 

 trials, but also a large percentage that make 

 no perfect trials. When they once learn the 

 way, the hybrids are more apt to make re- 

 peated perfect trials than either parent race, 

 but they also are less apt to make a single 

 perfect trial. This may be supposed to be 

 due to the greater sensitiveness of the hy- 

 brids, appearing as fear; once this fear is 

 overcome they show their greater ability to 

 form and retain the habit. 



Whatever interpretations may be made, 

 the facts are clear that the mice of the first 

 filial generation are neither intermediate be- 

 tween the parents nor like either parent. 

 Rather do they show a greater capacity for 

 learning accurately, when they learn at all, 

 and on the average they take less time per 

 trial. This demonstrates that the phenomenon of heterosis, so often 

 shown in physical characters, may also be found in behavior. With the 

 indications of the phenomenon of dominance in the dogs, and heterosis in 

 the mice, as applied to traits of behavior, two steps towards the analysis of 

 the genetic basis of behavior have been made. 



Heredity in Sheep. 



The experiments on heredity of twinning in sheep were continued. 

 The triplet ram of last year was continued as the sire of the flock and 

 the fiock was fed heavily, even on grain, during September and Octo- 

 ber. But despite this precaution the average number of lambs per 

 fertile ewe was 1.43, the lowest rate for many years. Also the still- 

 birth rate was exceptionally high, being 30 per cent of all lambs born. 

 For next season's lambs a sturdy single lamb of twin stock is to be used. 



Fig. \.—Maze used for training mice. — • 

 E indicates the entrance to the maze; 

 Ri and Li indicate right and left exits 

 of the first compartment; R2 and L2 

 indicate right and left exits of the 

 second compartment; G indicates 

 glass slide used in closing pathway; 

 F indicates food; C indicates correct 

 pathway through maze. 



GERMINAL AND SOMATIC VARIATIONS. 



Studies on the Vascular Anatomy of Normal and Teratological Seedlings of 



Phaseolus vulgaris. 



These investigations have been continued by Dr. Harris along lines 

 indicated in previous reports (Year Book Carnegie Inst. Wash. 19 : 139) . 

 Harris, Sinnott, Pennypacker, and Durham have published the results 

 for variation in the vascular structure of dimerous and trimerous 



