The Process of Heredity 555 



105. The individuals of both hybrid strains show all degrees of 

 blending of these two rates, each strain showing an average not 

 far removed from the ideal mean, 125 beats per minute. 



The effects of these two cases of blending are far-reaching, and 

 are quite different in the two hybrid strains. 



The H hybrid strain shows the F. majalis influence in acquir- 

 ing a heart rhythm about ten or twelve hours later than the H 

 pures. The process of development in an embryo is greatly 

 accelerated by the establishment of a circulation, and as a conse- 

 quence the H pure strain gains markedly upon the H hybrids 

 at this time (compare H^ and /z5, Plate III). When, however, 

 the heart rhythm of the H hybrids is established, it is distinctly 

 more rapid on the average than that of the H pures and this 

 greater rapidity, other things being equal, initiates more rapid 

 development. Consequently the speed of development, pre- 

 viously retarded by the belated establishment of the heart rhythm, 

 is now accelerated by the introduction of a more rapid heart rate, 

 and, as a result, all of the healthy H hybrids gain markedly upon 

 the pure bred embryos and present, after a period of eight or ten 

 hours, the relative average conditions seen in H6 and h6 (Plate 

 III). The hybrid specimens that inherit the least retardation in 

 the establishment of a heart rhythm and the greatest acceleration 

 in the rate of heart-beat, actually overtake the best of the H pures 

 before hatching, a few hatching as early as any of the H pures. 

 After hatching these same fortunate hybrid specimens outgrow 

 and outlive the best of the H pures, and these are the specimens 

 that show the most marked condition of hyperdominance with 

 regard to rate of development and resistance to adverse conditions. 

 The specimens which, on the other hand, have inherited the maxi- 

 mum retardation in the establishment of a heart rhythm and the 

 minimum acceleration in the rate of heart-beat are too severely 

 handicapped to succeed in the race, and they lag far behind, and 

 become weaklings or monstrosities, carrying the character of ill- 

 health to an extreme not met with in either of the pure breeds. 

 This condition might also be considered as a case of hyperdom- 

 inance. 



The case is entirely different for the M hybrids. Here also we 



