FERTILITY OF PARENT SPECIES AND HYBRIDS. 97 



mating them to the tame guinea-pigs we should expect them to breed 

 true to fertility on this hypothesis. In spite of hypotheses, when fertile 

 males occurred and were bred to guinea-pigs the male offspring were 

 not all completely fertile, as will be shown. Hence we can not regard 

 the fertile males as simple, ultimate recessives in a Mendelian sense. 

 There is evidence of segregation of factors for fertility, but the case is 

 more complicated than the strict hjrpothesis of 8 factors allows.^ What 

 part interaction of factors plays, we do not know. Nor do we know 

 that all guinea-pigs carry the absence of factors disturbiiig fertility in 

 these crosses. 



It may be added that some dejfinite characters from the wild were 

 surely compatible with fertility, because males with the "wild agouti" 

 were also fertile. 



THE MALE OFFSPRING OF FERTILE MALE HYBRIDS. 



Offspring of fertile male hybrids were also tested. They may be 

 <iivided into two classes: the offspring of fertile male hybrids and 

 female hybrids, and the offspring of fertile male hybrids and guinea- 

 pigs. It seems that when male hybrids were fertile they could be bred 

 to any sort of fertile female (see table 78). Male guinea-pigs have 

 been bred to all classes of female hybrids from the Fi to the Fg genera- 

 tion inclusive. Male hybrids of every class from the F3 through Fe 

 were bred successfully to guinea-pig females. Male hybrids in each 

 generation from the F3 to the F7 inclusive were successful sires in 

 matings with female hybrids of the same or different generations. In 

 this last class of matings such diverse crosses as the following were 

 possible: Fs males were bred to Fi ,F4, F5, and Fe female hybrids, while 

 Fe males were bred to F2, F5, Fe, and F7 females. A | wild female, 5 

 years old, was impregnated by her great great grandson. A J wild 

 female was successfully mated with a -^ wild male. The different 

 possible combinations of successful matings indicate that fertile male 

 hybrids of any blood dilution can impregnate any sort of fertile female. 



Febt!L-5 Male Hybrids in Crosses with Female Hybrids. 



In all, 39 offspring from this sort of mating were tested (see table 

 72) ; 36 received only a microscopic test, while 3 received both tests. 

 Tables 74 and 76 show that all classes of males were produced, ranging 



' The percentages of males with many motile sperm in the Fj and F? generations were 69.4 

 per cent and 73.3 per cent respectively (table 75). As a matter of fact, these percentages do 

 not correspond to the expectations based on 8 factors (table 77), but are nearer the results one 

 would expect with 12 or 20 factors in the Fb and F7 generations respectively. This can be 

 readily computed from the formula given on page 95. 



Fe....J ^°~^~^ lp = .694 (.96875)P = .694 p log .96875= log .694 p =11.51 

 F, (.98438)P = .733 p log .98438= log .733 p = 19.72 



