236 GENETICS AND EUGENICS 



hybrids, three-fourths bloods may be produced which are 

 almost as variable as a true F2 generation. If the products 

 of this cross are shown to possess economic advantages over 

 domestic cattle (which seems very doubtful) a fertile hybrid 

 race will doubtless be established in the near future. How 

 this can be done is shown in experiments made by Dr. Det- 

 lefsen and myself in crossing the guinea-pig with a wild 

 Brazilian species of cavy, Cavia rufescens. The Fi individuals 

 surpass either parent species in size and vigor, but the males 

 are fully sterile, the females, however, being fertile. After 

 two back-crosses of female hybrids with the guinea-pig a 

 few fertile males were obtained, whose descendants were also 

 fertile. But they possess certain Mendelizing characters de- 

 rived from the wild parent, Cavia rufescens. The skeletal 

 characters of the hybrids are a blend. The great vigor of the 

 Fi hybrids is not shown in the fertile hybrids obtained by 

 back-crossing. As regards size and vigor they are not su- 

 perior to guinea-pigs. If the Mendelizing color characters 

 possessed economic value, the hybrid race could now be 

 easily continued. As in the case of the cattle-bison cross, the 

 economic value of the Fi generation is not sufficient to war- 

 rant the expense of its continued production. 



Hybrids which are feeble as well as sterile have, of course, 

 no economic value. They are scientifically interesting as 

 showing how, when the difference between gametes becomes 

 too great, they can no longer form a vigorous zygote. Few, 

 if any, animal hybrids of this sort are known, but many 

 plant hybrids of this sort have been produced, among them 

 being some of the first produced hybrids obtained by cross- 

 ing different species of Nicotiana (tobacco). See Fig. 26a, 

 East's repetition of Kolreuter's pioneer experiment. 



5. WTien organisms are crossed which differ more widely 

 than do ordinary species, so that they are referable to differ- 

 ent genera or families, the production of a hybrid organism 

 does not follow, apparently because the uniting gametes are 

 too unlike to be capable of continued existence together in the 

 same cell. Nevertheless a parthenogenetic development of the 



