I20 Experimental Zoology 



ancestry. If the various sires are arranged "in the order of the 

 respective amounts of polydactylous ancestry which they possess, 

 we see at once that this is not the order of their potencies, for 

 those having the same amount of polydactylous ancestry often 

 differ much in the potency with which they transmit the poly- 

 dactyl character." 



When polydactylous individuals were mated with normal ones, 

 the results were far from being uniform. Some of the offspring 

 have the extra toes greatly weakened ; in other cases there is no 

 toe at all, while in still other cases the extra toe may be fairly 

 well developed. "The inheritance is neither sharply alterna- 

 tive (MendeHan) nor completely blending." It is clear that in 

 its inheritance the extra toe of these guinea pigs does not follow 

 Mendel's law. Castle concludes that the extra toe is inherited 

 in a manner intermediate between blending and alternative in- 

 heritance. The gametes, he thinks, only partially blend in the 

 zygote, producing a variable result. "If the inheritance were 

 sharply alternative, we should expect to get, not a series of gradu- 

 ated forms, but two or at most three sharply distinct groups, 

 but this is not the result observed. If, on the other hand, the 

 inheritance were fully blending, all the offspring of two pure par- 

 ents, or of two cross-bred parents should be alike, but this is not 

 the result observed. We are forced to conclude, therefore, that 

 there occurs a partial blending of gametes [characters] in the 

 zygote, and a partial segregation as the zygote gives off 

 gametes." 



Castle points out, further, that partial blending is the more 

 common result of hybridizing, since both sharply alternative 

 inheritance and complete blending are rare. By selection the 

 breeder is able to produce an almost pure race by picking out the 

 more potent individuals in each generation. It is interesting to 

 note that the potency of the male is a germinal variation, tend- 

 ing toward determinate inheritance, and not simply an extreme 

 fluctuating variation due to external conditions. Hence, in 

 selecting prepotent individuals the process involves the choice of 

 certain individuals that transmit certain qualities in a high degree. 



