POLYDACTYLISM. 



21 



Table 12 will enable us to analyze the diflference of the proportions in 

 tables 10 and 11. 



Table 12. Percentages of the various types of toes in F^ and F, of the polydactyl hybrids compared. 



* Reduced duplex and triplex toes classified as typical duplex and triplex. 



These tables >deld several points of interest. First, although the pro- 

 portions of normal and extra toe in table 12, a and c, are not MendeUan, 

 yet the average increase, from F, to F^ in the proportion of the recessive 

 (4-toed) type is almost exactly what is called for by Mendel's law. That law 

 calls for an increase of 25 per cent. The actual average increase is 23.3 per 

 cent (20.1 and 26.5 in the two cases). It seems fair to conclude, conse- 

 quently, that Mendel's law does hold here, and that the 4-toed individuals of 

 F, are heterozygotes with imperfect dominance. The feet of most of the 

 4-toed Silkies of this generation belong, indeed, to the reduced 5-toed type 

 (table 10, B), and the reduced condition is prima facie evidence of hetero- 

 zygotism. In F, Silkies of the first hybrid generation, 20 per cent of the 

 feet exhibit "reduced" types of toes, but in F^ only 5 per cent; and 

 this might have been anticipated, since in Fj heterozygotes are relatively 

 only half as numerous as in F,. Again, in F^ we see reappearing the high 

 ancestral toe-numbers (practically lost in the heterozygotes of F table 

 12, b). These I interpret as extracted dominants. 6-toed extracts are more 

 numerous among the Silkie than the Houdan hybrids, because the Silkie 

 ancestors were 6-toed and the Houdan ancestors only 5-toed. However, 

 only a small proportion of the extracted Silkie dominants have as many 

 toes as the original Silkie ancestors, and this indicates a permanent regres- 

 sion (through the contaminating influence of hybridization?) toward the 

 normal condition of toes. It will be observed that, although 6 toes are not 

 found in the Silkie hybrids of Fj, many of these heterozygotes are of the 

 reduced triplex tyipe. Classifying them as virtually 6-toed, we find (table 

 12, c) 14.5 per cent of the 6-toed type in the F, generation. 



Among the extracted dominants of F^ are a few showing more toes 

 than appeared in the ancestors (table 12, a; there was also one 7-toed Fj 

 Silkie hybrid, not recorded in the table). It is this sort of an advance in 

 F2 that permits the breeder to make a forward step. Theoretically, the 

 appearance of this more aberrant class is probably due to the greater 

 numbers of progeny than of ancestors, since the extracted dominants of 



