POLYDACTYLISM. 23 



Fj are seven times as numerous as their extra-toed grandparents. Here, 

 as elsewhere, the absolute range of variability depends upon the number of 

 individuals observed. 



As we have seen, failure of dominance is much more complete in some 

 of the individuals of F^, namely, those with 4 toes, than others. There is 

 a variation in "potency." Is the degree of potency inherited? Do the 

 4-toed heterozygotes produce a larger proportion of imperfect dominants 

 in Fj than the 5- toed heterozygotes? The answer to this question should 

 be given by the correlation between total number of toes in the two parents 

 and average number of toes in their offspring, as given in table 11. In the 

 case of the Houdan crosses there is a strong positive correlation, measured 

 by 0.683 ±0.092; but the correlation is insignificant in the Silkie crosses 

 (— 0.085±0.032). This lack of correlation in the Silkie hybrids is perhaps 

 due to the heavy regression in toe-number characteristic of the second hybrid 

 generation. In general, there seems to be an inheritance of potency. 



It now remains to test our conclusions by reference to the mating of 

 the heterozygote with the dominant and with the recessive types, respec- 

 tively. An examination of tables 13 to 15, particularly the last, reveals 

 several points of interest. MendeUan expectation in the DR X R cross is 

 50 per cent of the recessive (4-4) type. Actually, in the two crosses, A and B, 

 68 per cent and 67 per cent, respectively, were obtained. But recalling 

 that of these amounts one-half of 27.3, or 13.71, and one-half of 9.4, or 

 4.7, are respectively due to failure to develop the extra-toe in heterozy- 

 gotes, there remain 54 per cent and 62 per cent, respectively, of 4-toed off- 

 spring, which doubtless represent the extracted RIl type and approach 

 the expected proportions. 



Mendelian expectation in the DR X D cross (table 15) is 50 per cent 

 heterozygotes and 50 per cent extracted dominants. Of the heterozygotes 

 some 14 per cent may be expected to show 4r-4 toes; that the percentage 

 is much less than that is doubtless due to the small numbers involved. 

 What is striking is the reappearance, in the second generation, of large 

 proportions of the extreme dominant type. These results thus confirm 

 those of the F^ generation. 



Since extra-toe frequently fails to dominate, there should be certain 

 4-toed heterozygotes which throw extra-toe offspring, and such are found. 

 In table 16 are given six matings of 4-toed DR's. One sees that they pro- 

 duce some 5-toed offspring. On the other hand, extracted 4-toed recessives 

 are obtained, as table 17 shows. 



Finally, we must consider whether, among the polydactyl birds of one 

 class, e. g., Houdans or Silkies, there is any difference in the "centgener 

 power" of parents corresponding to the degree of development of their 

 extra toes. This inquiry is suggested by Castle's study (1906, p. 20) of 

 polydactyl guinea-pigs. He finds that when the extra toes of the mothers 



