It now remains to test our conclusions by reference to the mating of the heterozygote with the dominant and with the recessive types, respectively. An examination of tables 13 to 15, particularly the last, reveals several points of interest. Mendelian expectation in the DR × 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 heterozygotes, there remain 54 per cent and 62 per cent, respectively, of 4-toed offspring, which doubtless represent the extracted RR type and approach the expected proportions.

Mendelian expectation in the DR × 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 4-4 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 F2 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 produce 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 are graded into the 5 classes, good (G), fair (F), poor (P), normal though of abnormal ancestry (N), and normal of normal ancestry (N'), it follows: "first, that the proportion of polydactylous young produced by a male decreases in the successive classes from G to N'; and, secondly, that the degree of development of the toes produced on those polydactylous young diminishes in the same order." It is possible to test this conclusion in poultry because, inside of any one type of extra-toe, e. g., the triplex type, variation appears in the absolute size of the toes and in the degree of their separateness. Our questions, then, are: (1) does the proportion of polydactyl young produced by a pair of birds of any type diminish with the degree of development of toes inside of that type, and (2) does the degree of development of the toes produced on the polydactylous offspring diminish in the same order?

Table 16.—Distribution of toe-numbers in the offspring of 4-toed heterozygotes.

Pen No.Mother.Father.Offspring.Nature of
mating.
No.Races.No. of toes.No.Races.No. of toes.4-4 toes.4-5 toes.5-5 toes.
637685Houd. × Min.4-4570Houd.×Min.4-4 512DR × DR
729913Houd. × Min.4-4936Houd.×Legh.4-4381319DR × DR
7292269Do.4-4936Do.4-415510DR × DR
7292324Do.4-4936Do.4-43053DR × R
642750Min. × Polish4-4647Do.4-410...3R × DR
671742Min. × Brah.4-4352Houd.×Brah.4-4822R × DR

Table 17.—Distribution of toe-numbers in the offspring of extracted 4-toed parents.

Pen No.Mother.Father.Offspring.Nature of
mating.
No.Races.No. of toes.No.Races.No. of toes.4-4 toes.4-5 toes.5-5 toes.
7622011Polish × Min.4-4444F2 Houd.×Legh.4-4 10......R × R
2614Do.4-4444Do.4-46......R × R
2333Do.4-4444Do.4-416......R × R
2618Do.4-4444Do.4-42......R × R
3776Do.4-4444Do.4-42......R × R

Two sets of data are available for answering these questions. The most direct set includes the data derived from crossing "pure-bred" polydactyl birds and the other includes the data derived from using hybrids between normal-toed and polydactyl ancestors. The latter data have the advantage that the parents offer a greater variability; but they have the disadvantage that the germinal condition of those parents is incompletely known.

The pure races may be considered first. Eight matings of Houdans, each parent with 5 toes, gave 122 offspring, of which 116 had 5-5 toes, 3 had 4-5 toes, and 3 had 4-4 toes. The variability of the toes is not great in the parent Houdans. But, arranging them in the order of development of the toes, the most developed first, the series of table 18 results.