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._ +-----+-------------------------+-----------------------+-----------------+--------+ | | Mother. | Father. | Offspring. | | | Pen +----+-------------+------+----+-----------+------+-----+-----+-----+ Nature | | No. | No.| Races. |No. of| No.| Races. |No. of| 4-4 | 4-5 | 5-5 | of | | | | | toes.| | | toes.|toes.|toes.|toes.| mating.| +-----+----+-------------+------+----+-----------+------------+-----+-----+--------+ | 637 | 685|Houd. × Min. | 4-4 | 570|Houd.×Min. | 4-4 | 5 | 1 | 2 |DR × DR | | 729 | 913|Houd. × Min. | 4-4 | 936|Houd.×Legh.| 4-4 | 38 | 13 | 19 |DR × DR | | 729 |2269| Do. | 4-4 | 936| Do. | 4-4 | 15 | 5 | 10 |DR × DR | | 729 |2324| Do. | 4-4 | 936| Do. | 4-4 | 30 | 5 | 3 |DR × R | | 642 | 750|Min. × Polish| 4-4 | 647| Do. | 4-4 | 10 | .. | 3 | R × DR | | 671 | 742|Min. × Brah. | 4-4 | 352|Houd.×Brah.| 4-4 | 8 | 2 | 2 | R × DR | +-----+----+-------------+------+----+-----------+------+-----+-----+-----+--------+ TABLE 17.—--_Distribution of toe-numbers in the offspring of extracted 4-toed parents._