A. STATEMENT OF PROBLEM.

In man, various mammals, and some birds two or more adjacent fingers are sometimes intimately connected by an extension of the web that is normally a mere rudiment at their base. Such a condition is known as syndactylism. A good introductory account of syndactylism is given by Bateson (1904, pp. 356-358). Taking a number of cases of syndactylism together, he says: "A progressive series may be arranged showing every condition, beginning from an imperfect webbing together of the proximal phalanges to the state in which two digits are intimately united even in their bones, and perhaps even to the condition in which two digits are represented by a single digit." He also calls attention to the fact that in the human hand "there is a considerable preponderance of cases of union between the digits iii and iv;" while in the foot the united digits "are nearly always ii and iii." The matter of syndactylism in birds has a peculiar interest because of the fact that among wading and swimming birds syndactylism has become a normal condition of the feet, and, moreover, just this feature is one that has become classical in evolutionary history, because Lamarck thought it well illustrated his idea of the origin of an organ by effort and use.

Concerning the cause of syndactylism little can be said. Both in mammals and birds the digits are indicated before they are freed from lateral tissue connections. The linear development of the fingers is in part accompanied by a cutting back of this primordial web, in part by a growth beyond it. In syndactylism growth of the web keeps pace with that of the fingers. From this point of view syndactylism may be regarded as due to a peculiar excessive development of the web.[5] In some human cases adhesions of the apex of the appendage to the embryonic membranes has stimulated the growth of the interdigital membrane, resulting in syndactylism. But it would be absurd to attempt to explain syndactylism in general on this ground. The more "normal" forms of syndactylism, as seen in poultry, still want for a causal explanation.

Most of the cases of syndactylism whose inheritance is about to be described arose in a single strain of fowl and can, indeed, be traced back to a single bird. This ancestor is No. 121, a Dark Brahma hen described in a previous report.[6] It was only in the search for the origin of the exaggerated forms of syndactylism observed in some of her descendants that an unusually great extension of the web in her feet was noticed. The syndactyl condition of my birds did not, thus, arise de novo, but had its origin antecedent to the beginning of the breeding experiments. In addition to this main strain a slight degree of syndactylism has appeared among some of my Cochin bantams.

Table 23.—Ancestry of syndactyl fowl and the results of various matings involving syndactylism.

[Abbreviations: Abα, Abβ, etc., types of syndactylism ([p. 32]); F, father; FF, father's father; FM, father's mother; M, mother; MF, mother's father; MM, mother's mother; M × P, hybrid of Minorca and Polish races; Synd., syndactyl (type unknown). f, foot. In Nos. 24 to 42 two cocks (Nos. 242 and 3116, and 5399 and 4562, respectively) were at different times used.]

Serial No.Pen No.First mating.Second mating.
Ancestry.Offspring.Ancestry.Offspring.Average
per cent
syndactyl.
M's No.MM.MF.F's No.FM. FF.Syndactyl.M's No.MM.MF.F's No.FM. FF.Syndactyl.
2f.1f.0f.2f.1f.0f.
1a, b627302[1]121[2]8a180[1]121[2]8a0034302[1]121[2]8a242[1]121[2]8a302910.3
2a, b6272801218a1801218a00232801218a2421218a20219.5
3a, b6271811218a1801218a00201811218a2421218a30339.1
4a, b6273541218a1801218a00243541218a2421218a10372.6
5a, b6271781218a1801218a00201781218a2421218a0042...
6a, b6271901218a1801218a10241901218a2421218a006...
7a, b...3531218a1801218a00133531218a2421218a0022...
8a, b...3001211a1801218a00233001211a2421218a0037...
Totals (182)10181Totals (236)90227
Percentages0.55099.45Percentages3.81096.19
[1] No. 121 is a Dark Brahma.[2] No. 8A is a Tosa fowl (Game).
[3] (White Leghorn × Rose Comb Black Minorca) × Dark Brahma.[4] Dark Brahma.
[5] See supra.[6] 121♂ Dark Brahma × 8A Tosa.
[7] F2 (White Leghorn × Dark Brahma).
Serial No.Pen No.Mother.Father.Offspring.
No.Bred in pen No.Toes.No.Bred in pen No.Toes.Syndactyl.Classification.
2f. 1f. 0f. P. ct. Aaα. Abα. Abβ. Abβ´ . Bbα.
97472526[3]658Normal.1888[3]658Normal.90950.0...216......
107472831658Do1888658Do.60650.0...75......
117472652658Do.1888658Do.302510.7...6.........
127473541658Do.1888658Do.40418.9143......
137471892658Do.1888658Do.40477.8...............
147471872658Do.1888658Do.00280.0...............
157471874658Do.1888658Do.00280.0...............
26018412.4
167032353D. Br.Do.122D. Br.Do.10614.3...2.........
177032030D. Br.Do.122D. Br.Do.211220.0...5.........
211220.0...
187543126[4]627Normal.871[4]627Normal.1213030.2...1312......
197543175627Do.871627Do.30827.3 ...33......
20754873627Do.871627Do.[2](?)(?)(?) ......4......
217541052627Do.871627Do.00170.0...............
22754853627Do.871627Do.00190.0...............
23754862627Do.871627Do.00270.0...............
15110113.7
247672526[3]658Normal.3116D. Br. Synd.502218.5116...2
25767872[5]627 Abβ242[5]513Normal.10150.0...1...1...
25a767872627Abβ3116D. Br.Synd.713021.0353...4
267672104[7]608Normal.3116D. Br.Do.301814.3...22...2
277672831[3]658Do.3116D. Br.Do.30328.6...6.........
28767181[6]513Do.242513Normal.10224.42............
28a767181513Do.3116D. Br.Synd.11603.2...11...1
29 767190[5]520Do.242513Normal. 11286.71.........2
29a767190520Do.3116D. Br.Synd.4...497.6...34...1
Syndactyl (242 ♂)31517.3
Syndactyl (3116 ♂)2322119.4

Table 23.—Ancestry of syndactyl fowl and the results of various matings involving syndactylism—Continued.

Serial No.Pen No.Mother.Father.Offspring.
No.Bred in pen No.Toes.No.Bred in pen No.Toes.Syndactyl.Classification.
2f. 1f. 0f. P. ct. Aaα. Abα. Abβ. Abβ´ . Bbα.
308014569767Abα5399747Abα200100.010300
30a8014569767Abα4562767Normal.02250.0...11......
318016843767Normal.4562767Do.13266.7...221...
32801872627Abβ5399747Abα1241159.33911...5
32a801872627Abβ4562767Normal.711240.0 2841...
338015515767Bbα5399747Abα40736.4...26......
33a8015515767Bbα4562767Normal.12537.5211......
348017528767Abβ5399747Abα100100.0...2.........
34a8017528767Abβ4562767Normal.21730.0...14......
358016861767Normal.4562767Do.10325.0...2.........
368016869767Do.5399747Abα01325.01............
36a8016869767Do.4562767Normal.10420.0......2......
378012831658Do.5399747Abα311818.2...4......3
37a8012831658Do.4562767Normal.211121.4...2......3
388012526658Do.5399747Abα0050.0...............
38a8012526658Do.4562767Normal.10233.3...11......
398014570767Do.5399747Abα01516.71............
39a8014570767Do.4562767Normal.021710.511.........
408011892658Do.5399747Abα0090.0...............
40a8011892658Do.4562767Normal.10325.0...2.........
418014263767Do.5399747Abα01420.0...1.........
41a8014263767Do.4562767Normal.00100.0...............
428016872767Do.4562767Do.0060.0...............
Syndactyl (5399 ♂)2286232.6
Syndactyl (4562 ♂)17128425.7
437762291Coch.Normal.2732Coch.Normal.20625.0...2......2
447762574Coch.Do.2732Coch.Do...2910.0...1.........
457762570Coch.Do.2732Coch.Do....1118.3...1.........
467762297Coch.Do.2732Coch.Do....1127.7............1
477762299Coch.Do.2732Coch.Do.10165.9...2.........
487762904Coch.Do.2732Coch.Do.0060.0...............
497762937Coch.Do.2732Coch.Do.0070.0...............
507762300Coch.Do.2732Coch.Do.00150.0...............
517762736Coch.Do.2732Coch.Do00180.0...............
331005.7
52816121D. Br.Abα122D. Br.Normal.311028.6...1...24
52a816121D. Br.Abα4912M × PDo.00130.0...............
538165835D. Br.Normal.122D. Br.Do.10614.3 ...2.........
548162353D. Br.Do.122D. Br.Do.0070.0...............
54a8162353D. Br.Do.4912M × PDo.0040.0...............
Syndactyl ( 122 ♂)4123 17.9
Syndactyl (4912 ♂)00170.0

The types of syndactylism which have appeared in my flock form a rather extensive series. First, (A) the single web, which, in my specimens, always occupies the interspace between digits iii and iv. This is the same interval which is most apt to show the web in syndactylism of the human hand, and, it is suggestive to note, it is this interval that is filled in those wading birds that have the single web only between the toes (e.g., Cursorius, Glareola, Vanellus, Squatarola, Charadrius, Limosa, Machetes, Himantopus); second, there is (B) the double web, one-seventh as common, which always occupies the interspaces between the digits ii-iii and iii-iv.

On another basis, the syndactyl feet may be classified as: (a) toes adherent, web small in extent, and (b) toes distant, web broad. I have found the narrow web only between digits iii and iv. It is one-eighth as common as the broad-webbed type. The broad, double web approaches closely to the type found normally in swans, geese, and ducks.

Finally, the syndactyl feet may be classified as: α, straight-toed, or β, curve-toed. Class α is to class β in frequency as 2:1. In the typical curve-toed syndactyl foot the web between iii and iv is complete to the nails of each; in fact, in extreme cases the nails of the two toes are more or less fused together. From the fused nails the middle toe, being the longer, passes in a curve to the distal end of the metatarsus. The D-shaped interspace between the curved iii and straight iv toe is filled with the web. In other cases the nails are merely approximated and the middle toe is slightly curved. In three instances (4 per cent of all) the outer toe (iv) is curved toward the (straight) median toe (class β´).

As stated, the polydactyl offspring trace back their ancestry to No. 121; her feet both show the double, broad, straight-toed type (Bbα). We shall attempt in the following paragraphs to trace the heredity of her type of polydactylism and of the others that have subsequently arisen.