B. RESULTS OF HYBRIDIZATION.

In taking up the results of breeding experiments to test the method of inheritance of syndactylism, it will be best first to give in a table all pens in which the character showed itself, with the frequency of the different types of foot in them (table 23).

The history of the syndactyl strain begins with No. 121 ♀ and in the matings 1 to 8 are given the results of crossing together some of her progeny derived from a normal-toed father. This father was either No. 8a or 1a, both full-blooded Tosa (Japanese Game) fowl and without suspicion in either soma or offspring of syndactyl taint. There is no record of trace of syndactylism in the progeny of 121 × 8a (or 1a); but a slightly developed condition of syndactylism may very well have been overlooked by me in this F1 generation (as I had never thought of such an abnormality), even as I at first overlooked the syndactylism visible in No. 121. But when these F1 hybrids were mated together (pen 627, serial Nos. 1 to 8) I got, in the different families, from 10 per cent syndactyl offspring down to none at all.

At first sight the suggestion arises that, if inheritance is at all Mendelian, the normal condition is dominant and that the heterozygotes throw again, in pen 627, the syndactylous condition. If this hypothesis were true it would follow that syndactyls bred together should, sometimes at least, throw, even in large families, 100 per cent syndactyl offspring. But only 2 families, Nos. 30 and 34, have yielded 100 per cent syndactyls, and these contained 2 and 1 offspring, respectively; so they are not significant. On the other hand, there are numerous matings of 2 extracted normal-toed parents that have produced only normal-toed offspring (families Nos. 14, 15, 21, 22, 23, including 119 individuals). Consequently the conclusion is favored that normal-foot is recessive and syndactyl-foot dominant, and this shall be our working hypothesis.

On our hypothesis, No. 121 is probably a heterozygote. Mated with the recessive normal, expectation is 50 per cent heterozygous, showing syndactylism; the remainder normal-toed. But dominance is here, as in polydactylism, very imperfect. For this reason and because it was not looked for, no syndactylism was noted in the first hybrid generation. The offspring prove to be of two sorts, however. No. 180 ♂ is a pure recessive, and in 8 matings with as many different sisters of his he got 184 normal-toed to 1 syndactyl. These same sisters, mated to another brother, No. 242, in some cases gave 9 per cent and 10 per cent syndactyl. No. 242 is, consequently, probably a DR and, mated to DR sisters (which constitute according to expectation about one-half of all) gives some DD's, part of which constitute the 9 to 10 per cent of syndactyls. Of course, 25 per cent DD is to be expected; the difference gives a measure in this instance of the imperfection of dominance in the "extracted" as well as "heterozygous" condition.

Matings 9 to 15 (pen 747) are instructive in comparison with the foregoing case. Both parents are derived from pen 658, which contained as breeders a heterozygous Dark Brahma male (No. 146) and various females of non-booted races far removed from suspicion of syndactylism; expectation being an equal number of DR and RR offspring. In pen 747 No. 1888 ♂ acts like a DR, and so do the hens in matings 9 to 13, while the hens in the other 2 matings are doubtless RR's. The former give 17 per cent syndactyl offspring, the latter none at all (in 56 individuals).

Matings 16 and 17 (pen 703) are between pure-bred Dark Brahmas that are probably DR's. About 22 per cent of their offspring are syndactyl—a rather higher proportion than we have found before. Matings 18 to 19 are between progeny of pen 627. In mating 20 the normals were not recorded. The cock in this pen, No. 871, is probably heterozygous, as are also the first two hens, so that nearly 30 per cent of their progeny are syndactyl. From the other 3 hens no syndactyl offspring were obtained. Evidently the two sets of hens have a very different gametic constitution. The existence of two sorts of families is one of the strong arguments for the segregation of this character.

We next come to the pens (matings Nos. 24 to 42) which were especially mated to study the inheritance of syndactylism. I had now, for the first time, two parents with syndactylic feet.

On account of imperfection of dominance decision as to gametic composition of any parent must largely rest on the make-up of the progeny. Table 24 gives the most reasonable classification of the parentages.

Table 24.

DD × DD (SYNDACTYL × SYNDACTYL).
Family No.Mother's No.Bred in pen No.Toes.Father's No.Bred in pen No.Toes.Syndactyl.
2t. 1t. 0t. P. ct.
304569767Abα5399747Abα200100.0
347528767Abβ5399747Abα100100.0
32872627Abβ5399747Abα1241159.3
335515767Bbα5399747Abα40736.4
Totals1941874.2
DD × DR.
316843767Normal.4562767Normal.13266.7
30a4569767Abα4562767Do.02250.0
33a5515767Bbα4562767Do12544.4
32a872627Abβ4562767Do.711242.9
34a7528767Abβ4562767Do.21730.0
366869767Normal.5399747Abα01325.0
25a872627Abβ3116D. Br.Synd.713021.1
414263767Normal. 5399747Abα01420.0
372831658Do.5399747Abα311818.2
394570658Do. 5399747Abα01516.7
401892658Do.5399747Abα0090.0
Totals21149726.5
DR × DR.
38a2526658Normal.4562767Normal.10233.3
356861767Do.4562767Do.10325.0
40a1892658Do.4562767Do.10325.0
37a2831658Do.4562767Do.211121.4
36a6869767Do.4562767Do.10420.0
242526658Do.3116D. Br.Synd.502218.5
262104608Do.3116Do.Do.301814.3
39a4570767Do.4562767Do.021710.5
272831658Do.3116D. Br.Do.30328.6
29a190520Do.3116D. Br.Do.40497.6
29767190Do.242513Do.11286.7
28a181513Do.3116Do.Do.11603.2
Totals23524910.1
RR × DR.
426872767Normal.4562767Normal.0060.0
41a4263767Do.4562767Do.00100.0
Totals00160.0

Summarizing the foregoing, and comparing the totals with Mendelian expectation, we get the result shown in table 25.

A comparison of realization and expectation in table 25 shows that the proportion of syndactyls is always less than expectation, not only for dominants and heterozygotes together, but even for pure dominants alone. The proportion of syndactyls obtained diminishes, to be sure, in accordance with expectation (on the assumption that they are pure dominants), but the numbers lag behind, in the higher proportions 40 to 25 per cent. So we reach the conclusion that, as in polydactylism, so in syndactylism dominance is very imperfect. But there is this difference, that in syndactylism dominance is so imperfect that the dominant condition rarely shows itself in heterozygotes and even fails in many pure dominants. The striking fact, the one that assures us the segregation is nevertheless occurring in this case too, is that some families (whose two parents are extracted recessives) throw 100 per cent recessives.

Table 25.

Nature of mating. f Expectation.Realization.
Dominants +
heterozygotes.		Pure dominants.Syndactyls.
P. ct.P. ct.P. ct.
DD × DD41100.0100.056.1
DD × DR132100.050.026.5
DR × DR27775.0 25.010.1
RR × DR1650.00.00.0
RR × RR 1190.00.00.0

These studies on syndactylism in poultry may be used for a critical examination of the recent work of Lewis and Embleton (1908) on syndactylism in man. The cases described by them follow the types I have just described in poultry. Their fig. 18 corresponds to my types a and α; figs. 10 and 11 to my type β. The "crossbones" referred to by the authors correspond to bones of the "curved toe." The facts presented by the authors support the idea that syndactylism is dominant rather than recessive, but they deny the application of Mendelian principles to this case. Actually, the foot deformities described by Lewis and Embleton are inherited much like syndactylism in poultry. No extracted normal (recessive) extremity produces the abnormal condition. Heterozygotes show much variation, from very abnormal to slightly abnormal (possibly perfectly normal?) appendages. Dominance is, indeed, much more potent than in poultry.

The authors' denial of the application of Mendelism to this case seems to be based on an all too superficial consideration of the hereditary behavior of the character and a tendency to "mass" statistics—a procedure that tends to obscure the interpretation of the data of heredity.

As to the inheritance of type, my statistics are not extensive enough to give a final answer, but if all types be grouped into those with straight and those with curved toes, then in crosses of straight-toed syndactyl and normal 33 per cent of the offspring were of the curved type, whereas in crosses of curved-toed syndactyls and normal 45 per cent were of the curved type. These averages depend on 22 and 15 individuals, respectively. They lead us to look for an inheritance of type when more extensive data shall be available.

Syndactylism is a typical sport, that is, a rather large mutation having a teratological aspect. The question arises, Does it prove to be prejudicial to the welfare of the species? The breeder who has only a few individuals of a rare sport feels their loss more than that of normals and the general impression left in his mind is that the sport is less capable of maintaining itself than the normal form. Assembling the data, consisting of about 40 individuals of each kind, it appears that the death-rate is not very different in the two lots; the slight excess of that of the syndactyls is sufficiently accounted for by the circumstance that no normals were reared during the period of greatest mortality (the summer), but were destroyed or given away as soon as hatched. It is probable, therefore, that syndactylism, under the conditions of the poultry-yard, has little life and death significance, but is one of those neutral characters whose existence Darwin clearly recognized.

CHAPTER IV.
RUMPLESSNESS.

The tail of vertebrates is, historically, the post-anal part of the trunk. Containing no longer any part of the alimentary canal, it has lost much of its primitive importance, so that its disappearance in any case is a matter of relatively little importance. Accordingly we find groups of animals in which it is rudimentary or wholly absent, such as many amphibia and the anthropoid apes and man. In all recent birds the tail is a distinct but much reduced organ—the uropygium—which contains several vertebræ in a degenerate condition. The uropygium supports the tail feathers, which are of much use in directing the bird in flight, but in ground birds, such as the grouse and poultry, seem to function only for display in the male and, in the female, to facilitate copulation.

Now, among various typically tailed vertebrates the tail is sometimes absent. Tailless dogs, cats, sheep, and horses are known; on the other hand, several cases of tails in man have been described (Harrison, 1901). Thus the tail is a part of the body subject to sporting; and it has also become the differential character for some specific groups. In other words, it is an organ that has played an important part in evolution and consequently its method of inheritance is a matter of great interest.

The origin of the tailless poultry which I have bred has been twofold. The most important strain is that referred to in an earlier report[7] as Bantam Games. The second lot consists of rumpless fowl that have arisen in my yards, spontaneously, from normal blood. Of these more later.

The two rumpless Game cocks bore the numbers 117 and 116. Dr. A. G. Phelps, of Glens Falls, New York, from whom the birds were purchased, wrote that he had imported No. 117 from England, and No. 116 was its son. The birds were very closely similar in all external features.

The matings made with No. 117 and their results are given in table 26.

Table 26.—Progeny of tailless cock and tailed hens.

Serial No.Pen No.Father.Mother.Offspring.
No. Races.Condition of uropygium.Per cent
rumpless.
Present.Small.Absent.
1525117114Nankin.3...00
252611720aFrizzle.8...00
3532117...Bl. Coch.14...00
4532a117127Wh. Legh.19...00
4a653117508Bl. Coch. × Wh. Legh.8300
Totals52300

In 25 cases of the 52 an oil-gland was looked for and, in every case, it was found to be missing.

Table 26, the conclusions from which were drawn in my 1906 report, seemed to indicate the dominance of tail over its absence. On this hypothesis I suspected that if No. 117 were bred to his (tailed) offspring about 50 per cent of the progeny would be tailless, and if the tailed hybrids of the F1 were bred together about 25 per cent of their progeny should be tailless. The actual result of such matings is shown in table 27.

Table 27.—Heterozygotes mated with father.

Serial No.Pen No.Tailless cock × heterozygotes.Offspring.
Father.Mother.Condition of uropygium.
No.From
pen No.		No.From
pen No.		Present.Small.Absent.
5653117Original.577532610
6653117Do.587532820
7653117Do.635532700
8653117Do.691532520
9653117Do.6525321500
10653117Do.691532520
11653117Do.705532920
12653117Do.713532720
13653117Do.7605321320
14653117Do.799532700
Total82130

Table 28.—Heterozygotes mated inter se.

Serial No.Pen No.Father.Mother.Condition of uropygium in offspring.
Frequency.Percentage.
No.From
pen No.		No.From
pen No.		Present.Small.Absent.Present.Small.Absent.
15661466526401a52650010000
1666146652663553250010000
1766146652669153240010000
1866146652679953241080200
19649516532a521532a174081190
20649516532a565532a247077230
21649516532a665532a114073270
22649516532a692532a18109550
2365234352534452582080200
2466142852663553240010000
2566142852669153230010000
2666142852679953250010000
Total10819085150

The results given in tables 27 and 28 are remarkable. Neither in the DR × R nor the DR × DR crosses did the tail fail to develop. The tailless condition, that I had strongly suspected of being recessive and expected in 25 per cent to 50 per cent of the offspring, never once appeared. The only point of variation in the uropygium of the chicks derived from the back cross or from F1's bred inter se was that in some the uropygium seemed distinctly smaller than in the others. This small uropygium was as a matter of fact recorded chiefly in chicks that failed to hatch, but it was occasionally noticed in older birds, being then usually associated with a slight convexity of the back. In some of the families the uropygium is recorded as small in suspiciously close to 25 per cent of the offspring. There is little doubt in my mind that this small uropygium represents in some way the "absence" of tail that was expected.

The next step was to cross the other rumpless bantam (No. 116), to see if he behaved like his father. Accordingly, in pen 653, I replaced the cock No. 117 by 116, the hens remaining the same, and got the result shown in table 29.

Table 29.—Heterozygotes mated with No. 116.

Serial No.Father's No.Mother's No.Condition of uropygium in offspring.
Present.Small.Absent.Per cent
absent.
27116508521059
2811657730350
2911658731450
3011665240233
3111670531556
3211671310267
3311676040233
Totals (55)2342851

Here we get a result almost exactly in accord with Mendelian expectation. Having, now, obtained rumpless hens, it became possible for the first time to test the inheritance of rumplessness in both parents. The result is shown in the table 30.

Table 30.—Rumpless fowl mated inter se.

[A] Both from chicks that died in shell.
[B] From a hatched chicken.
Serial No.Pen No.Father.Mother.Condition of tail in offspring.
No. From
Serial No.		 No. From
Serial No.		Present.Small.Absent.
34742297827260129004
35854297827343027009
367422978273430...[A]207
37854297827297727[B]101
Total3021

Table 30 is unfortunately small; one may say, fragmentary. Rumpless hens are incapable of copulating unless the tail coverts are trimmed; moreover my birds have been so much inbred that they are very weak; finally, the chicks are so small that it is impracticable to rear them in brooders and the eggs are particularly apt to be broken by the brooding hens. However, it suffices to show that two tailless fowl are able to throw some tailed offspring.

The second lot of rumpless fowl, namely, those that arose de novo in my yards, must now be considered. In 1906, 2 birds hatched out from ordinary tailed strains. As one was a cock and the other a hen these were mated in 1907. The cock (No. 2464) came from No. 71♀ (a pure White Leghorn bred by myself from original White Leghorn stock described in my 1906 report) and No. 235♂ (an F1 hybrid between one of these White Leghorns and my original Rose-comb Black Minorca). The hen was No. 1636. Her mother (No. 618) was an F1 hybrid between a Minorca and Dark Brahma of series V, 1906 report, and her father (No. 637) had the same origin. Thus the parents and grandparents of both of these new rumpless birds were well known to me and known to be fully tailed and to throw only tailed birds, with the exception of these two birds.

The result of the mating of Nos. 2464 and 1636 in pen 736 was 25 chicks, of which 24 had tails and 1 (No. 5335) was without tail or oil-gland. This, unfortunately, died early, so it was impossible to breed it. In 1908, the hen No. 1636 having in the meantime died, I mated No. 2464♂ to 6 of his (tailed) daughters. He was not well and soon died, leaving no descendants by them, but 5 offspring by a female cousin, all tailed. Then one of his sons (tailed) was mated to its own sisters and produced 49 offspring, all tailed. Thus the strain seems to have died out. The whole history is important both because an apparently new mutation had taken place and because it was, in a degree, "hereditary."

How, if at all, can this case and those of the bantams be brought under known laws of inheritance? First of all, it must be confessed that the provisional hypothesis, suggested in my earlier report, that rumplessness is in my strain recessive has not been supported by the newer facts. In the light of the principle of imperfect dominance to which the facts of the last two chapters have led us, everything receives a satisfactory explanation. The only conclusion that meets all the facts is this: The inhibitor of tail development—the tailless factor—is dominant; its absence—permitting a continuation of the normal development of the tail region—is recessive.

The application of this hypothesis to the various matings may now be attempted. No. 117 is to be regarded as a heterozygote. The matings with tailed birds is of the order DR × R, and expectation in the typical case is 50 per cent DR (interrupted tail) and 50 per cent RR (non-interrupted). But, owing to the relatively weak potency of the interrupter derived from No. 117, growth of the tail is not interrupted in the heterozygous offspring. These offspring are, by hypothesis, so far as their gametes go, of two equally numerous sorts, DR and RR. Mated to No. 117, two sorts of families are to be expected, namely, the products of DR × RR (=50 per cent DR, 50 per cent RR) and the products of DR × DR (=25 per cent DD, 50 per cent DR, 25 per cent RR). The first lot of families might be expected to resemble the preceding generation in consisting entirely of tailed birds; the latter might be expected to show in the 25 per cent extracted DD's evidence of the presence of the undiluted interrupter. Actually in matings of the latter sort (table 27) 3 families show no trace of the tail-interrupter, but in 7 there is evidence of a disturbance, as shown by the small size of the uropygium and the bent back. In these families there are 13 cases of small uropygium to 53 of large, being about 20 per cent of the affected uropygium where 25 per cent was to be looked for—not a wide departure, considering the liability of not recognizing the reduced uropygium as such. This failure even of the extracted dominants completely to stop the development of the tail gives a measure of the weakness of the inhibitor in this case. Also, in table 28, matings are varied. Some are probably matings of two heterozygotes, others of two recessives, and others still of a recessive with a heterozygote. On our hypothesis we should expect some of the families of the mated hybrids to show evidence of the inhibiting factor and others to show no such evidence. In those families in which small tail appears it is found in about 19 per cent of the cases. On account of this weakness of the inhibitor in the germ-plasm of No. 117 that inhibitor is rarely fully activated. Only in one case out of the 250 or more in which that germ-plasm is used is the development of the tail completely stopped. In this case a hybrid cock derived from pen 526 (series 2, table 26) was crossed with various birds of tailed races (probable RR's), and produced in addition to 20 tailed offspring 1 devoid of uropygium and oil-gland. In this case we may conceive that an unusually potent condition of the inhibitor wholly stopped the development of the tail.

The behavior of No. 116 is that of a pure dominant. Mated to DR (and some RR?) females he produces pure dominants and heterozygotes. His inhibiting factor is potent enough to be active in the DD offspring at least; as a matter of fact 47 per cent of his get have their tails inhibited. Even in the DR's the inhibitor may sometimes work itself out. Thus No. 116 crossed on No. 508, without tailless ancestry, had 56 per cent of the progeny without tail. Since tailless birds may be either pure dominants or DR's, we may expect families of two sorts when two such are bred together—those containing only tailless offspring and those containing only 75 per cent or less of such. Both sorts of families are to be expected in a table with the composition of table 30, and both appear there.

The case of the rumpless fowl that arose de novo will be explained, then, as follows: Even in normal RR matings the inhibiting factor may arise by mutation. But even when two of these inhibiting factors are paired they show themselves so weak as not to appear in 25 per cent, much less the typical 75 per cent of cases, but, as in our case, merely 4 per cent. The strain takes on, indeed, the essential features of the "eversporting varieties" of De Vries (1905). It seems probable, therefore, that even in eversporting varieties inheritance may be Mendelian, modified by variations in "potency" as shown by irregularities in dominance.

CHAPTER V.
WINGLESSNESS.

The entire absence of appendages is a rare monstrosity, few cases having been cited even for man. In my experience with poultry, out of about 14,000 birds I have obtained one that had no wing on one side of the body, but this unfortunately died before being bred from. A second bird was given to me by a fancier. The bird was an Indian Game, a vigorous cock, which was handicapped by his abnormality in two ways. First, whenever he fell upon his side or back he was unable to get upon his feet without aid. On several occasions he evidently had spent hours upon the ground before he was discovered and picked up. The wings are thus clearly most important to the fowl in enabling it to regain its feet after having become prone. Secondly, he was unable to tread a hen, since this act requires the use of wings as balancers. He was, however, able to copulate with small birds without leaving the ground. Thus in two respects his abnormality would have proved fatal in nature. First, because of the personal risk, the greater since a prone bird must fall an easy prey to predaceous enemies; and secondly, because of the risk to his germ-plasm. Little wonder, then, that this abnormality should not be known among wild ground-birds.

Mated to 6 hens this wingless cock produced 130 chicks in 1907, of which all had two wings. The following year he was mated to his daughters, but died without leaving offspring. So I used a son of his to mate with his own sisters and half-sisters. The progeny in this F2 generation consisted of 223 chicks, all of which had two wings. Thus, no trace of winglessness appeared in any of the descendants of the wingless cock.

The explanation of this case is not very certain, in view of the limited data. It seems to resemble the behavior of No. 117, the rumpless cock. And following the interpretation given in his case I would conclude that winglessness is dominant to the normal condition, that the original wingless cock was a heterozygote, and that the dominance of winglessness was imperfect in the first generation. On this hypothesis his son may well have been a pure recessive, and then all of his descendants, in turn, would be either recessives or heterozygotes (with imperfect dominance). It is, on the other hand, possible that the wingless cock was a pure dominant, but that the potency of the inhibitor was so slight as not to appear in the heterozygotes or even in extracted dominants.

CHAPTER VI.
BOOTING.

The method of inheritance of the feathering on the feet of some poultry has already been made the subject of much study. Hurst (1905, p. 152) crossed booted and non-booted birds and bred the hybrids together. He concluded that "the Mendelian principles are at work in these aberrant phenomena, but are masked by something not yet perceived." My own conclusion (1906, p. 72) was: "Booting is dominant, but usually imperfectly so." A more extended study has been desirable.

Booting is variable in amount. To indicate its degree I have had recourse to an artificial scale. I recognize 11 grades, running from 0 to 10. The grade 0 implies no feathers whatsoever. Grade 10 implies heavy booting extending over the front half of the shank. Grade 5 implies an extent of only half of the maximum, i. e., the outer front quarter of the shank. Intermediate grades indicate intermediate extension of the feathered area.