By crossing dancing mice with common white mice both Haacke and von Guaita obtained gray or black mice which are very similar to the wild house mouse in general appearance and behavior. The characteristic movements of the dancers do not appear. As the result of a long series of breeding experiments, Darbishire (16 pp. 26, 27) says: "When the race of waltzing mice is crossed with albino mice which do not waltz, the waltzing habit disappears in the resulting young, so that waltzing is completely recessive in Mendel's sense; the eye-color of the hybrids is always dark; the coat-color is variable, generally a mixture of wild-gray and white, the character of the coat being distinctly correlated with characters transmitted both by the albino and by the colored parent." When hybrids produced by the cross described by Darbishire are paired, they produce dancers in the proportion of about one to five.
Bateson (5 p. 93, footnote), in discussing the results obtained by Haacke, von Guaita, and Darbishire, writes: "As regards the waltzing character, von Guaita's experiments agree with Darbishire's in showing that it was always recessive to the normal. No individual in F1 [thus the first hybrid generation is designated] or in families produced by crossing F1 with the pure normal, waltzed. In Darbishire's experiments F1 x F1 [first hybrids mated] gave 8 waltzers in 37 offspring, indicating 1 in 4 as the probable average. From von Guaita's matings in the form DR x DR the totals of families were 117 normal and 21 waltzers…. There is therefore a large excess of normals over the expected 3 to 1. This is possibly due to the delicacy of the waltzers, which are certainly much more difficult to rear than normals are. The small number in von Guaita's litters makes it very likely that many were lost before such a character as this could be determined."
Bateson does not hazard a guess at the origin of the dancer, but merely remarks (5 p. 86) that the exact physiological basis of the dancing character is uncertain and the origin of this curious variation in behavior still more obscure. "Mouse fanciers have assured me," he continues, "that something like it may appear in strains inbred from the normal type, though I cannot find an indubitable case. Such an occurrence may be nothing but the appearance of a rare recessive form. Certainly it is not a necessary consequence of inbreeding, witness von Guaita's long series of inbred albinos." (von Guaita (17 p. 319) inbred for twenty-eight generations.)
From the foregoing survey of the available sources of information concerning the origin and history of the race of dancing mice the following important facts appear. There are four theories of the origin of the race: (1) origin by selectional breeding (Haacke, Zoth, Milne- Edwards); (2) origin through the inheritance of an acquired character (Kishi); (3) origin by mutation, inheritance, and selectional breeding (Saint Loup, Kammerer, Cyon); (4) origin by natural selection, and inheritance, favored by selectional breeding (Landois). Everything indicates that the race originated in China. It is fairly certain that individuals with a tendency to move in circles appear at rare intervals in races of common mice. It seems highly probable, in view of these facts, that the Chinese took advantage of a deviation from the usual form of behavior to develop by means of careful and patient selectional breeding a race of mice which is remarkable for its dancing. Even if it should be proved that the mutation as it appears among common mice is not inherited, the view that slight deviations were taken advantage of by the breeders would still be tenable. The dancing tendency is such in nature as to unfit an individual for the usual conditions of mouse existence, hence, in all probability human care alone could have produced and preserved the race of dancers.
In answer to the question, how and when did the race of dancers originate, it may be said that historical research indicates that a structural variation or mutation which occasionally appears in Mus musculus, and causes those peculiarities of movement which are known as dancing, has been preserved and accentuated through selectional breeding by the Chinese and Japanese, until finally a distinct race of mice which breeds true to the dance character has been established. The age of the race is not definitely known, but it is supposed to have existed for several centuries.
CHAPTER II
FEEDING, BREEDING, AND DEVELOPMENT OF THE YOUNG
In this chapter I shall report, for the benefit of those who may wish to know how to take care of dancing mice, my experience in keeping and breeding the animals, and my observations concerning the development of the young. It is commonly stated that the dancer is extremely delicate, subject to diseases to an unusual degree and difficult to breed. I have not found this to be true. At first I failed to get them to breed, but this was due, as I discovered later, to the lack of proper food. For three years my mice have bred frequently and reared almost all of their young. During one year, after I had learned how to care for the animals, when the maximum number under observation at any time was fifty and the total number for the year about one hundred, I lost two by disease and one by an accident. I very much doubt whether I could have done better with any species of mouse. There can be no doubt, however, that the dancer is delicate and demands more careful attention than do most mice. In March, 1907, I lost almost all of my dancers from what appeared to be an intestinal trouble, but with this exception I have had remarkably good luck in breeding and rearing them.
My dancers usually were kept in the type of cage of which Figure 2 is a photograph.[1] Four of these double cages, 70 cm. long, 45 cm. wide, and 10 cm. deep in front, were supported by a frame as is shown in Figure 3. The fact that the covers of these cages cannot be left open is of practical importance. A similar type of cage, which I have used to some extent, consists of a wooden box 30 by 30 cm. by 15 cm. deep, without any bottom, and with a hinged cover made in part of 1 cm. mesh wire netting. Such a cage may be placed upon a piece of tin or board, or simply on a newspaper spread out on a table. The advantage of the loose bottom is that the box may be lifted off at any time, and the bottom thoroughly cleansed. I have had this type of cage constructed in blocks of four so that a single bottom and cover sufficed for the block. If the mice are being kept for show or for the observation of their movements, at least one side of the cages should be of wire netting, and, as Kishi suggests, such objects as a wheel, a tower, a tunnel, a bridge, and a turntable, if placed in the cage, will give the animals excellent opportunity to exhibit their capacity for varied forms of activity.
[Footnote 1: This cage was devised by Professors W.E. Castle and E.L.
Mark, and has been used in the Zoological Laboratories of Harvard
University for several years.]