it is not at all probable that this is true of all of them. We may,
therefore, accept the statement that dancing individuals now and then
appear in various races of mice. They are usually spoken of as freaks,
and, because of their inability to thrive under the conditions of life of
the race in which they happen to appear, they soon perish.

Another and a strikingly different notion of the origin of the race of
dancers from those already mentioned is that of Cyon (11 p. 443) who
argues that it is not a natural variety of mouse, as one might at first
suppose it to be, but instead a pathological variation. The pathological
nature of the animals is indicated, he points out, by the exceptionally
high degree of variability of certain portions of the body. According to
this view the dancing is due to certain pathological structural conditions
which are inherited. Cyon's belief raises the interesting question, are
the mice normal or abnormal, healthy or pathological? That the question
cannot be answered with certainty off-hand will be apparent after we have
considered the facts of structure and function which this volume presents.

Everything organic sooner or later is accounted for, in some one's mind,
by the action of natural selection. The dancing mouse is no exception, for
Landois (22 p. 62) thinks that it is the product of natural selection and
heredity, favored, possibly, by selectional breeding in China. He further
maintains that the Chinese dancer is a variety of _Mus musculus L._ in
which certain peculiarities of behavior appear because of bilateral
defects in the brain. This author is not alone in his belief that the
brain of the dancer is defective, but so far as I have been able to
discover he is the only scientist who has had the temerity to appeal to
natural selection as an explanation of the origin of the race.

Milne-Edwards, as quoted by Schlumberger (29 p. 63), is of the opinion