tion
if I neglected to include some important work which has led some of my
fellow-workers to a very different conclusion.

[Illustration: FIG. 88. Scheme to show classes of hooded rats used by
Castle. (After Castle.)]

Castle in particular is the champion of a view based on his results with
hooded rats. Starting with individuals which have a narrow black stripe
down the back he selected for a narrower stripe in one direction and for a
broader stripe in the other. As the diagram shows (fig. 88) Castle has
succeeded in producing in one direction a race in which the dorsal stripe
has disappeared and in the other direction a race in which the black has
extended over the back and sides, leaving only a white mark on the belly.
Neither of these extremes occurs, he believes, in the ordinary hooded race
of domesticated rats. In other words no matter how many of them came under
observation the extreme types of his experiment would not be found.

Castle claims that the factor for hoodedness must be a single Mendelian
unit, because if hooded rats are crossed to wild gray rats with uniform
coat and their offspring are inbred there are produced in F_2 three uniform
rats to one hooded rat. Castle advances the hypothesis that factors--by
which he means Mendelian factors--may themselves vary in much the same way
as do the characters that they stand for. He argues, in so many words, that
since we judge a factor by the kind of character it produces, when the
character varies the factor that stands for it may have changed.

As early as 1903 Cuenot had carried out experiments with spotted mice
similar to those of Castle with rats. Cuenot found that spotted crossed to
uniform coat color gave in F_2 a ratio of three uniform to one spotted, yet
selection of those spotted mice with more white in their coat produced mice
in successive generations that had more and more white. Conversely Cuenot
showed that selection of those spotted mice that had more color in their
coat produced mice with more and more color and less white. Cuenot does not
however bring up in this connection the question as to how selection in
these spotted mice brings about its results.

Without attempting to discuss these results at the length that they deserve
let me briefly state why I think Castle's evidence fails to establish his
conclusion.

In the first place one of the premises may be wrong. The three to one ratio
in F_2 by no means proves that all conditions of h