258 Referate. 
discontinuous variations be modified by cross breeding? These questions 
the authors have attempted to answer by several series of experiments on 
rats and guinea pigs. As regards the pattern relations of colour and white 
four classes may be distinguished in the rat, viz: 
(1) Self, whole body pigmented. 
(2) Irish, whole body pigmented except more or less of the ventral 
surface. 
(3) Hooded, only the head, shoulders, and usually a median dorsal 
stripe pigmented. 
(4) Albino, no pigmentation. 
Incidentally the authors confirm the work of previous observers who 
have shewn that pigmentation is dominant to albinism, that self colour is 
more or less completely dominant to the Irish and hooded patterns, and 
that Irish is on the whole dominant to hooded. Moreover the albino may 
be shewn to contain either the self colour or either of the pattern characters. — 
A series of experiments was made in which a hooded strain was crossed 
with Irish. Several hundred individuals in all were bred and the authors 
conclude as the result of these experiments that the effect of crossing the 
hooded with the more fully pigmented Irish form is to produce a strain 
of hooded with a distinctly greater amount of pigmentation in the dorsal 
stripe than the original strain exhibited. 
“A cross with the Irish stock raises considerably the average size of 
„the dorsal stripe in hooded rats, as well as the range of variation upward 
„in size of stripe’. The hooded rats so produced were bred inter se (p. 12) 
for thee further generations. In the first two of these the average increased 
size of the dorsal stripe was maintained but in the third it became distinctly 
smaller, smaller even than it was in the original hooded strain before the 
Irish cross was made. In view of this result we think that caution should 
be exercised in drawing any deduction of a far reaching nature from these 
experiments. 
A further series of experiments deals with the selection of strains of 
hooded rats for reduced and for increased size of the dorsal stripe. Selection 
of rats in which the dorsal stripe was reduced led in four generations to 
a strain in which it was very much smaller than in the normals. The 
reduction process was apparently a steady one and the absence of what 
may be called regression has led the authors to the conclusion that the 
effects of selection ,,in reducing the extent of the pigmentation will be 
„permament, that is, that a stable, narrow-striped variety of hooded rats 
„can be established by selection, and that this variety will breed true”. — 
A similar set of experiments in which selection was for increased size 
of the dorsal stripe led to a similar conclusion. While recognising the 
need of further experiment the authors are inclined to consider that 
, selection is a most important factor, not only in the isolation of discon- 
tinuous variations, but also in their production“. And here it may be 
suggested that Mac Curdy and Castle’s results are not inconsistent with the 
idea that pure lines differing in the extent of the dorsal stripe may exist 
in hooded rats similar to the pure lines which Johannsen was able to 
demonstrate in his beans. If this were so selection would be concerned 
here also with the isolation of preexisting discontinuous variations. It 
would be of great interest to establish a race of hooded rats breeding true 
to the minimum amount of pigmentation, and then to select these for 
fuller pigmentation. If, as Mac Curdy and Castle are inclined to maintain, 
