Referate. 257 
both B1 and B2 behave as simple recessives to the fully ;igmented state B. 
But what the relation between B! and B?2 is has not yet been worked out. 
Castle suggests that the various colour varieties have all arisen by 
loss, partial or complete, of one or more of the independent factors which 
contribute to the production of the grey coat of wild rabbits. He proceeds 
to an interesting comparison of the coat-colours in other rodents. 
In guinea-pigs the three main colour factors viz, black, yellow, and 
the barring factor are also present. But a point of difference between this 
animal and the rabbit, so far as is hitherto known, is that the black factor 
may be wanting. Such animals have a chocolate coat and their eyes are 
brown instead of black. Dilute chocolates are yellow with brown eyes, 
while dilute agoutis (wild type) and blacks are in ap; earance black-eyed 
yellows. The chocolate stock originally used did not contain the barring 
factor (A), but Castle suggests that there seems no reason to doubt that 
such chocolates may be made by appropriate matings. They would of 
course give agoutis when crossed with blacks. 
In rats and in mice the relation of the wild grey and black coats is 
the same as in the rabbit. But in mice we have to deal with two other 
forms of pigmented coat, viz. chocolate and yellow. The chocolate in the 
mouse appears to bear the same relation to the black as it does in the 
guinea pig, i. e. chocolate arises from black by the loss of the black factor B. 
When the chocolate carries the barring factor the animal is spoken of as 
a cinnamon agouti and such mice when crossed with blacks revert to the 
wild grey type. 
Yellow in the mouse offers a remarkable contrast to yellow in the 
rabbit and the guinea pig. In the first place it is dominant to all other 
colours, and in the second place the various observers who have worked 
with this colour are agreed that it does not occur in a homozygous condition. 
The existing evidence points to the so-called yellow in the rabbit and the 
guinea pig being a dilute form of grey or agouti (and in the guinea pig ot 
black also), and it seems most natural to regard some dilute form of grey 
in the mouse as comparable to the yellow of the other two rodents. 
Yellow in the mouse may possibly be of a different nature and further 
experimental work must be awaited before we can hope to assign to it 
its place in the general colour scheme. 
Summing up we may say that in all the four species of rodents dealt 
with the black form has arisen from the wild type by the loss of a 
dominant barring factor (A). Further loss of the black factor (B) leads 
to the production of a chocolate but this is only known to occur certainly 
in guinea pigs and mice. The three main colours grey (or agouti), black, 
and chocolate may be further modified by dilution factors. Of such factors 
two certainly occur in the rabbit and one in the guinea pig, while there 
may be several in mice. Yellow in rabbits and guinea pigs arises through 
the operation of a dilution factor but the jeculiarities of behaviour of 
yellow in the mouse suggest that the nature of the pigmentation is here 
different, but before the nature of this difference can be cleared up further 
experimental work is required. ey 62 Bunnect 
Mac Curdy, H. and Castle, W. E. Selection and Crossbreeding in Relation 
to the Inheritance of Coat Pigments and Coat Patterns in Rats and 
Guinea-Pigs. Contrib. Mus. Comparat. Zoolog. Harvard. 70, 1907. 
The authors of this paper start by formulating two questions, Viz. 
(1) Can®discontinuous variations be modified by selection alone? (2) Can 
