IV] Colotivs of Mice 83 



On microscopical examination the dilution of the pig- 

 ment seems to consist in a diminution in the number of 

 the pigment granules, and not in a reduction of their size. 

 It is interesting to notice that many different animals have 

 varieties in which the dilution has proceeded to the same 

 extent. For example the particular dilution of black which 

 we call blue is known as characterizing definite varieties in 

 both rabbits, cats and mice, not to mention other cases less 

 certainly comparable. 



As was mentioned in the description of the Andalusian 

 fowl and its genetic features the blue colour of that breed is 

 not comparable with the blues we have been discussing. 

 In the mouse the blue is gametic, being a condition which 

 can be carried by the germ cells, while in the Andalusian 

 the blue is zygotic and depends on the collocation in one 

 individual of one germ-cell bearing black with another 

 which does not bear black. It is interesting, as exemplify- 

 ing the danger of reasoning from analogy where genetic 

 phenomena are concerned, that the blue-roan of cattle should 

 not follow the same rules as the other mammalian blue 

 varieties, but should conform rather to the Andalusian 

 system. Of course blue roan is, even to the eye, not the 

 homogeneous blue of the blue cat or mouse, but a mixture 

 of white or whitish hairs among blue and black ones. Still 

 on analogy we might have expected the blue of cattle to 

 be capable of representation in the germ-cells, but the 

 facts, so far as I can discover, afford no support to that 

 supposition^. 



* From such meagre evidence as I have obtained it is likely that the 

 blue, or blue dapple, of Dachshunds and other hounds is also a heterozygous 

 combination. As to the blue of Greyhounds and Great Danes I have no 

 iniormation, but I suspect it to be a dilute black capable of bemg bred 

 true. 



6—2 



