4 Result of Crossing Japanese Waltzing with Albino Mice 
Let us imagine that our object is to make crosses between albino and Japanese 
waltzing mice, and that we are provided with a stock of waltzing mice which we 
know to breed true, and of albino mice of authentic pedigree. Two sorts of books 
are necessary — (i) a book recording the matings, and later on (ii) an illustrated 
catalogue. The entry of the cross in the mating book is quite simple. The date 
underneath the description refers to the day on which the mice were put together ; 
and the occurrences which come to pass on the subsequent dates are also recorded. 
Gross 1. 
$ Albino (pure bred) from Mr Steer. 
(/ Japanese waltzer. 
Dec. 24. 
removed, Jan. 10th. 
8 young born, Jan. 16th. 
% removed, Feb. 8th. 
Young sexed, Feb. 25th. 
The offspring are catalogued in two places for reasons which will shortly be 
explained : first they are catalogued as soon as the colour of their eyes and coat is 
discernible ; this might quite well be done in the mating book ; but I find it more 
convenient to have a separate book (First Catalogue) for the purpose : the coat- 
colour and eye- colour of each mouse are entered directly they can be plainly seen, 
so that a minimum number of records is lost through cannibalism or disease : 
nothing else is recorded ; the sex is not (for it cannot be) determined. Such an 
entry appears as follows :— 
Cross 1. 5c, 5c, 5c, 3c, 2c, 2c, Ic (ix. 21). 
The entry in brackets gives the date on which the mice were catalogued ; if 
this is not done and a note is made in the mating book that a young one of a 
certain description has escaped or died, doubt would arise in the mind of the 
observer as to whether that mouse had been catalogued or not. 
When the young are sexed they are entered in the illustrated catalogue, 
which, beside the record of matings (already described), is the second kind of 
book necessary for the observations. In this book each mouse is given a name : 
thus the first mouse which happens to be described, in the litter of the imaginary 
cross which we entered in the mating book above, is 1^', the second I''", and so on. 
The " a " signifies that the individual belonged to the first litter produced by 
those parents ; individuals of the second litter produced by the same parents 
would receive the names 1'^', 1**", 1^'", etc. Roman numerals are used to indicate 
the individuals of the family because they are never likely to become too large to 
be convenient, for litters very rarely exceed, and, in fact, very seldom attain, the 
number of ten, and l*"" is quite convenient to write. In cases of albinos and self- 
coloured mice, in which the tail is uniformly covered with pigment, I have 
merely written the description of the mouse, as a picture of such a one would 
