Studies on variation and selection. 165 



male, bred from a wild Mus norvegicus and a female "rat de labora- 

 toire". The rats were numbered, and classed in a catalogue, which 

 gave on each half page the number of a rat, its colour, and sex and 

 the number of its parents, the animals it was mated to, and those 

 of its young, produced with any of these. A picture of the rat was 

 filled in in an outline, printed on the page from a rubber stamp. 

 Thus, it was easy, for any given rat, to look up its ascendants as 

 well as its descendants. 



The breeding-experiments ended in a catastrophe, only one single 

 rat surviving an all-devastating epidemic. 



The records, however complete, were far from giving us a clear 

 oversight over the results so far obtained. When however, Prof. 

 Castle published his paper in the American Breeders Magazine, and 

 thus showed still to adhere to the belief, that he had, by selection, 

 modified a gene (or in his own words a unit-character) we thought 

 it worth while to put the pictures of our rats together, and arrange 

 them in pedigrees, to show clearly what selection had accomplished 

 in the few generations we had bred. 



The result is shown in the two figures. Of each family the 

 parents are figured at the left, all the children born from them at 

 the right. Where a rat figures more than once in the plate, it has 

 been given a distinctive sign, to facilitate looking up its descendance. 



In the first figure is shown the genealogy of a family of Hooded 

 rats, originating from a brother and sister, of which we know nothing 

 further, than that they were produced from two hooded parents, 

 and that the litter contained some albinos. These two rats, N°s 47 

 and 3 have given eleven Hooded children, of which some were as 

 dark as the parents or darker (e. g. 46 and 125) and some much 

 lighter (45, 44, 50). 



The variation in this family is discontinuous. 



Two of the darkest, N°s 49 and 125 have been mated (46 and 

 125 being of the same sex) and they have given three dark hooded 

 young. Two of the lightest, 44 and 50, have given only two light 

 Hooded young. The two extremes, 44 and 46 have been mated, 

 producing dark and light Hooded young. The two lightest, 44 and 

 45, have given fourteen children, all Hooded, and all light. One of 

 the dark sons of 44 and 46, N° 150, has been mated back to his 

 mother, 46. They have given dark young chiefly, but also some 

 hghts, 261 and 268. Of these, two dark ones have been selected, 

 23, 263 and 260. These have given six young, all dark. 



