Ijß Hagedoorn. 



Factor I. 



All the factors of which we have treated until now are present 

 in most wild Mus muscubis, and it can be safely assumed, that indivi- 

 duals lacking any one of this factors were produced by the loss of 

 this factor from a gamete (Mutation). It is only necessary to assume 

 that each of these eight factors was lost once, to have an explanation 

 of the origin of the hundrees of differently coloured types in mice. 

 I have myself a few times witnessed such a mutation, the sudden 

 loss of one genetic factor from at least one gamete, produced by an 

 individual, homozygous for this factor, under conditions of rigorous 

 control. Several analogous cases of a mutation are on record, each 

 time the loss of one factor at the time. I want to draw attention 

 to the fact, that never as yet has there been found a case of the 

 spontaneous acquisition of a hitherto absent genetic factor. Never- 

 theless there exist several groups, in which some types, apparently 

 derived from one original wild type, possess genetic factors, not present 

 in this wild type. In such cases the question is an open one, whether 

 such factors have indeed been acquired spontaneously, being created 

 at the formation of the new type, or whether they have been acquired 

 by a cross from some other form, possessing them. So, for instance, 

 have the young from the cross between the straight-tailed wild boar 

 and a curledtailed domestic swine, all curled tails. Apparently these 

 domestic swine have at some time or other spontaneously asquired 

 a genetic factor not present in their wild ancestor. But it seems to 

 me even more probable that this factor was acquired from some other 

 wild species of Sus with a curled tail, by crossing. 



In the domestic varieties of chickens, there exist factors, which 

 seem not to be present in the wild Gallus femigineus, the alleged 

 ancestor of all our domesticated pultry. So, for instance have black 

 fowls a factor, which such chickens as are coloured like ferrugineus 

 have not. And a certain factor, producing, when present a barring 

 of the leathers, also seems to be absent from ferrugineus. It seems 

 to me perfectly plausible to assume that these two factors have been 

 acquired, not as the result of a mutation, but as the result of a cross 

 with some other species, in which they were present, the first one 

 e. g. from G alius pur pur eus, the second one from G soeneratti. Darwin 

 founded his idea that the races of domestic poultry are exclusively 

 derived from one single species, ferrugineus, on the fact that some 

 peculiarities of other wild species of Gallus are not found in domestic 

 poultry. But since we know that genetic factors are independently 



