ii6 COMMON MODES OF INHERITANCE 



thousand in man, in which either the two irises differ in colour, 

 or the one iris shows different patches of colour " (Pearson, 1900, 

 p. 452). The case of an English sheep-dog with a paternal eye 

 on the one side of its head and a maternal eye on the other is 

 vivid enough. 



In hybrids there are many instances of particulate inheritance. 

 Thus Sanson notes that in the hybrid cattle of Nivernais, the 

 same beast may have a horn of the Durham type on one side 

 and a horn of the Charolais type on the other. We have already 

 alluded to the hybrid Adam's laburnum {Cytisus adami), which 

 bore pink flowers (a blend) on some branches, yellow flowers 

 (of one parent, C. laburnum) on other branches, and purple 

 flowers (of the other parent, C. purpureas) on other branches. 



When there is a marked difference in the pedigree, the vigour, 

 the age — in short, in the constitution — of the two parents, the 

 same mode of inheritance may be illustrated in a succession of 

 offspring. Thus an inbred sire, paired with a commonplace mate, 

 may be prepotent birth after birth ; or a young mother mated 

 with a worn-out male may have it all her own way in regard to 

 inheritance, as well as in much else. On the other hand, when 

 there are no such marked differences between the parents, the 

 inheritance may be a blend in one offspring, exclusive in another, 

 particulate in a third. Moreover, in the same offspring, different 

 sets of characters may illustrate different modes of inheritance. 

 Thus we see that these modes of inheritance are merely useful 

 descriptive terms, helping us to keep our facts in order, but not 

 directly aiding us in their interpretation. They point to the 

 need of some unifying conception, which shall enable us to 

 understand how all these alternatives are possible. 



Postponing this, however, we shall simply notice a few facts 

 which indicate that much depends on the condition of the two 

 gametes when they unite in amphimixis. 



In large families there is sometimes observable an interesting 

 change in the direction of preponderance in the successive 



