MENDELISM GENERAL DISCUSSION 69 



which "blends" on crossing. A marriage of a white 

 with a mulatto gives " quadroon " children, which are 

 usually recognisable as such, and are generally about 

 intermediate as regards skin colour. 



Such ordinary observation, however, is not sufficient 

 to enable us to say that Mendel's law does not apply. 

 It is possible that the case is only slightly more com- 

 plicated than that of the inheritance of red colour in 

 wheat. And without full and reliable statistics, which 

 are difficult to obtain, we cannot fairly set down this 

 instance as an exception to Mendel's law. 



A case in which much better evidence is available, 

 is that of the inheritance of ear length in rabbits. The 

 well-known " lop " variety is characterised by its large 

 size and the extraordinary length of its ears, which 

 causes them to hang down to the ground. In the speci- 

 mens used by Castle in his experiments the average ear 

 length was about 220 millimetres, while that of the 

 ordinary variety with which they were crossed was 

 about 110 millimetres. The hybrid between these two 

 types had ears about 160 millimetres long, or nearly 

 intermediate between the parent lengths. These 

 rabbits of the first hybrid generation were interbred, 

 but the offspring instead of showing ordinary segrega- 

 tion into 1 long, 2 intermediate, and 1 short, were all 

 about intermediate as regards ear length. Indeed, 

 although the number of individuals was too small to 

 base statistical conclusions on, there appeared to be 

 no greater variation in the second than in the first 

 hybrid generation. 



This experiment has aroused a good deal of dis- 

 cussion, and the real meaning of the results obtained 

 is doubtful. It is certain, however, that they cannot 

 be held as demonstrating the absence of segregation 

 with regard to ear length. It may well be that this 

 character depends on a considerable number of inde- 

 pendent factor pairs, so that a very large number of 

 individuals would require to be bred in order to have 

 any considerable chance of sensible segregation. 



Similar results have been obtained in the crossing 



