HEREDITY IN MAN 357 



We have to think of the germinal constitution as containing 

 a very hxrge number of factors — predispositions in other words. ' 

 The characters to which these factors give rise may be the 

 characters which we see when we examine any individual, but 

 more often they are not. There are, for instance, cases in which 

 the colour of a flower or of the coat of a mammal are unit-characters 

 but more often such visible characters are the product of two or 

 more miit-factors. There are many cases now known in which 

 the visible character is in this fashion due to the presence of 

 several unit-factors. With regard to this hypothesis in general 

 all that can be said is that at present no facts are known which 

 are definitely in contradiction to it. It is the only hypothesis 

 which holds the field. ^ 



Up to the present very few unit-factors have been distinguished 

 in man. It has been found that brachydactyly — a peculiar mal- 

 formation of the hand — presenile cataract, tylosis — a thickening of 

 the hands and of the soles of the feet — epidermolysis bullosa — a 

 blistering of the skin — and night blindness behave as simple 

 Mendelian characters. So too does eye colour, pigment in front 

 of the iris being dominant to its absence — in other words, brown, 

 green, and hazel being dominant to pure grey and blue. There 

 is also some reason for thinking that musical ability is a recessive 

 character. On the hypothesis outlined above it must be supposed 

 that most of the visible characters of man are the product of 

 several unit-factors which have not yet been identified. What 

 appears to happen when matings take place between people 

 of different colour, stature, and so on is that there is a 

 blending of characters in the offspring. Such appearances are 

 not incompatible with the hypothesis — the assumption being that 

 many unit- characters are concerned. 



This question as to the ultimate nature of inheritance has been 

 introduced here because of its inherent interest. It is not as a 

 matter of fact strictly relevant. What we require to know is^ 

 rather what predispositions are present in the germinal consti- 

 tution. It is this which is essential ; the precise manner in which 

 the predispositions are represented in the gametes, whether by 



* The ' Ancestral Law of Heredity ', which attributes on the average half the 

 germinal constitution to the parents, a quarter to the grandparents, an eighth 

 to the great-grandparents, and so on, is not incompatible with Mendelian inheri- 

 tance as a general statistical result. The conception of the nature of inheritaivco 

 involved in this theory is, however, incompatible with the Mendelian conception of 

 unit-characters, which conception, it may be said, is the only one which fits the facts. 



