48 THE PRINCIPLES OF HEREDITY 



other colour. " In breeding Game-fowls ... if you cross a 

 black with a white game, you get birds of both breeds of the 

 clearest colour . . . when turnspit dogs and Ancon sheep, 

 both of which have dwarfed limbs, are crossed with common 

 breeds, the offspring are not intermediate in structure but 

 take after either parent." l Long-established races such as 

 .natural varieties are usually prepotent, though seldom entirely 

 so over newer races such as domestic varieties. Thus the ass 

 is usually prepotent over the horse, and the jackall over the 

 dog. Very inbred stock are also said to be prepotent over 

 less inbred stock. 2 When varieties are crossed there is a 

 tendency for descendants to take exclusively after one or the 

 other form. But subsequent descendants occasionally revert 

 from one to the other parent variety. 



(c) In particulate inheritance there is exclusive inheritance 

 not as regards the whole organism, but as regards particular 

 characters. Thus a child may inherit the mother's eyes, but 

 the father's nose. Thus also a horse may not blend the 

 colours of its parents, or inherit exclusively the colour of one 

 of them ; he may be a piebald, and display both colours 

 unblended in patches. It is conceivable that all blended 

 inheritance is only a form of particulate inheritance in which 

 the patches are invisibly small. 



82. Now sexual reproduction is plainly a cause of varia- 

 tions, since in blended and particulate inheritance the offspring 

 varies from both parents, and since in exclusive inheritance he 

 varies from one of them. Even if, bearing in mind the con- 

 ception of the mid-parent 3 we agree not to regard blended 

 inheritance as a variation, yet exclusive inheritance is certainly 

 a variation from one parent, and particulate inheritance is a 

 variation from both. If of several offspring one exhibits 

 blended inheritance, a second inclusive inheritance, and a 

 third particulate inheritance, they vary amongst themselves, 

 and it is difficult to understand how they can do this without 



1 Animals and Plants, vol. ii., p. 70. 



2 The Penicuik Experiments, p. xlix. 



3 " The word * Mid-Parent ' . . . expresses an ideal person of com- 

 posite sex, whose stature is half-way between the stature of the father 

 and the transmuted stature of the mother." (Natural Inheritance, p. 87. 

 See also The Grammar of Science, p. 470.) Similarly a mid-grandparent 

 is a conception obtained by striking an average between the characters of 

 the four grandparents, and a mid-ancestor of any generation by striking 

 an average between all the ancestors of that generation. The expression 

 is a useful one as saving much circumlocution. Except when otherwise 

 indicated by the context the terms parent, grandparent, and ancestor are 

 always intended by the present writer to imply mid-parent, mid-grand- 

 parent, and mid-ancestor. 



