452 THE GRAMMAR OF SCIENCE 



z enables us to see something of the nature of heredity. 

 The a's define the inheritance from the male, the ^'s from 

 the female element. If both exist for any special 

 character z, then the inheritance is said to be blended. If 

 one or other set are numerically much larger than the re- 

 maining one, then the male or female element, as the case 

 may be, is said to be prepotent. If the a's or the /S's 

 are sensibly zero, then the inheritance is spoken of as 

 exclusive. In some cases the character is neither a 

 parental blend, nor is there absolute prepotency, i.e. 

 exclusive inheritance. There is an exclusive inheritance 

 first from one parent and then from another not neces- 

 sarily of different organs, but in parts of the same organ 

 or character. Thus coat-colour in horses may blend, but 

 we also get piebald horses. Eye - colour is generally 

 exclusive, but we get one or two cases per thousand in 

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

 one iris shows different patches of colour. The same 

 phenomenon is more common with the eyes of English 

 sheep dogs. Or, again, in the flowers of dahlias, where 

 there has been a cross between a light and dark flowered 

 variety, we find not only variegated and blended colours, 

 but flowers with marked patches of the two parental 

 colours. Such forms of inheritance are termed particulate. 



Thus we have three chief forms of inheritance : {a) 

 Blended Inheritance ; {h) Exclusive Inheritance ; (c) Particu- 

 late InJieritance. All three require careful statistical study, 

 and at present very little has been done except for the 

 case of blended inheritance. 



Returning now to the other symbols in our expression 

 of ^, namely, the -r's and the j''s, which give the characters 

 of the individual spermatozoon and ovum, we may remark 

 one or two points about them : — 



(i.) Are they purely determined by bathmic influences 

 within the individual, or are they related in any way to 

 their environment ? Does, in particular, the growth and 

 nourishment of the individual influence the reproductive 

 organs and so vary the character of the gametes produced 

 under different circumstances ? Many writers have held 



