I] INTRODUCTION 6 



have obtained a form different from either parent. 

 But if we mate the same black parent with another 

 white individual, it may happen that all the offspring 

 are black, and instead of reverting to the wild form 

 they all follow one parent. If either the greys or 

 blacks produced in this way are mated together^ 

 some of their young will be white ; although none 

 of the children of the original white individual 

 resembled their white parent in colour, yet the white 

 has appeared again among the grandchildren after 

 skipping a generation. In man, a colour-blind father 

 rarely has colour-blind children, but some of his 

 nephews and male grandchildren through the female 

 line are usually affected ; that is to say, the disease 

 appears in males but is transmitted by females. 



It is clear from this short list of examples that 

 there are a number of different forms of hereditary 

 transmission, and our object must be, first to classify 

 them into groups in which the behaviour is similar, 

 and next to attempt to bring them under a common 

 scheme. And it is also clear that the different kinds 

 of heredity are associated with different kinds of 

 variation ; for example variation in height in man is 

 inherited diffferently from variation in colour-vision, 

 and both differ fi'om variation of coat-colour in 

 rabbits, in their inheritance. 



A question of a different kind is the cause of 

 inherited diff'erences, and whether differences due to 



