74 THE FIRST PRINCIPLES OF HEREDITY 



certain stage, we may get either paternal or maternal 

 characters expressed, according as the paternal or maternal 

 determinants of a certain cell-stage act homodynamously 

 together or not. To make the case clear, we may take 

 Weismann's illustration of a plant species A, which has 

 an old-established form of the flower, but a newly acquired 

 form of the leaf ; while in the crossed species B the form of 

 the flower is new, but the form of the leaf old. There will 

 be, therefore, in species A more homodynamous deter- 

 minants for the flower than for the leaf ; while in species B 

 there are more homodynamous determinants for the leaf 

 than for the flower. The hybrid of both will, therefore, 

 have a great tendency to have the form of the flower 

 from A, but that of the leaf from B. 



(d) Individual Traits. 



The germ-plasm of aU the individuals belonging to 

 the same species or race contains the same number of 

 Idants and Ids ; and, furthermore, all the determinants, 

 be it from the paternal or maternal side, must be homol- 

 ogous. 



Blended inheritance of individual traits, then, would 

 simply depend on the combined action of the determinants 

 from father and mother, which, being equal in number 

 and determining power, would bring about an intermediate 

 result between the characteristics of the parents. 



More difficult to explain is the case where the child re- 

 sembles only one parent — let us say the father. At first 

 sight it seems impossible that the child should have no 

 resemblance to the other parent, seeing that the mother, 

 too, furnishes the same quota of hereditary substance, 

 there being an equal number of Ids in the germ-plasm of 

 both parents. Furthermore, as the child receives only 

 half the number of chromosomes from the father, we must 

 presume that half the number of Ids of the father are 

 sufficient to express his full characteristics. All that can be 



