70 THE FIRST PRINCIPLES OF HEREDITY 



crossing of species and varieties, and then with the indi- 

 vidual characters, as exhibited by the offspring of parents 

 of the same species.* 



{a) Blended Inheritance. 



We must assume with Weismann that the majority 

 of Ids, if not all, contained in the germ-plasm of any 

 species, consist of determinants representing all the racial 

 characters of that species. They all have the tendency to 

 determine identical or " homologous " parts of the body, 

 and are therefore called by Weismann " homologous Ids," 

 containing homologous determinants. As these homol- 

 ogous units all strive to express identical cells or cell- 

 groups, their action will tend in the same direction, and the 

 effect of their determining power will be the combined 

 result of all. On the other hand, if the determinants 

 represent different parts of the body (as will happen with 

 determinants of widely distant species), the determinants 

 are said to be " heterologous"; they will not combine in 

 their action, but mutually inhibit each other's tendencies. 

 This explains, according to Weismann's theory, why 

 widely different species are not fertile with each other, 

 because harmonious combination between the very heterol- 

 ogous determinants of such species is impossible, each set 

 of determinants trying to impress during the development 

 of the organism a widely different constitution on the 

 successive organic parts. 



While the homologous determinants are identical in 

 their effect as regards the part of the organism which they 

 determine, they may differ in allied species or varieties 

 with regard to the quality they impress on that same part. 



* Much of the remainder of this chapter is very compUcated, 

 but has to be given for the sake of completeness. The beginner 

 should not despair, but may, if unable to understand it, leave it 

 out, and reserve it for later perusal, when he has mastered the 

 subject better. 



