50 GENETICS AND EUGENICS 



(" lata mutant ") contains one more chromosome than the 

 parent species from which it has been observed repeatedly to 

 arise. Another type of mutant in this same group of plants 

 contains twice the ordinary number of chromosomes (" gig as 

 mutant," Gates, 1915). The fact that visible characters of 

 the organism vary simultaneously with variation in the 

 chromosomes creates a presumption that the relationship is 

 a causal one. 



3. The experimental evidence shows that in general the 

 father is just as influential as the mother in determining the 

 inheritance of the children. But the egg-cell is vastly larger 

 than the sperm-cell. Therefore much of the substance of the 

 egg cannot be concerned in heredity. What the egg and 

 sperm-cell have in common consists more largely of chromatin 

 than of any other substance. This makes it seem probable that 

 chromatin is concerned in heredity. 



4. There exists a parallelism between the behavior of the 

 chromosomes in the development of the germ-cells and that 

 of certain characteristics in heredity. It is supposed, there- 

 fore, that the chromosomes actually contain chemical sub- 

 stances necessary for the development of these inherited 

 characters and in this sense are determiners of heredity. 



The assumption of Weismann that heredity is due to deter- 

 miners contained in the germ-cell, like the pangenesis theory 

 of Darwin, has encountered many difficulties. Consequently 

 numerous supplementary hypotheses have been found neces- 

 sary to enable it to feature as a general explanation of the 

 facts of inheritance. 



Difficulties Encountered by Weismann's Theory 



1. Development (ontogeny). The first diflSculty en- 

 countered lay in the explanation of the development of the 

 individual from the egg. Weismann assumed that each cell 

 owes its peculiar form and activities to the determiners which 

 it contains, these being located in its chromosomes. Since 

 the cells composing the different parts and tissues of the body 

 differ in their forms and activities, it was necessary to assume 



